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NAME
erl_driver - API functions for an Erlang driver
DESCRIPTION
An Erlang driver is a library containing a set of native driver callback functions that the Erlang VM
calls when certain events occur. There may be multiple instances of a driver, each instance is associated
with an Erlang port.
Warning:
Use this functionality with extreme care!
A driver callback is executed as a direct extension of the native code of the VM. Execution is not made
in a safe environment. The VM can not provide the same services as provided when executing Erlang code,
such as preemptive scheduling or memory protection. If the driver callback function doesn't behave well,
the whole VM will misbehave.
* A driver callback that crash will crash the whole VM.
* An erroneously implemented driver callback might cause a VM internal state inconsistency which may
cause a crash of the VM, or miscellaneous misbehaviors of the VM at any point after the call to the
driver callback.
* A driver callback that do lengthy work before returning will degrade responsiveness of the VM, and
may cause miscellaneous strange behaviors. Such strange behaviors include, but are not limited to,
extreme memory usage, and bad load balancing between schedulers. Strange behaviors that might occur
due to lengthy work may also vary between OTP releases.
As of erts version 5.5.3 the driver interface has been extended (see extended marker). The extended
interface introduce version management, the possibility to pass capability flags (see driver flags) to
the runtime system at driver initialization, and some new driver API functions.
Note:
As of erts version 5.9 old drivers have to be recompiled and have to use the extended interface. They
also have to be adjusted to the 64-bit capable driver interface.
The driver calls back to the emulator, using the API functions declared in erl_driver.h. They are used
for outputting data from the driver, using timers, etc.
Each driver instance is associated with a port. Every port has a port owner process. Communication with
the port is normally done through the port owner process. Most of the functions take the port handle as
an argument. This identifies the driver instance. Note that this port handle must be stored by the
driver, it is not given when the driver is called from the emulator (see driver_entry).
Some of the functions take a parameter of type ErlDrvBinary, a driver binary. It should be both allocated
and freed by the caller. Using a binary directly avoids one extra copying of data.
Many of the output functions have a "header buffer", with hbuf and hlen parameters. This buffer is sent
as a list before the binary (or list, depending on port mode) that is sent. This is convenient when
matching on messages received from the port. (Although in the latest versions of Erlang, there is the
binary syntax, that enables you to match on the beginning of a binary.)
In the runtime system with SMP support, drivers are locked either on driver level or port level (driver
instance level). By default driver level locking will be used, i.e., only one emulator thread will
execute code in the driver at a time. If port level locking is used, multiple emulator threads may
execute code in the driver at the same time. There will only be one thread at a time calling driver call-
backs corresponding to the same port, though. In order to enable port level locking set the
ERL_DRV_FLAG_USE_PORT_LOCKING driver flag in the driver_entry used by the driver. When port level locking
is used it is the responsibility of the driver writer to synchronize all accesses to data shared by the
ports (driver instances).
Most drivers written before the runtime system with SMP support existed will be able to run in the
runtime system with SMP support without being rewritten if driver level locking is used.
Note:
It is assumed that drivers do not access other drivers. If drivers should access each other they have to
provide their own mechanism for thread safe synchronization. Such "inter driver communication" is
strongly discouraged.
Previously, in the runtime system without SMP support, specific driver call-backs were always called from
the same thread. This is not the case in the runtime system with SMP support. Regardless of locking
scheme used, calls to driver call-backs may be made from different threads, e.g., two consecutive calls
to exactly the same call-back for exactly the same port may be made from two different threads. This will
for most drivers not be a problem, but it might. Drivers that depend on all call-backs being called in
the same thread, have to be rewritten before being used in the runtime system with SMP support.
Note:
Regardless of locking scheme used, calls to driver call-backs may be made from different threads.
Most functions in this API are not thread-safe, i.e., they may not be called from an arbitrary thread.
Functions that are not documented as thread-safe may only be called from driver call-backs or function
calls descending from a driver call-back call. Note that driver call-backs may be called from different
threads. This, however, is not a problem for any function in this API, since the emulator has control
over these threads.
Warning:
Functions not explicitly documented as thread safe are not thread safe. Also note that some functions are
only thread safe when used in a runtime system with SMP support.
A function not explicitly documented as thread safe may at some point in time have a thread safe
implementation in the runtime system. Such an implementation may however change to a thread unsafe
implementation at any time without any notice at all.
Only use functions explicitly documented as thread safe from arbitrary threads.
As mentioned in the warning text at the beginning of this document it is of vital importance that a
driver callback does return relatively fast. It is hard to give an exact maximum amount of time that a
driver callback is allowed to work, but as a rule of thumb a well behaving driver callback should return
before a millisecond has passed. This can be achieved using different approaches. If you have full
control over the code that are to execute in the driver callback, the best approach is to divide the work
into multiple chunks of work and trigger multiple calls to the timeout callback using zero timeouts. The
erl_drv_consume_timeslice() function can be useful in order to determine when to trigger such timeout
callback calls. It might, however, not always be possible to implement it this way, e.g. when calling
third party libraries. In this case you typically want to dispatch the work to another thread.
Information about thread primitives can be found below.
FUNCTIONALITY
All functions that a driver needs to do with Erlang are performed through driver API functions. There are
functions for the following functionality:
Timer functions:
Timer functions are used to control the timer that a driver may use. The timer will have the emulator
call the timeout entry function after a specified time. Only one timer is available for each driver
instance.
Queue handling:
Every driver instance has an associated queue. This queue is a SysIOVec that works as a buffer. It's
mostly used for the driver to buffer data that should be written to a device, it is a byte stream. If
the port owner process closes the driver, and the queue is not empty, the driver will not be closed.
This enables the driver to flush its buffers before closing.
The queue can be manipulated from arbitrary threads if a port data lock is used. See documentation of
the ErlDrvPDL type for more information.
Output functions:
With the output functions, the driver sends data back to the emulator. They will be received as
messages by the port owner process, see open_port/2. The vector function and the function taking a
driver binary are faster, because they avoid copying the data buffer. There is also a fast way of
sending terms from the driver, without going through the binary term format.
Failure:
The driver can exit and signal errors up to Erlang. This is only for severe errors, when the driver
can't possibly keep open.
Asynchronous calls:
The latest Erlang versions (R7B and later) has provision for asynchronous function calls, using a
thread pool provided by Erlang. There is also a select call, that can be used for asynchronous
drivers.
Multi-threading:
A POSIX thread like API for multi-threading is provided. The Erlang driver thread API only provide a
subset of the functionality provided by the POSIX thread API. The subset provided is more or less the
basic functionality needed for multi-threaded programming:
* Threads
* Mutexes
* Condition variables
* Read/Write locks
* Thread specific data
The Erlang driver thread API can be used in conjunction with the POSIX thread API on UN-ices and with
the Windows native thread API on Windows. The Erlang driver thread API has the advantage of being
portable, but there might exist situations where you want to use functionality from the POSIX thread
API or the Windows native thread API.
The Erlang driver thread API only returns error codes when it is reasonable to recover from an error
condition. If it isn't reasonable to recover from an error condition, the whole runtime system is
terminated. For example, if a create mutex operation fails, an error code is returned, but if a lock
operation on a mutex fails, the whole runtime system is terminated.
Note that there exists no "condition variable wait with timeout" in the Erlang driver thread API.
This is due to issues with pthread_cond_timedwait(). When the system clock suddenly is changed, it
isn't always guaranteed that you will wake up from the call as expected. An Erlang runtime system has
to be able to cope with sudden changes of the system clock. Therefore, we have omitted it from the
Erlang driver thread API. In the Erlang driver case, timeouts can and should be handled with the
timer functionality of the Erlang driver API.
In order for the Erlang driver thread API to function, thread support has to be enabled in the
runtime system. An Erlang driver can check if thread support is enabled by use of
driver_system_info(). Note that some functions in the Erlang driver API are thread-safe only when the
runtime system has SMP support, also this information can be retrieved via driver_system_info(). Also
note that a lot of functions in the Erlang driver API are not thread-safe regardless of whether SMP
support is enabled or not. If a function isn't documented as thread-safe it is not thread-safe.
NOTE: When executing in an emulator thread, it is very important that you unlock all locks you have
locked before letting the thread out of your control; otherwise, you are very likely to deadlock the
whole emulator. If you need to use thread specific data in an emulator thread, only have the thread
specific data set while the thread is under your control, and clear the thread specific data before
you let the thread out of your control.
In the future there will probably be debug functionality integrated with the Erlang driver thread
API. All functions that create entities take a name argument. Currently the name argument is unused,
but it will be used when the debug functionality has been implemented. If you name all entities
created well, the debug functionality will be able to give you better error reports.
Adding / removing drivers:
A driver can add and later remove drivers.
Monitoring processes:
A driver can monitor a process that does not own a port.
Version management:
Version management is enabled for drivers that have set the extended_marker field of their
driver_entry to ERL_DRV_EXTENDED_MARKER. erl_driver.h defines ERL_DRV_EXTENDED_MARKER,
ERL_DRV_EXTENDED_MAJOR_VERSION, and ERL_DRV_EXTENDED_MINOR_VERSION. ERL_DRV_EXTENDED_MAJOR_VERSION
will be incremented when driver incompatible changes are made to the Erlang runtime system. Normally
it will suffice to recompile drivers when the ERL_DRV_EXTENDED_MAJOR_VERSION has changed, but it
could, under rare circumstances, mean that drivers have to be slightly modified. If so, this will of
course be documented. ERL_DRV_EXTENDED_MINOR_VERSION will be incremented when new features are added.
The runtime system uses the minor version of the driver to determine what features to use. The
runtime system will refuse to load a driver if the major versions differ, or if the major versions
are equal and the minor version used by the driver is greater than the one used by the runtime
system.
The emulator will refuse to load a driver that does not use the extended driver interface, to allow
for 64-bit capable drivers, since incompatible type changes for the callbacks output, control and
call were introduced in release R15B. A driver written with the old types would compile with warnings
and when called return garbage sizes to the emulator causing it to read random memory and create huge
incorrect result blobs.
Therefore it is not enough to just recompile drivers written with version management for pre-R15B
types; the types have to be changed in the driver suggesting other rewrites especially regarding size
variables. Investigate all warnings when recompiling!
Also, the API driver functions driver_output*, driver_vec_to_buf, driver_alloc/realloc* and the
driver_* queue functions were changed to have larger length arguments and return values. This is a
lesser problem since code that passes smaller types will get them auto converted in the calls and as
long as the driver does not handle sizes that overflow an int all will work as before.
REWRITES FOR 64-BIT DRIVER INTERFACE
"
For erts-5.9 two new integer types ErlDrvSizeT and ErlDrvSSizeT were introduced that can hold 64-bit
sizes if necessary.
To not update a driver and just recompile it probably works when building for a 32-bit machine creating a
false sense of security. Hopefully that will generate many important warnings. But when recompiling the
same driver later on for a 64-bit machine there will be warnings and almost certainly crashes. So it is a
BAD idea to postpone updating the driver and not fixing the warnings!
When recompiling with gcc use the -Wstrict-prototypes flag to get better warnings. Try to find a similar
flag if you are using some other compiler.
Here follows a checklist for rewriting a pre erts-5.9 driver, most important first.
Return types for driver callbacks:
Rewrite driver callback control to use return type ErlDrvSSizeT instead of int.
Rewrite driver callback call to use return type ErlDrvSSizeT instead of int.
Note:
These changes are essential to not crash the emulator or worse cause malfunction. Without them a driver
may return garbage in the high 32 bits to the emulator causing it to build a huge result from random
bytes either crashing on memory allocation or succeeding with a random result from the driver call.
Arguments to driver callbacks:
Driver callback output now gets ErlDrvSizeT as 3rd argument instead of previously int.
Driver callback control now gets ErlDrvSizeT as 4th and 6th arguments instead of previously int.
Driver callback call now gets ErlDrvSizeT as 4th and 6th arguments instead of previously int.
Sane compiler's calling conventions probably make these changes necessary only for a driver to handle
data chunks that require 64-bit size fields (mostly larger than 2 GB since that is what an int of 32
bits can hold). But it is possible to think of non-sane calling conventions that would make the
driver callbacks mix up the arguments causing malfunction.
Note:
The argument type change is from signed to unsigned which may cause problems for e.g. loop termination
conditions or error conditions if you just change the types all over the place.
Larger size field in ErlIOVec:
The size field in ErlIOVec has been changed to ErlDrvSizeT from int. Check all code that use that
field.
Automatic type casting probably makes these changes necessary only for a driver that encounters sizes
larger than 32 bits.
Note:
The size field changed from signed to unsigned which may cause problems for e.g. loop termination
conditions or error conditions if you just change the types all over the place.
Arguments and return values in the driver API:
Many driver API functions have changed argument type and/or return value to ErlDrvSizeT from mostly
int. Automatic type casting probably makes these changes necessary only for a driver that encounters
sizes larger than 32 bits.
driver_output:
3rd argument
driver_output2:
3rd and 5th arguments
driver_output_binary:
3rd 5th and 6th arguments
driver_outputv:
3rd and 5th arguments
driver_vec_to_buf:
3rd argument and return value
driver_alloc:
1st argument
driver_realloc:
2nd argument
driver_alloc_binary:
1st argument
driver_realloc_binary:
2nd argument
driver_enq:
3rd argument
driver_pushq:
3rd argument
driver_deq:
2nd argument and return value
driver_sizeq:
return value
driver_enq_bin:
3rd and 4th argument
driver_pushq_bin:
3rd and 4th argument
driver_enqv:
3rd argument
driver_pushqv:
3rd argument
driver_peekqv:
return value
Note:
This is a change from signed to unsigned which may cause problems for e.g. loop termination conditions
and error conditions if you just change the types all over the place.
DATA TYPES
ErlDrvSizeT:
An unsigned integer type to be used as size_t
ErlDrvSSizeT:
A signed integer type the size of ErlDrvSizeT
ErlDrvSysInfo:
typedef struct ErlDrvSysInfo {
int driver_major_version;
int driver_minor_version;
char *erts_version;
char *otp_release;
int thread_support;
int smp_support;
int async_threads;
int scheduler_threads;
int nif_major_version;
int nif_minor_version;
} ErlDrvSysInfo;
The ErlDrvSysInfo structure is used for storage of information about the Erlang runtime system.
driver_system_info() will write the system information when passed a reference to a ErlDrvSysInfo
structure. A description of the fields in the structure follows:
driver_major_version:
The value of ERL_DRV_EXTENDED_MAJOR_VERSION when the runtime system was compiled. This value is the
same as the value of ERL_DRV_EXTENDED_MAJOR_VERSION used when compiling the driver; otherwise, the
runtime system would have refused to load the driver.
driver_minor_version:
The value of ERL_DRV_EXTENDED_MINOR_VERSION when the runtime system was compiled. This value might
differ from the value of ERL_DRV_EXTENDED_MINOR_VERSION used when compiling the driver.
erts_version:
A string containing the version number of the runtime system (the same as returned by
erlang:system_info(version)).
otp_release:
A string containing the OTP release number (the same as returned by
erlang:system_info(otp_release)).
thread_support:
A value != 0 if the runtime system has thread support; otherwise, 0.
smp_support:
A value != 0 if the runtime system has SMP support; otherwise, 0.
async_threads:
The number of async threads in the async thread pool used by driver_async() (the same as returned
by erlang:system_info(thread_pool_size)).
scheduler_threads:
The number of scheduler threads used by the runtime system (the same as returned by
erlang:system_info(schedulers)).
nif_major_version:
The value of ERL_NIF_MAJOR_VERSION when the runtime system was compiled.
nif_minor_version:
The value of ERL_NIF_MINOR_VERSION when the runtime system was compiled.
ErlDrvBinary:
typedef struct ErlDrvBinary {
ErlDrvSint orig_size;
char orig_bytes[];
} ErlDrvBinary;
The ErlDrvBinary structure is a binary, as sent between the emulator and the driver. All binaries are
reference counted; when driver_binary_free is called, the reference count is decremented, when it
reaches zero, the binary is deallocated. The orig_size is the size of the binary, and orig_bytes is
the buffer. The ErlDrvBinary does not have a fixed size, its size is orig_size + 2 * sizeof(int).
Note:
The refc field has been removed. The reference count of an ErlDrvBinary is now stored elsewhere. The
reference count of an ErlDrvBinary can be accessed via driver_binary_get_refc(),
driver_binary_inc_refc(), and driver_binary_dec_refc().
Some driver calls, such as driver_enq_binary, increment the driver reference count, and others, such
as driver_deq decrement it.
Using a driver binary instead of a normal buffer, is often faster, since the emulator doesn't need to
copy the data, only the pointer is used.
A driver binary allocated in the driver, with driver_alloc_binary, should be freed in the driver
(unless otherwise stated), with driver_free_binary. (Note that this doesn't necessarily deallocate
it, if the driver is still referred in the emulator, the ref-count will not go to zero.)
Driver binaries are used in the driver_output2 and driver_outputv calls, and in the queue. Also the
driver call-back outputv uses driver binaries.
If the driver for some reason or another, wants to keep a driver binary around, in a static variable
for instance, the reference count should be incremented, and the binary can later be freed in the
stop call-back, with driver_free_binary.
Note that since a driver binary is shared by the driver and the emulator, a binary received from the
emulator or sent to the emulator, must not be changed by the driver.
Since erts version 5.5 (OTP release R11B), orig_bytes is guaranteed to be properly aligned for
storage of an array of doubles (usually 8-byte aligned).
ErlDrvData:
The ErlDrvData is a handle to driver-specific data, passed to the driver call-backs. It is a pointer,
and is most often type cast to a specific pointer in the driver.
SysIOVec:
This is a system I/O vector, as used by writev on unix and WSASend on Win32. It is used in ErlIOVec.
ErlIOVec:
typedef struct ErlIOVec {
int vsize;
ErlDrvSizeT size;
SysIOVec* iov;
ErlDrvBinary** binv;
} ErlIOVec;
The I/O vector used by the emulator and drivers, is a list of binaries, with a SysIOVec pointing to
the buffers of the binaries. It is used in driver_outputv and the outputv driver call-back. Also, the
driver queue is an ErlIOVec.
ErlDrvMonitor:
When a driver creates a monitor for a process, a ErlDrvMonitor is filled in. This is an opaque data-
type which can be assigned to but not compared without using the supplied compare function (i.e. it
behaves like a struct).
The driver writer should provide the memory for storing the monitor when calling
driver_monitor_process. The address of the data is not stored outside of the driver, so the
ErlDrvMonitor can be used as any other datum, it can be copied, moved in memory, forgotten etc.
ErlDrvNowData:
The ErlDrvNowData structure holds a timestamp consisting of three values measured from some arbitrary
point in the past. The three structure members are:
megasecs:
The number of whole megaseconds elapsed since the arbitrary point in time
secs:
The number of whole seconds elapsed since the arbitrary point in time
microsecs:
The number of whole microseconds elapsed since the arbitrary point in time
ErlDrvPDL:
If certain port specific data have to be accessed from other threads than those calling the driver
call-backs, a port data lock can be used in order to synchronize the operations on the data.
Currently, the only port specific data that the emulator associates with the port data lock is the
driver queue.
Normally a driver instance does not have a port data lock. If the driver instance wants to use a port
data lock, it has to create the port data lock by calling driver_pdl_create(). NOTE: Once the port
data lock has been created, every access to data associated with the port data lock has to be done
while having the port data lock locked. The port data lock is locked, and unlocked, respectively, by
use of driver_pdl_lock(), and driver_pdl_unlock().
A port data lock is reference counted, and when the reference count reaches zero, it will be
destroyed. The emulator will at least increment the reference count once when the lock is created and
decrement it once when the port associated with the lock terminates. The emulator will also increment
the reference count when an async job is enqueued and decrement it after an async job has been
invoked, or canceled. Besides this, it is the responsibility of the driver to ensure that the
reference count does not reach zero before the last use of the lock by the driver has been made. The
reference count can be read, incremented, and decremented, respectively, by use of
driver_pdl_get_refc(), driver_pdl_inc_refc(), and driver_pdl_dec_refc().
ErlDrvTid:
Thread identifier.
See also: erl_drv_thread_create(), erl_drv_thread_exit(), erl_drv_thread_join(),
erl_drv_thread_self(), and erl_drv_equal_tids().
ErlDrvThreadOpts:
int suggested_stack_size;
Thread options structure passed to erl_drv_thread_create(). Currently the following fields exist:
suggested_stack_size:
A suggestion, in kilo-words, on how large a stack to use. A value less than zero means default
size.
See also: erl_drv_thread_opts_create(), erl_drv_thread_opts_destroy(), and erl_drv_thread_create().
ErlDrvMutex:
Mutual exclusion lock. Used for synchronizing access to shared data. Only one thread at a time can
lock a mutex.
See also: erl_drv_mutex_create(), erl_drv_mutex_destroy(), erl_drv_mutex_lock(),
erl_drv_mutex_trylock(), and erl_drv_mutex_unlock().
ErlDrvCond:
Condition variable. Used when threads need to wait for a specific condition to appear before
continuing execution. Condition variables need to be used with associated mutexes.
See also: erl_drv_cond_create(), erl_drv_cond_destroy(), erl_drv_cond_signal(),
erl_drv_cond_broadcast(), and erl_drv_cond_wait().
ErlDrvRWLock:
Read/write lock. Used to allow multiple threads to read shared data while only allowing one thread to
write the same data. Multiple threads can read lock an rwlock at the same time, while only one thread
can read/write lock an rwlock at a time.
See also: erl_drv_rwlock_create(), erl_drv_rwlock_destroy(), erl_drv_rwlock_rlock(),
erl_drv_rwlock_tryrlock(), erl_drv_rwlock_runlock(), erl_drv_rwlock_rwlock(),
erl_drv_rwlock_tryrwlock(), and erl_drv_rwlock_rwunlock().
ErlDrvTSDKey:
Key which thread specific data can be associated with.
See also: erl_drv_tsd_key_create(), erl_drv_tsd_key_destroy(), erl_drv_tsd_set(), and
erl_drv_tsd_get().
EXPORTS
void driver_system_info(ErlDrvSysInfo *sys_info_ptr, size_t size)
This function will write information about the Erlang runtime system into the ErlDrvSysInfo
structure referred to by the first argument. The second argument should be the size of the
ErlDrvSysInfo structure, i.e., sizeof(ErlDrvSysInfo).
See the documentation of the ErlDrvSysInfo structure for information about specific fields.
int driver_output(ErlDrvPort port, char *buf, ErlDrvSizeT len)
The driver_output function is used to send data from the driver up to the emulator. The data will
be received as terms or binary data, depending on how the driver port was opened.
The data is queued in the port owner process' message queue. Note that this does not yield to the
emulator. (Since the driver and the emulator run in the same thread.)
The parameter buf points to the data to send, and len is the number of bytes.
The return value for all output functions is 0. (Unless the driver is used for distribution, in
which case it can fail and return -1. For normal use, the output function always returns 0.)
int driver_output2(ErlDrvPort port, char *hbuf, ErlDrvSizeT hlen, char *buf, ErlDrvSizeT len)
The driver_output2 function first sends hbuf (length in hlen) data as a list, regardless of port
settings. Then buf is sent as a binary or list. E.g. if hlen is 3 then the port owner process will
receive [H1, H2, H3 | T].
The point of sending data as a list header, is to facilitate matching on the data received.
The return value is 0 for normal use.
int driver_output_binary(ErlDrvPort port, char *hbuf, ErlDrvSizeT hlen, ErlDrvBinary* bin, ErlDrvSizeT
offset, ErlDrvSizeT len)
This function sends data to port owner process from a driver binary, it has a header buffer (hbuf
and hlen) just like driver_output2. The hbuf parameter can be NULL.
The parameter offset is an offset into the binary and len is the number of bytes to send.
Driver binaries are created with driver_alloc_binary.
The data in the header is sent as a list and the binary as an Erlang binary in the tail of the
list.
E.g. if hlen is 2, then the port owner process will receive [H1, H2 | <<T>>].
The return value is 0 for normal use.
Note that, using the binary syntax in Erlang, the driver application can match the header directly
from the binary, so the header can be put in the binary, and hlen can be set to 0.
int driver_outputv(ErlDrvPort port, char* hbuf, ErlDrvSizeT hlen, ErlIOVec *ev, ErlDrvSizeT skip)
This function sends data from an IO vector, ev, to the port owner process. It has a header buffer
(hbuf and hlen), just like driver_output2.
The skip parameter is a number of bytes to skip of the ev vector from the head.
You get vectors of ErlIOVec type from the driver queue (see below), and the outputv driver entry
function. You can also make them yourself, if you want to send several ErlDrvBinary buffers at
once. Often it is faster to use driver_output or driver_output_binary.
E.g. if hlen is 2 and ev points to an array of three binaries, the port owner process will receive
[H1, H2, <<B1>>, <<B2>> | <<B3>>].
The return value is 0 for normal use.
The comment for driver_output_binary applies for driver_outputv too.
ErlDrvSizeT driver_vec_to_buf(ErlIOVec *ev, char *buf, ErlDrvSizeT len)
This function collects several segments of data, referenced by ev, by copying them in order to the
buffer buf, of the size len.
If the data is to be sent from the driver to the port owner process, it is faster to use
driver_outputv.
The return value is the space left in the buffer, i.e. if the ev contains less than len bytes it's
the difference, and if ev contains len bytes or more, it's 0. This is faster if there is more than
one header byte, since the binary syntax can construct integers directly from the binary.
int driver_set_timer(ErlDrvPort port, unsigned long time)
This function sets a timer on the driver, which will count down and call the driver when it is
timed out. The time parameter is the time in milliseconds before the timer expires.
When the timer reaches 0 and expires, the driver entry function timeout is called.
Note that there is only one timer on each driver instance; setting a new timer will replace an
older one.
Return value is 0 (-1 only when the timeout driver function is NULL).
int driver_cancel_timer(ErlDrvPort port)
This function cancels a timer set with driver_set_timer.
The return value is 0.
int driver_read_timer(ErlDrvPort port, unsigned long *time_left)
This function reads the current time of a timer, and places the result in time_left. This is the
time in milliseconds, before the timeout will occur.
The return value is 0.
int driver_get_now(ErlDrvNowData *now)
This function reads a timestamp into the memory pointed to by the parameter now. See the
description of ErlDrvNowData for specification of its fields.
The return value is 0 unless the now pointer is not valid, in which case it is < 0.
int driver_select(ErlDrvPort port, ErlDrvEvent event, int mode, int on)
This function is used by drivers to provide the emulator with events to check for. This enables
the emulator to call the driver when something has happened asynchronously.
The event argument identifies an OS-specific event object. On Unix systems, the functions
select/poll are used. The event object must be a socket or pipe (or other object that select/poll
can use). On windows, the Win32 API function WaitForMultipleObjects is used. This places other
restrictions on the event object. Refer to the Win32 SDK documentation.
The on parameter should be 1 for setting events and 0 for clearing them.
The mode argument is a bitwise-or combination of ERL_DRV_READ, ERL_DRV_WRITE and ERL_DRV_USE. The
first two specify whether to wait for read events and/or write events. A fired read event will
call ready_input while a fired write event will call ready_output.
Note:
Some OS (Windows) do not differentiate between read and write events. The call-back for a fired
event then only depends on the value of mode.
ERL_DRV_USE specifies if we are using the event object or if we want to close it. On an emulator
with SMP support, it is not safe to clear all events and then close the event object after
driver_select has returned. Another thread may still be using the event object internally. To
safely close an event object call driver_select with ERL_DRV_USE and on==0. That will clear all
events and then call stop_select when it is safe to close the event object. ERL_DRV_USE should be
set together with the first event for an event object. It is harmless to set ERL_DRV_USE even
though it already has been done. Clearing all events but keeping ERL_DRV_USE set will indicate
that we are using the event object and probably will set events for it again.
Note:
ERL_DRV_USE was added in OTP release R13. Old drivers will still work as before. But it is
recommended to update them to use ERL_DRV_USE and stop_select to make sure that event objects are
closed in a safe way.
The return value is 0 (failure, -1, only if the ready_input/ready_output is NULL).
void *driver_alloc(ErlDrvSizeT size)
This function allocates a memory block of the size specified in size, and returns it. This only
fails on out of memory, in that case NULL is returned. (This is most often a wrapper for malloc).
Memory allocated must be explicitly freed with a corresponding call to driver_free (unless
otherwise stated).
This function is thread-safe.
void *driver_realloc(void *ptr, ErlDrvSizeT size)
This function resizes a memory block, either in place, or by allocating a new block, copying the
data and freeing the old block. A pointer is returned to the reallocated memory. On failure (out
of memory), NULL is returned. (This is most often a wrapper for realloc.)
This function is thread-safe.
void driver_free(void *ptr)
This function frees the memory pointed to by ptr. The memory should have been allocated with
driver_alloc. All allocated memory should be deallocated, just once. There is no garbage
collection in drivers.
This function is thread-safe.
ErlDrvBinary *driver_alloc_binary(ErlDrvSizeT size)
This function allocates a driver binary with a memory block of at least size bytes, and returns a
pointer to it, or NULL on failure (out of memory). When a driver binary has been sent to the
emulator, it must not be altered. Every allocated binary should be freed by a corresponding call
to driver_free_binary (unless otherwise stated).
Note that a driver binary has an internal reference counter, this means that calling
driver_free_binary it may not actually dispose of it. If it's sent to the emulator, it may be
referenced there.
The driver binary has a field, orig_bytes, which marks the start of the data in the binary.
This function is thread-safe.
ErlDrvBinary *driver_realloc_binary(ErlDrvBinary *bin, ErlDrvSizeT size)
This function resizes a driver binary, while keeping the data. The resized driver binary is
returned. On failure (out of memory), NULL is returned.
This function is only thread-safe when the emulator with SMP support is used.
void driver_free_binary(ErlDrvBinary *bin)
This function frees a driver binary bin, allocated previously with driver_alloc_binary. Since
binaries in Erlang are reference counted, the binary may still be around.
This function is only thread-safe when the emulator with SMP support is used.
long driver_binary_get_refc(ErlDrvBinary *bin)
Returns current reference count on bin.
This function is only thread-safe when the emulator with SMP support is used.
long driver_binary_inc_refc(ErlDrvBinary *bin)
Increments the reference count on bin and returns the reference count reached after the increment.
This function is only thread-safe when the emulator with SMP support is used.
long driver_binary_dec_refc(ErlDrvBinary *bin)
Decrements the reference count on bin and returns the reference count reached after the decrement.
This function is only thread-safe when the emulator with SMP support is used.
Note:
You should normally decrement the reference count of a driver binary by calling
driver_free_binary(). driver_binary_dec_refc() does not free the binary if the reference count
reaches zero. Only use driver_binary_dec_refc() when you are sure not to reach a reference count
of zero.
int driver_enq(ErlDrvPort port, char* buf, ErlDrvSizeT len)
This function enqueues data in the driver queue. The data in buf is copied (len bytes) and placed
at the end of the driver queue. The driver queue is normally used in a FIFO way.
The driver queue is available to queue output from the emulator to the driver (data from the
driver to the emulator is queued by the emulator in normal erlang message queues). This can be
useful if the driver has to wait for slow devices etc, and wants to yield back to the emulator.
The driver queue is implemented as an ErlIOVec.
When the queue contains data, the driver won't close, until the queue is empty.
The return value is 0.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
int driver_pushq(ErlDrvPort port, char* buf, ErlDrvSizeT len)
This function puts data at the head of the driver queue. The data in buf is copied (len bytes) and
placed at the beginning of the queue.
The return value is 0.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
ErlDrvSizeT driver_deq(ErlDrvPort port, ErlDrvSizeT size)
This function dequeues data by moving the head pointer forward in the driver queue by size bytes.
The data in the queue will be deallocated.
The return value is the number of bytes remaining in the queue or -1 on failure.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
ErlDrvSizeT driver_sizeq(ErlDrvPort port)
This function returns the number of bytes currently in the driver queue.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
int driver_enq_bin(ErlDrvPort port, ErlDrvBinary *bin, ErlDrvSizeT offset, ErlDrvSizeT len)
This function enqueues a driver binary in the driver queue. The data in bin at offset with length
len is placed at the end of the queue. This function is most often faster than driver_enq, because
the data doesn't have to be copied.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
The return value is 0.
int driver_pushq_bin(ErlDrvPort port, ErlDrvBinary *bin, ErlDrvSizeT offset, ErlDrvSizeT len)
This function puts data in the binary bin, at offset with length len at the head of the driver
queue. It is most often faster than driver_pushq, because the data doesn't have to be copied.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
The return value is 0.
ErlDrvSizeT driver_peekqv(ErlDrvPort port, ErlIOVec *ev)
This function retrieves the driver queue into a supplied ErlIOVec ev. It also returns the queue
size. This is one of two ways to get data out of the queue.
If ev is NULL all ones i.e. -1 type cast to ErlDrvSizeT is returned.
Nothing is removed from the queue by this function, that must be done with driver_deq.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
SysIOVec *driver_peekq(ErlDrvPort port, int *vlen)
This function retrieves the driver queue as a pointer to an array of SysIOVecs. It also returns
the number of elements in vlen. This is one of two ways to get data out of the queue.
Nothing is removed from the queue by this function, that must be done with driver_deq.
The returned array is suitable to use with the Unix system call writev.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
int driver_enqv(ErlDrvPort port, ErlIOVec *ev, ErlDrvSizeT skip)
This function enqueues the data in ev, skipping the first skip bytes of it, at the end of the
driver queue. It is faster than driver_enq, because the data doesn't have to be copied.
The return value is 0.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
int driver_pushqv(ErlDrvPort port, ErlIOVec *ev, ErlDrvSizeT skip)
This function puts the data in ev, skipping the first skip bytes of it, at the head of the driver
queue. It is faster than driver_pushq, because the data doesn't have to be copied.
The return value is 0.
This function can be called from an arbitrary thread if a port data lock associated with the port
is locked by the calling thread during the call.
ErlDrvPDL driver_pdl_create(ErlDrvPort port)
This function creates a port data lock associated with the port. NOTE: Once a port data lock has
been created, it has to be locked during all operations on the driver queue of the port.
On success a newly created port data lock is returned. On failure NULL is returned.
driver_pdl_create() will fail if port is invalid or if a port data lock already has been
associated with the port.
void driver_pdl_lock(ErlDrvPDL pdl)
This function locks the port data lock passed as argument (pdl).
This function is thread-safe.
void driver_pdl_unlock(ErlDrvPDL pdl)
This function unlocks the port data lock passed as argument (pdl).
This function is thread-safe.
long driver_pdl_get_refc(ErlDrvPDL pdl)
This function returns the current reference count of the port data lock passed as argument (pdl).
This function is thread-safe.
long driver_pdl_inc_refc(ErlDrvPDL pdl)
This function increments the reference count of the port data lock passed as argument (pdl).
The current reference count after the increment has been performed is returned.
This function is thread-safe.
long driver_pdl_dec_refc(ErlDrvPDL pdl)
This function decrements the reference count of the port data lock passed as argument (pdl).
The current reference count after the decrement has been performed is returned.
This function is thread-safe.
int driver_monitor_process(ErlDrvPort port, ErlDrvTermData process, ErlDrvMonitor *monitor)
Start monitoring a process from a driver. When a process is monitored, a process exit will result
in a call to the provided process_exit call-back in the ErlDrvEntry structure. The ErlDrvMonitor
structure is filled in, for later removal or compare.
The process parameter should be the return value of an earlier call to driver_caller or
driver_connected call.
The function returns 0 on success, < 0 if no call-back is provided and > 0 if the process is no
longer alive.
int driver_demonitor_process(ErlDrvPort port, const ErlDrvMonitor *monitor)
This function cancels a monitor created earlier.
The function returns 0 if a monitor was removed and > 0 if the monitor did no longer exist.
ErlDrvTermData driver_get_monitored_process(ErlDrvPort port, const ErlDrvMonitor *monitor)
The function returns the process id associated with a living monitor. It can be used in the
process_exit call-back to get the process identification for the exiting process.
The function returns driver_term_nil if the monitor no longer exists.
int driver_compare_monitors(const ErlDrvMonitor *monitor1, const ErlDrvMonitor *monitor2)
This function is used to compare two ErlDrvMonitors. It can also be used to imply some artificial
order on monitors, for whatever reason.
The function returns 0 if monitor1 and monitor2 are equal, < 0 if monitor1 is less than monitor2
and > 0 if monitor1 is greater than monitor2.
void add_driver_entry(ErlDrvEntry *de)
This function adds a driver entry to the list of drivers known by Erlang. The init function of the
de parameter is called.
Note:
To use this function for adding drivers residing in dynamically loaded code is dangerous. If the
driver code for the added driver resides in the same dynamically loaded module (i.e. .so file) as
a normal dynamically loaded driver (loaded with the erl_ddll interface), the caller should call
driver_lock_driver before adding driver entries.
Use of this function is generally deprecated.
int remove_driver_entry(ErlDrvEntry *de)
This function removes a driver entry de previously added with add_driver_entry.
Driver entries added by the erl_ddll erlang interface can not be removed by using this interface.
char *erl_errno_id(int error)
This function returns the atom name of the erlang error, given the error number in error. Error
atoms are: einval, enoent, etc. It can be used to make error terms from the driver.
void erl_drv_busy_msgq_limits(ErlDrvPort port, ErlDrvSizeT *low, ErlDrvSizeT *high)
Sets and gets limits that will be used for controling the busy state of the port message queue.
The port message queue will be set into a busy state when the amount of command data queued on the
message queue reaches the high limit. The port message queue will be set into a not busy state
when the amount of command data queued on the message queue falls below the low limit. Command
data is in this context data passed to the port using either Port ! {Owner, {command, Data}}, or
port_command/[2,3]. Note that these limits only concerns command data that have not yet reached
the port. The busy port feature can be used for data that has reached the port.
Valid limits are values in the range [ERL_DRV_BUSY_MSGQ_LIM_MIN, ERL_DRV_BUSY_MSGQ_LIM_MAX].
Limits will be automatically adjusted to be sane. That is, the system will adjust values so that
the low limit used is lower than or equal to the high limit used. By default the high limit will
be 8 kB and the low limit will be 4 kB.
By passing a pointer to an integer variable containing the value ERL_DRV_BUSY_MSGQ_READ_ONLY,
currently used limit will be read and written back to the integer variable. A new limit can be set
by passing a pointer to an integer variable containing a valid limit. The passed value will be
written to the internal limit. The internal limit will then be adjusted. After this the adjusted
limit will be written back to the integer variable from which the new value was read. Values are
in bytes.
The busy message queue feature can be disabled either by setting the ERL_DRV_FLAG_NO_BUSY_MSGQ
driver flag in the driver_entry used by the driver, or by calling this function with
ERL_DRV_BUSY_MSGQ_DISABLED as a limit (either low or high). When this feature has been disabled it
cannot be enabled again. When reading the limits both of them will be ERL_DRV_BUSY_MSGQ_DISABLED,
if this feature has been disabled.
Processes sending command data to the port will be suspended if either the port is busy or if the
port message queue is busy. Suspended processes will be resumed when neither the port is busy, nor
the port message queue is busy.
For information about busy port functionality see the documentation of the set_busy_port()
function.
void set_busy_port(ErlDrvPort port, int on)
This function set and unset the busy state of the port. If on is non-zero, the port is set to
busy, if it's zero the port is set to not busy. You typically want to combine this feature with
the busy port message queue functionality.
Processes sending command data to the port will be suspended if either the port is busy or if the
port message queue is busy. Suspended processes will be resumed when neither the port is busy, nor
the port message queue is busy. Command data is in this context data passed to the port using
either Port ! {Owner, {command, Data}}, or port_command/[2,3].
If the ERL_DRV_FLAG_SOFT_BUSY has been set in the driver_entry, data can be forced into the driver
via port_command(Port, Data, [force]) even though the driver has signaled that it is busy.
For information about busy port message queue functionality see the documentation of the
erl_drv_busy_msgq_limits() function.
void set_port_control_flags(ErlDrvPort port, int flags)
This function sets flags for how the control driver entry function will return data to the port
owner process. (The control function is called from port_control/3 in erlang.)
Currently there are only two meaningful values for flags: 0 means that data is returned in a list,
and PORT_CONTROL_FLAG_BINARY means data is returned as a binary from control.
int driver_failure_eof(ErlDrvPort port)
This function signals to erlang that the driver has encountered an EOF and should be closed,
unless the port was opened with the eof option, in that case eof is sent to the port. Otherwise,
the port is closed and an 'EXIT' message is sent to the port owner process.
The return value is 0.
int driver_failure_atom(ErlDrvPort port, char *string)
int driver_failure_posix(ErlDrvPort port, int error)
int driver_failure(ErlDrvPort port, int error)
These functions signal to Erlang that the driver has encountered an error and should be closed.
The port is closed and the tuple {'EXIT', error, Err}, is sent to the port owner process, where
error is an error atom (driver_failure_atom and driver_failure_posix), or an integer
(driver_failure).
The driver should fail only when in severe error situations, when the driver cannot possibly keep
open, for instance buffer allocation gets out of memory. For normal errors it is more appropriate
to send error codes with driver_output.
The return value is 0.
ErlDrvTermData driver_connected(ErlDrvPort port)
This function returns the port owner process.
Note that this function is not thread-safe, not even when the emulator with SMP support is used.
ErlDrvTermData driver_caller(ErlDrvPort port)
This function returns the process id of the process that made the current call to the driver. The
process id can be used with driver_send_term to send back data to the caller. driver_caller() only
returns valid data when currently executing in one of the following driver callbacks:
start:
Called from open_port/2.
output:
Called from erlang:send/2, and erlang:port_command/2
outputv:
Called from erlang:send/2, and erlang:port_command/2
control:
Called from erlang:port_control/3
call:
Called from erlang:port_call/3
Note that this function is not thread-safe, not even when the emulator with SMP support is used.
int erl_drv_output_term(ErlDrvTermData port, ErlDrvTermData* term, int n)
This functions sends data in the special driver term format to the port owner process. This is a
fast way to deliver term data from a driver. It also needs no binary conversion, so the port owner
process receives data as normal Erlang terms. The erl_drv_send_term() functions can be used for
sending to any arbitrary process on the local node.
Note:
Note that the port parameter is not an ordinary port handle, but a port handle converted using
driver_mk_port().
The term parameter points to an array of ErlDrvTermData, with n elements. This array contains
terms described in the driver term format. Every term consists of one to four elements in the
array. The term first has a term type, and then arguments. The port parameter specifies the
sending port.
Tuple and lists (with the exception of strings, see below), are built in reverse polish notation,
so that to build a tuple, the elements are given first, and then the tuple term, with a count.
Likewise for lists.
A tuple must be specified with the number of elements. (The elements precede the ERL_DRV_TUPLE
term.)
A list must be specified with the number of elements, including the tail, which is the last term
preceding ERL_DRV_LIST.
The special term ERL_DRV_STRING_CONS is used to "splice" in a string in a list, a string given
this way is not a list per se, but the elements are elements of the surrounding list.
Term type Argument(s)
===========================================
ERL_DRV_NIL
ERL_DRV_ATOM ErlDrvTermData atom (from driver_mk_atom(char *string))
ERL_DRV_INT ErlDrvSInt integer
ERL_DRV_UINT ErlDrvUInt integer
ERL_DRV_INT64 ErlDrvSInt64 *integer_ptr
ERL_DRV_UINT64 ErlDrvUInt64 *integer_ptr
ERL_DRV_PORT ErlDrvTermData port (from driver_mk_port(ErlDrvPort port))
ERL_DRV_BINARY ErlDrvBinary *bin, ErlDrvUInt len, ErlDrvUInt offset
ERL_DRV_BUF2BINARY char *buf, ErlDrvUInt len
ERL_DRV_STRING char *str, int len
ERL_DRV_TUPLE int sz
ERL_DRV_LIST int sz
ERL_DRV_PID ErlDrvTermData pid (from driver_connected(ErlDrvPort port) or driver_caller(ErlDrvPort port))
ERL_DRV_STRING_CONS char *str, int len
ERL_DRV_FLOAT double *dbl
ERL_DRV_EXT2TERM char *buf, ErlDrvUInt len
The unsigned integer data type ErlDrvUInt and the signed integer data type ErlDrvSInt are 64 bits
wide on a 64 bit runtime system and 32 bits wide on a 32 bit runtime system. They were introduced
in erts version 5.6, and replaced some of the int arguments in the list above.
The unsigned integer data type ErlDrvUInt64 and the signed integer data type ErlDrvSInt64 are
always 64 bits wide. They were introduced in erts version 5.7.4.
To build the tuple {tcp, Port, [100 | Binary]}, the following call could be made.
ErlDrvBinary* bin = ...
ErlDrvPort port = ...
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("tcp"),
ERL_DRV_PORT, driver_mk_port(drvport),
ERL_DRV_INT, 100,
ERL_DRV_BINARY, bin, 50, 0,
ERL_DRV_LIST, 2,
ERL_DRV_TUPLE, 3,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
Where bin is a driver binary of length at least 50 and drvport is a port handle. Note that the
ERL_DRV_LIST comes after the elements of the list, likewise the ERL_DRV_TUPLE.
The term ERL_DRV_STRING_CONS is a way to construct strings. It works differently from how
ERL_DRV_STRING works. ERL_DRV_STRING_CONS builds a string list in reverse order, (as opposed to
how ERL_DRV_LIST works), concatenating the strings added to a list. The tail must be given before
ERL_DRV_STRING_CONS.
The ERL_DRV_STRING constructs a string, and ends it. (So it's the same as ERL_DRV_NIL followed by
ERL_DRV_STRING_CONS.)
/* to send [x, "abc", y] to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("x"),
ERL_DRV_STRING, (ErlDrvTermData)"abc", 3,
ERL_DRV_ATOM, driver_mk_atom("y"),
ERL_DRV_NIL,
ERL_DRV_LIST, 4
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
/* to send "abc123" to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_NIL, /* with STRING_CONS, the tail comes first */
ERL_DRV_STRING_CONS, (ErlDrvTermData)"123", 3,
ERL_DRV_STRING_CONS, (ErlDrvTermData)"abc", 3,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
The ERL_DRV_EXT2TERM term type is used for passing a term encoded with the external format, i.e.,
a term that has been encoded by erlang:term_to_binary, erl_interface, etc. For example, if binp is
a pointer to an ErlDrvBinary that contains the term {17, 4711} encoded with the external format
and you want to wrap it in a two tuple with the tag my_tag, i.e., {my_tag, {17, 4711}}, you can do
as follows:
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("my_tag"),
ERL_DRV_EXT2TERM, (ErlDrvTermData) binp->orig_bytes, binp->orig_size
ERL_DRV_TUPLE, 2,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
If you want to pass a binary and don't already have the content of the binary in an ErlDrvBinary,
you can benefit from using ERL_DRV_BUF2BINARY instead of creating an ErlDrvBinary via
driver_alloc_binary() and then pass the binary via ERL_DRV_BINARY. The runtime system will often
allocate binaries smarter if ERL_DRV_BUF2BINARY is used. However, if the content of the binary to
pass already resides in an ErlDrvBinary, it is normally better to pass the binary using
ERL_DRV_BINARY and the ErlDrvBinary in question.
The ERL_DRV_UINT, ERL_DRV_BUF2BINARY, and ERL_DRV_EXT2TERM term types were introduced in the 5.6
version of erts.
This function is only thread-safe when the emulator with SMP support is used.
int driver_output_term(ErlDrvPort port, ErlDrvTermData* term, int n)
Warning:
driver_output_term() is deprecated and will be removed in the OTP-R17 release. Use
erl_drv_output_term() instead.
The parameters term and n do the same thing as in erl_drv_output_term().
Note that this function is not thread-safe, not even when the emulator with SMP support is used.
ErlDrvTermData driver_mk_atom(char* string)
This function returns an atom given a name string. The atom is created and won't change, so the
return value may be saved and reused, which is faster than looking up the atom several times.
Note that this function is not thread-safe, not even when the emulator with SMP support is used.
ErlDrvTermData driver_mk_port(ErlDrvPort port)
This function converts a port handle to the erlang term format, usable in the
erl_drv_output_term(), and erl_drv_send_term() functions.
Note that this function is not thread-safe, not even when the emulator with SMP support is used.
int erl_drv_send_term(ErlDrvTermData port, ErlDrvTermData receiver, ErlDrvTermData* term, int n)
This function is the only way for a driver to send data to other processes than the port owner
process. The receiver parameter specifies the process to receive the data.
Note:
Note that the port parameter is not an ordinary port handle, but a port handle converted using
driver_mk_port().
The parameters port, term and n do the same thing as in erl_drv_output_term().
This function is only thread-safe when the emulator with SMP support is used.
int driver_send_term(ErlDrvPort port, ErlDrvTermData receiver, ErlDrvTermData* term, int n)
Warning:
driver_send_term() is deprecated and will be removed in the OTP-R17 release. Use
erl_drv_send_term() instead.
Also note that parameters of driver_send_term() cannot be properly checked by the runtime system
when executed by arbitrary threads. This may cause the driver_send_term() function not to fail
when it should.
The parameters term and n do the same thing as in erl_drv_output_term().
This function is only thread-safe when the emulator with SMP support is used.
long driver_async (ErlDrvPort port, unsigned int* key, void (*async_invoke)(void*), void* async_data,
void (*async_free)(void*))
This function performs an asynchronous call. The function async_invoke is invoked in a thread
separate from the emulator thread. This enables the driver to perform time-consuming, blocking
operations without blocking the emulator.
The async thread pool size can be set with the +A command line argument of erl(1). If no async
thread pool is available, the call is made synchronously in the thread calling driver_async(). The
current number of async threads in the async thread pool can be retrieved via
driver_system_info().
If there is a thread pool available, a thread will be used. If the key argument is null, the
threads from the pool are used in a round-robin way, each call to driver_async uses the next
thread in the pool. With the key argument set, this behaviour is changed. The two same values of
*key always get the same thread.
To make sure that a driver instance always uses the same thread, the following call can be used:
unsigned int myKey = driver_async_port_key(myPort);
r = driver_async(myPort, &myKey, myData, myFunc);
It is enough to initialize myKey once for each driver instance.
If a thread is already working, the calls will be queued up and executed in order. Using the same
thread for each driver instance ensures that the calls will be made in sequence.
The async_data is the argument to the functions async_invoke and async_free. It's typically a
pointer to a structure that contains a pipe or event that can be used to signal that the async
operation completed. The data should be freed in async_free, because it's called if
driver_async_cancel is called.
When the async operation is done, ready_async driver entry function is called. If ready_async is
null in the driver entry, the async_free function is called instead.
The return value is a handle to the asynchronous task, which can be used as argument to
driver_async_cancel.
Note:
As of erts version 5.5.4.3 the default stack size for threads in the async-thread pool is 16
kilowords, i.e., 64 kilobyte on 32-bit architectures. This small default size has been chosen
since the amount of async-threads might be quite large. The default stack size is enough for
drivers delivered with Erlang/OTP, but might not be sufficiently large for other dynamically
linked in drivers that use the driver_async() functionality. A suggested stack size for threads in
the async-thread pool can be configured via the +a command line argument of erl(1).
unsigned int driver_async_port_key (ErlDrvPort port)
This function calculates a key for later use in driver_async(). The keys are evenly distributed so
that a fair mapping between port id's and async thread id's is achieved.
Note:
Before OTP-R16, the actual port id could be used as a key with proper casting, but after the
rewrite of the port subsystem, this is no longer the case. With this function, you can achieve the
same distribution based on port id's as before OTP-R16.
int driver_async_cancel(long id)
This function used to cancel a scheduled asynchronous operation, if it was still in the queue. It
returned 1 if it succeeded, and 0 if it failed.
Since it could not guarantee success, it was more or less useless. The user had to implement
synchronization of cancellation anyway. It also unnecessarily complicated the implementation.
Therefore, as of OTP-R15B driver_async_cancel() is deprecated, and scheduled for removal in OTP-
R17. It will currently always fail, and return 0.
Warning:
driver_async_cancel() is deprecated and will be removed in the OTP-R17 release.
int driver_lock_driver(ErlDrvPort port)
This function locks the driver used by the port port in memory for the rest of the emulator
process' lifetime. After this call, the driver behaves as one of Erlang's statically linked in
drivers.
ErlDrvPort driver_create_port(ErlDrvPort port, ErlDrvTermData owner_pid, char* name, ErlDrvData drv_data)
This function creates a new port executing the same driver code as the port creating the new port.
A short description of the arguments:
port:
The port handle of the port (driver instance) creating the new port.
owner_pid:
The process id of the Erlang process which will be owner of the new port. This process will be
linked to the new port. You usually want to use driver_caller(port) as owner_pid.
name:
The port name of the new port. You usually want to use the same port name as the driver name
(driver_name field of the driver_entry).
drv_data:
The driver defined handle that will be passed in subsequent calls to driver call-backs. Note,
that the driver start call-back will not be called for this new driver instance. The driver
defined handle is normally created in the driver start call-back when a port is created via
erlang:open_port/2.
The caller of driver_create_port() is allowed to manipulate the newly created port when
driver_create_port() has returned. When port level locking is used, the creating port is, however,
only allowed to manipulate the newly created port until the current driver call-back that was
called by the emulator returns.
Note:
When port level locking is used, the creating port is only allowed to manipulate the newly created
port until the current driver call-back returns.
int erl_drv_thread_create(char *name, ErlDrvTid *tid,
void * (*func)(void *), void *arg, ErlDrvThreadOpts
*opts)
Arguments:
name:
A string identifying the created thread. It will be used to identify the thread in planned
future debug functionality.
tid:
A pointer to a thread identifier variable.
func:
A pointer to a function to execute in the created thread.
arg:
A pointer to argument to the func function.
opts:
A pointer to thread options to use or NULL.
This function creates a new thread. On success 0 is returned; otherwise, an errno value is
returned to indicate the error. The newly created thread will begin executing in the function
pointed to by func, and func will be passed arg as argument. When erl_drv_thread_create() returns
the thread identifier of the newly created thread will be available in *tid. opts can be either a
NULL pointer, or a pointer to an ErlDrvThreadOpts structure. If opts is a NULL pointer, default
options will be used; otherwise, the passed options will be used.
Warning:
You are not allowed to allocate the ErlDrvThreadOpts structure by yourself. It has to be allocated
and initialized by erl_drv_thread_opts_create().
The created thread will terminate either when func returns or if erl_drv_thread_exit() is called
by the thread. The exit value of the thread is either returned from func or passed as argument to
erl_drv_thread_exit(). The driver creating the thread has the responsibility of joining the
thread, via erl_drv_thread_join(), before the driver is unloaded. It is not possible to create
"detached" threads, i.e., threads that don't need to be joined.
Warning:
All created threads need to be joined by the driver before it is unloaded. If the driver fails to
join all threads created before it is unloaded, the runtime system will most likely crash when the
code of the driver is unloaded.
This function is thread-safe.
ErlDrvThreadOpts *erl_drv_thread_opts_create(char *name)
Arguments:
name:
A string identifying the created thread options. It will be used to identify the thread
options in planned future debug functionality.
This function allocates and initialize a thread option structure. On failure NULL is returned. A
thread option structure is used for passing options to erl_drv_thread_create(). If the structure
isn't modified before it is passed to erl_drv_thread_create(), the default values will be used.
Warning:
You are not allowed to allocate the ErlDrvThreadOpts structure by yourself. It has to be allocated
and initialized by erl_drv_thread_opts_create().
This function is thread-safe.
void erl_drv_thread_opts_destroy(ErlDrvThreadOpts *opts)
Arguments:
opts:
A pointer to thread options to destroy.
This function destroys thread options previously created by erl_drv_thread_opts_create().
This function is thread-safe.
void erl_drv_thread_exit(void *exit_value)
Arguments:
exit_value:
A pointer to an exit value or NULL.
This function terminates the calling thread with the exit value passed as argument. You are only
allowed to terminate threads created with erl_drv_thread_create(). The exit value can later be
retrieved by another thread via erl_drv_thread_join().
This function is thread-safe.
int erl_drv_thread_join(ErlDrvTid tid, void **exit_value)
Arguments:
tid:
The thread identifier of the thread to join.
exit_value:
A pointer to a pointer to an exit value, or NULL.
This function joins the calling thread with another thread, i.e., the calling thread is blocked
until the thread identified by tid has terminated. On success 0 is returned; otherwise, an errno
value is returned to indicate the error. A thread can only be joined once. The behavior of joining
more than once is undefined, an emulator crash is likely. If exit_value == NULL, the exit value of
the terminated thread will be ignored; otherwise, the exit value of the terminated thread will be
stored at *exit_value.
This function is thread-safe.
ErlDrvTid erl_drv_thread_self(void)
This function returns the thread identifier of the calling thread.
This function is thread-safe.
int erl_drv_equal_tids(ErlDrvTid tid1, ErlDrvTid tid2)
Arguments:
tid1:
A thread identifier.
tid2:
A thread identifier.
This function compares two thread identifiers for equality, and returns 0 it they aren't equal,
and a value not equal to 0 if they are equal.
Note:
A Thread identifier may be reused very quickly after a thread has terminated. Therefore, if a
thread corresponding to one of the involved thread identifiers has terminated since the thread
identifier was saved, the result of erl_drv_equal_tids() might not give the expected result.
This function is thread-safe.
ErlDrvMutex *erl_drv_mutex_create(char *name)
Arguments:
name:
A string identifying the created mutex. It will be used to identify the mutex in planned
future debug functionality.
This function creates a mutex and returns a pointer to it. On failure NULL is returned. The driver
creating the mutex has the responsibility of destroying it before the driver is unloaded.
This function is thread-safe.
void erl_drv_mutex_destroy(ErlDrvMutex *mtx)
Arguments:
mtx:
A pointer to a mutex to destroy.
This function destroys a mutex previously created by erl_drv_mutex_create(). The mutex has to be
in an unlocked state before being destroyed.
This function is thread-safe.
void erl_drv_mutex_lock(ErlDrvMutex *mtx)
Arguments:
mtx:
A pointer to a mutex to lock.
This function locks a mutex. The calling thread will be blocked until the mutex has been locked. A
thread which currently has locked the mutex may not lock the same mutex again.
Warning:
If you leave a mutex locked in an emulator thread when you let the thread out of your control, you
will very likely deadlock the whole emulator.
This function is thread-safe.
int erl_drv_mutex_trylock(ErlDrvMutex *mtx)
Arguments:
mtx:
A pointer to a mutex to try to lock.
This function tries to lock a mutex. If successful 0, is returned; otherwise, EBUSY is returned. A
thread which currently has locked the mutex may not try to lock the same mutex again.
Warning:
If you leave a mutex locked in an emulator thread when you let the thread out of your control, you
will very likely deadlock the whole emulator.
This function is thread-safe.
void erl_drv_mutex_unlock(ErlDrvMutex *mtx)
Arguments:
mtx:
A pointer to a mutex to unlock.
This function unlocks a mutex. The mutex currently has to be locked by the calling thread.
This function is thread-safe.
ErlDrvCond *erl_drv_cond_create(char *name)
Arguments:
name:
A string identifying the created condition variable. It will be used to identify the condition
variable in planned future debug functionality.
This function creates a condition variable and returns a pointer to it. On failure NULL is
returned. The driver creating the condition variable has the responsibility of destroying it
before the driver is unloaded.
This function is thread-safe.
void erl_drv_cond_destroy(ErlDrvCond *cnd)
Arguments:
cnd:
A pointer to a condition variable to destroy.
This function destroys a condition variable previously created by erl_drv_cond_create().
This function is thread-safe.
void erl_drv_cond_signal(ErlDrvCond *cnd)
Arguments:
cnd:
A pointer to a condition variable to signal on.
This function signals on a condition variable. That is, if other threads are waiting on the
condition variable being signaled, one of them will be woken.
This function is thread-safe.
void erl_drv_cond_broadcast(ErlDrvCond *cnd)
Arguments:
cnd:
A pointer to a condition variable to broadcast on.
This function broadcasts on a condition variable. That is, if other threads are waiting on the
condition variable being broadcast on, all of them will be woken.
This function is thread-safe.
void erl_drv_cond_wait(ErlDrvCond *cnd, ErlDrvMutex *mtx)
Arguments:
cnd:
A pointer to a condition variable to wait on.
mtx:
A pointer to a mutex to unlock while waiting.
:
This function waits on a condition variable. The calling thread is blocked until another thread
wakes it by signaling or broadcasting on the condition variable. Before the calling thread is
blocked it unlocks the mutex passed as argument, and when the calling thread is woken it locks the
same mutex before returning. That is, the mutex currently has to be locked by the calling thread
when calling this function.
Note:
erl_drv_cond_wait() might return even though no-one has signaled or broadcast on the condition
variable. Code calling erl_drv_cond_wait() should always be prepared for erl_drv_cond_wait()
returning even though the condition that the thread was waiting for hasn't occurred. That is, when
returning from erl_drv_cond_wait() always check if the condition has occurred, and if not call
erl_drv_cond_wait() again.
This function is thread-safe.
ErlDrvRWLock *erl_drv_rwlock_create(char *name)
Arguments:
name:
A string identifying the created rwlock. It will be used to identify the rwlock in planned
future debug functionality.
This function creates an rwlock and returns a pointer to it. On failure NULL is returned. The
driver creating the rwlock has the responsibility of destroying it before the driver is unloaded.
This function is thread-safe.
void erl_drv_rwlock_destroy(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to destroy.
This function destroys an rwlock previously created by erl_drv_rwlock_create(). The rwlock has to
be in an unlocked state before being destroyed.
This function is thread-safe.
void erl_drv_rwlock_rlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to read lock.
This function read locks an rwlock. The calling thread will be blocked until the rwlock has been
read locked. A thread which currently has read or read/write locked the rwlock may not lock the
same rwlock again.
Warning:
If you leave an rwlock locked in an emulator thread when you let the thread out of your control,
you will very likely deadlock the whole emulator.
This function is thread-safe.
int erl_drv_rwlock_tryrlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to try to read lock.
This function tries to read lock an rwlock. If successful 0, is returned; otherwise, EBUSY is
returned. A thread which currently has read or read/write locked the rwlock may not try to lock
the same rwlock again.
Warning:
If you leave an rwlock locked in an emulator thread when you let the thread out of your control,
you will very likely deadlock the whole emulator.
This function is thread-safe.
void erl_drv_rwlock_runlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to read unlock.
This function read unlocks an rwlock. The rwlock currently has to be read locked by the calling
thread.
This function is thread-safe.
void erl_drv_rwlock_rwlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to read/write lock.
This function read/write locks an rwlock. The calling thread will be blocked until the rwlock has
been read/write locked. A thread which currently has read or read/write locked the rwlock may not
lock the same rwlock again.
Warning:
If you leave an rwlock locked in an emulator thread when you let the thread out of your control,
you will very likely deadlock the whole emulator.
This function is thread-safe.
int erl_drv_rwlock_tryrwlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to try to read/write lock.
This function tries to read/write lock an rwlock. If successful 0, is returned; otherwise, EBUSY
is returned. A thread which currently has read or read/write locked the rwlock may not try to lock
the same rwlock again.
Warning:
If you leave an rwlock locked in an emulator thread when you let the thread out of your control,
you will very likely deadlock the whole emulator.
This function is thread-safe.
void erl_drv_rwlock_rwunlock(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an rwlock to read/write unlock.
This function read/write unlocks an rwlock. The rwlock currently has to be read/write locked by
the calling thread.
This function is thread-safe.
int erl_drv_tsd_key_create(char *name, ErlDrvTSDKey *key)
Arguments:
name:
A string identifying the created key. It will be used to identify the key in planned future
debug functionality.
key:
A pointer to a thread specific data key variable.
This function creates a thread specific data key. On success 0 is returned; otherwise, an errno
value is returned to indicate the error. The driver creating the key has the responsibility of
destroying it before the driver is unloaded.
This function is thread-safe.
void erl_drv_tsd_key_destroy(ErlDrvTSDKey key)
Arguments:
key:
A thread specific data key to destroy.
This function destroys a thread specific data key previously created by erl_drv_tsd_key_create().
All thread specific data using this key in all threads have to be cleared (see erl_drv_tsd_set())
prior to the call to erl_drv_tsd_key_destroy().
Warning:
A destroyed key is very likely to be reused soon. Therefore, if you fail to clear the thread
specific data using this key in a thread prior to destroying the key, you will very likely get
unexpected errors in other parts of the system.
This function is thread-safe.
void erl_drv_tsd_set(ErlDrvTSDKey key, void *data)
Arguments:
key:
A thread specific data key.
data:
A pointer to data to associate with key in calling thread.
This function sets thread specific data associated with key for the calling thread. You are only
allowed to set thread specific data for threads while they are fully under your control. For
example, if you set thread specific data in a thread calling a driver call-back function, it has
to be cleared, i.e. set to NULL, before returning from the driver call-back function.
Warning:
If you fail to clear thread specific data in an emulator thread before letting it out of your
control, you might not ever be able to clear this data with later unexpected errors in other parts
of the system as a result.
This function is thread-safe.
void *erl_drv_tsd_get(ErlDrvTSDKey key)
Arguments:
key:
A thread specific data key.
This function returns the thread specific data associated with key for the calling thread. If no
data has been associated with key for the calling thread, NULL is returned.
This function is thread-safe.
int erl_drv_putenv(char *key, char *value)
Arguments:
key:
A null terminated string containing the name of the environment variable.
value:
A null terminated string containing the new value of the environment variable.
This function sets the value of an environment variable. It returns 0 on success, and a value != 0
on failure.
Note:
The result of passing the empty string ("") as a value is platform dependent. On some platforms
the value of the variable is set to the empty string, on others, the environment variable is
removed.
Warning:
Do not use libc's putenv or similar C library interfaces from a driver.
This function is thread-safe.
int erl_drv_getenv(char *key, char *value, size_t *value_size)
Arguments:
key:
A null terminated string containing the name of the environment variable.
value:
A pointer to an output buffer.
value_size:
A pointer to an integer. The integer is both used for passing input and output sizes (see
below).
This function retrieves the value of an environment variable. When called, *value_size should
contain the size of the value buffer. On success 0 is returned, the value of the environment
variable has been written to the value buffer, and *value_size contains the string length
(excluding the terminating null character) of the value written to the value buffer. On failure,
i.e., no such environment variable was found, a value less than 0 is returned. When the size of
the value buffer is too small, a value greater than 0 is returned and *value_size has been set to
the buffer size needed.
Warning:
Do not use libc's getenv or similar C library interfaces from a driver.
This function is thread-safe.
int erl_drv_consume_timeslice(ErlDrvPort port, int percent)
Arguments:
port:
Port handle of the executing port.
percent:
Approximate consumed fraction of a full time-slice in percent.
Give the runtime system a hint about how much CPU time the current driver callback call has
consumed since last hint, or since the start of the callback if no previous hint has been given.
The time is given as a fraction, in percent, of a full time-slice that a port is allowed to
execute before it should surrender the CPU to other runnable ports or processes. Valid range is
[1, 100]. The scheduling time-slice is not an exact entity, but can usually be approximated to
about 1 millisecond.
Note that it is up to the runtime system to determine if and how to use this information.
Implementations on some platforms may use other means in order to determine the consumed fraction
of the time-slice. Lengthy driver callbacks should regardless of this frequently call the
erl_drv_consume_timeslice() function in order to determine if it is allowed to continue execution
or not.
erl_drv_consume_timeslice() returns a non-zero value if the time-slice has been exhausted, and
zero if the callback is allowed to continue execution. If a non-zero value is returned the driver
callback should return as soon as possible in order for the port to be able to yield.
This function is provided to better support co-operative scheduling, improve system
responsiveness, and to make it easier to prevent misbehaviors of the VM due to a port monopolizing
a scheduler thread. It can be used when dividing length work into a number of repeated driver
callback calls without the need to use threads. Also see the important warning text at the
beginning of this document.
char *erl_drv_cond_name(ErlDrvCond *cnd)
Arguments:
cnd:
A pointer to an initialized condition.
Returns a pointer to the name of the condition.
Note:
This function is intended for debugging purposes only.
char *erl_drv_mutex_name(ErlDrvMutex *mtx)
Arguments:
mtx:
A pointer to an initialized mutex.
Returns a pointer to the name of the mutex.
Note:
This function is intended for debugging purposes only.
char *erl_drv_rwlock_name(ErlDrvRWLock *rwlck)
Arguments:
rwlck:
A pointer to an initialized r/w-lock.
Returns a pointer to the name of the r/w-lock.
Note:
This function is intended for debugging purposes only.
char *erl_drv_thread_name(ErlDrvTid tid)
Arguments:
tid:
A thread identifier.
Returns a pointer to the name of the thread.
Note:
This function is intended for debugging purposes only.
SEE ALSO
driver_entry(3erl), erl_ddll(3erl), erlang(3erl)
An Alternative Distribution Driver (ERTS User's Guide Ch. 3)
Ericsson AB erts 5.10.4 erl_driver(3erl)