Provided by: libpmem-dev_1.12.1-2_amd64 
NAME
libpmem - persistent memory support library
SYNOPSIS
#include <libpmem.h>
cc ... -lpmem
Library API versioning:
const char *pmem_check_version(
unsigned major_required,
unsigned minor_required);
Error handling:
const char *pmem_errormsg(void);
Other library functions:
A description of other libpmem functions can be found on the following manual pages:
• most commonly used functions: pmem_is_pmem(3)
• partial flushing operations: pmem_flush(3)
• copying to persistent memory: pmem_memmove_persist(3)
DESCRIPTION
libpmem provides low-level persistent memory (pmem) support for applications using direct
access storage (DAX), which is storage that supports load/store access without paging
blocks from a block storage device. Some types of non-volatile memory DIMMs (NVDIMMs)
provide this type of byte addressable access to storage. A persistent memory aware file
system is typically used to expose the direct access to applications. Memory mapping a
file from this type of file system results in the load/store, non-paged access to pmem.
This library is for applications that use persistent memory directly, without the help of
any library-supplied transactions or memory allocation. Higher-level libraries that build
on libpmem are available and are recommended for most applications, see:
• libpmemobj(7), a general use persistent memory API, providing memory allocation and
transactional operations on variable-sized objects.
• libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks with atomic updates.
• libpmemlog(7), providing a pmem-resident log file.
Under normal usage, libpmem will never print messages or intentionally cause the process
to exit. The only exception to this is the debugging information, when enabled, as
described under DEBUGGING AND ERROR HANDLING below.
CAVEATS
libpmem relies on the library destructor being called from the main thread. For this
reason, all functions that might trigger destruction (e.g. dlclose(3)) should be called
in the main thread. Otherwise some of the resources associated with that thread might not
be cleaned up properly.
LIBRARY API VERSIONING
This section describes how the library API is versioned, allowing applications to work
with an evolving API.
The pmem_check_version() function is used to determine whether the installed libpmem
supports the version of the library API required by an application. The easiest way to do
this is for the application to supply the compile-time version information, supplied by
defines in <libpmem.h>, like this:
reason = pmem_check_version(PMEM_MAJOR_VERSION,
PMEM_MINOR_VERSION);
if (reason != NULL) {
/* version check failed, reason string tells you why */
}
Any mismatch in the major version number is considered a failure, but a library with a
newer minor version number will pass this check since increasing minor versions imply
backwards compatibility.
An application can also check specifically for the existence of an interface by checking
for the version where that interface was introduced. These versions are documented in
this man page as follows: unless otherwise specified, all interfaces described here are
available in version 1.0 of the library. Interfaces added after version 1.0 will contain
the text introduced in version x.y in the section of this manual describing the feature.
When the version check performed by pmem_check_version() is successful, the return value
is NULL. Otherwise the return value is a static string describing the reason for failing
the version check. The string returned by pmem_check_version() must not be modified or
freed.
ENVIRONMENT
libpmem can change its default behavior based on the following environment variables.
These are largely intended for testing and are not normally required.
• PMEM_IS_PMEM_FORCE=val
If val is 0 (zero), then pmem_is_pmem(3) will always return false. Setting val to 1
causes pmem_is_pmem(3) to always return true. This variable is mostly used for testing
but can be used to force pmem behavior on a system where a range of pmem is not detectable
as pmem for some reason.
NOTE: Unlike the other variables, the value of PMEM_IS_PMEM_FORCE is not queried
(and cached) at library initialization time, but on the first call to
pmem_is_pmem(3). This means that in case of libpmemlog(7), libpmemblk(7), and
libpmemobj(7), PMEM_IS_PMEM_FORCE may still be set or modified by the program until
the first attempt to create or open the persistent memory pool.
• PMEM_NO_CLWB=1
Setting this environment variable to 1 forces libpmem to never issue the CLWB instruction
on Intel hardware, falling back to other cache flush instructions instead (CLFLUSHOPT or
CLFLUSH on Intel hardware). Without this environment variable, libpmem will always use
the CLWB instruction for flushing processor caches on platforms that support the
instruction. This variable is intended for use during library testing but may be required
for some rare cases where using CLWB has a negative impact on performance.
• PMEM_NO_CLFLUSHOPT=1
Setting this environment variable to 1 forces libpmem to never issue the CLFLUSHOPT
instruction on Intel hardware, falling back to the CLFLUSH instructions instead. Without
this environment variable, libpmem will always use the CLFLUSHOPT instruction for flushing
processor caches on platforms that support the instruction, but where CLWB is not
available. This variable is intended for use during library testing.
• PMEM_NO_FLUSH=1
Setting this environment variable to 1 forces most libpmem functions to never issue any of
CLFLUSH, CLFLUSHOPT or CLWB instructions on Intel hardware. The only exceptions are
pmem_deep_flush(3) and pmem_deep_persist(3) functions.
• PMEM_NO_FLUSH=0
Setting this environment variable to 0 forces to always flush CPU caches using one of
CLFLUSH, CLFLUSHOPT or CLWB instructions even if pmem_has_auto_flush(3) function returns
true and the platform supports flushing the processor caches on power loss or system
crash.
• PMEM_NO_MOVNT=1
Setting this environment variable to 1 forces libpmem to never use the non-temporal move
instructions on Intel hardware. Without this environment variable, libpmem will use the
non-temporal instructions for copying larger ranges to persistent memory on platforms that
support the instructions. This variable is intended for use during library testing.
• PMEM_MOVNT_THRESHOLD=val
This environment variable allows overriding the minimum length of the
pmem_memmove_persist(3) operations, for which libpmem uses non-temporal move instructions.
Setting this environment variable to 0 forces libpmem to always use the non-temporal move
instructions if available. It has no effect if PMEM_NO_MOVNT is set to 1. This variable
is intended for use during library testing.
• PMEM_MMAP_HINT=val
This environment variable allows overriding the hint address used by pmem_map_file(). If
set, it also disables mapping address randomization. This variable is intended for use
during library testing and debugging. Setting it to some fairly large value
(i.e. 0x10000000000) will very likely result in mapping the file at the specified address
(if not used) or at the first unused region above given address, without adding any random
offset. When debugging, this makes it easier to calculate the actual address of the
persistent memory block, based on its offset in the file. In case of libpmemobj it
simplifies conversion of a persistent object identifier (OID) into a direct pointer to the
object.
NOTE: Setting this environment variable affects all the PMDK libraries, disabling
mapping address randomization and causing the specified address to be used as a
hint about where to place the mapping.
DEBUGGING AND ERROR HANDLING
If an error is detected during the call to a libpmem function, the application may
retrieve an error message describing the reason of the failure from pmem_errormsg(). This
function returns a pointer to a static buffer containing the last error message logged for
the current thread. If errno was set, the error message may include a description of the
corresponding error code as returned by strerror(3). The error message buffer is
thread-local; errors encountered in one thread do not affect its value in other threads.
The buffer is never cleared by any library function; its content is significant only when
the return value of the immediately preceding call to a libpmem function indicated an
error. The application must not modify or free the error message string. Subsequent
calls to other library functions may modify the previous message.
Two versions of libpmem are typically available on a development system. The normal
version, accessed when a program is linked using the -lpmem option, is optimized for
performance. That version skips checks that impact performance and never logs any trace
information or performs any run-time assertions.
A second version of libpmem, accessed when a program uses the libraries under
/usr/lib/pmdk_debug, contains run-time assertions and trace points. The typical way to
access the debug version is to set the environment variable LD_LIBRARY_PATH to
/usr/lib/pmdk_debug or /usr/lib64/pmdk_debug, as appropriate. Debugging output is
controlled using the following environment variables. These variables have no effect on
the non-debug version of the library.
NOTE: On Debian/Ubuntu systems, this extra debug version of the library is shipped
in the respective -debug Debian package and placed in the /usr/lib/$ARCH/pmdk_dbg/
directory.
• PMEM_LOG_LEVEL
The value of PMEM_LOG_LEVEL enables trace points in the debug version of the library, as
follows:
• 0 - This is the default level when PMEM_LOG_LEVEL is not set. No log messages are
emitted at this level.
• 1 - Additional details on any errors detected are logged, in addition to returning the
errno-based errors as usual. The same information may be retrieved using
pmem_errormsg().
• 2 - A trace of basic operations is logged.
• 3 - Enables a very verbose amount of function call tracing in the library.
• 4 - Enables voluminous and fairly obscure tracing information that is likely only useful
to the libpmem developers.
Unless PMEM_LOG_FILE is set, debugging output is written to stderr.
• PMEM_LOG_FILE
Specifies the name of a file where all logging information should be written. If the last
character in the name is “-”, the PID of the current process will be appended to the file
name when the log file is created. If PMEM_LOG_FILE is not set, output is written to
stderr.
EXAMPLE
The following example uses libpmem to flush changes made to raw, memory-mapped persistent
memory.
WARNING: There is nothing transactional about the pmem_persist(3) or pmem_msync(3)
calls in this example. Interrupting the program may result in a partial write to
pmem. Use a transactional library such as libpmemobj(7) to avoid torn updates.
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libpmem.h>
/* using 4k of pmem for this example */
#define PMEM_LEN 4096
#define PATH "/pmem-fs/myfile"
int
main(int argc, char *argv[])
{
char *pmemaddr;
size_t mapped_len;
int is_pmem;
/* create a pmem file and memory map it */
if ((pmemaddr = pmem_map_file(PATH, PMEM_LEN, PMEM_FILE_CREATE,
0666, &mapped_len, &is_pmem)) == NULL) {
perror("pmem_map_file");
exit(1);
}
/* store a string to the persistent memory */
strcpy(pmemaddr, "hello, persistent memory");
/* flush above strcpy to persistence */
if (is_pmem)
pmem_persist(pmemaddr, mapped_len);
else
pmem_msync(pmemaddr, mapped_len);
/*
* Delete the mappings. The region is also
* automatically unmapped when the process is
* terminated.
*/
pmem_unmap(pmemaddr, mapped_len);
}
See <https://pmem.io/pmdk/libpmem> for more examples using the libpmem API.
ACKNOWLEDGEMENTS
libpmem builds on the persistent memory programming model recommended by the SNIA NVM
Programming Technical Work Group: <https://snia.org/nvmp>
SEE ALSO
dlclose(3), pmem_flush(3), pmem_is_pmem(3), pmem_memmove_persist(3), pmem_msync(3),
pmem_persist(3), strerror(3), libpmemblk(7), libpmemlog(7), libpmemobj(7) and
<https://pmem.io>