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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
posix_spawn, posix_spawnp — spawn a process (ADVANCED REALTIME)
SYNOPSIS
#include <spawn.h>
int posix_spawn(pid_t *restrict pid, const char *restrict path,
const posix_spawn_file_actions_t *file_actions,
const posix_spawnattr_t *restrict attrp,
char *const argv[restrict], char *const envp[restrict]);
int posix_spawnp(pid_t *restrict pid, const char *restrict file,
const posix_spawn_file_actions_t *file_actions,
const posix_spawnattr_t *restrict attrp,
char *const argv[restrict], char *const envp[restrict]);
DESCRIPTION
The posix_spawn() and posix_spawnp() functions shall create a new process (child process)
from the specified process image. The new process image shall be constructed from a
regular executable file called the new process image file.
When a C program is executed as the result of this call, it shall be entered as a C-
language function call as follows:
int main(int argc, char *argv[]);
where argc is the argument count and argv is an array of character pointers to the
arguments themselves. In addition, the following variable:
extern char **environ;
shall be initialized as a pointer to an array of character pointers to the environment
strings.
The argument argv is an array of character pointers to null-terminated strings. The last
member of this array shall be a null pointer and is not counted in argc. These strings
constitute the argument list available to the new process image. The value in argv[0]
should point to a filename string that is associated with the process image being started
by the posix_spawn() or posix_spawnp() function.
The argument envp is an array of character pointers to null-terminated strings. These
strings constitute the environment for the new process image. The environment array is
terminated by a null pointer.
The number of bytes available for the combined argument and environment lists of the child
process is {ARG_MAX}. The implementation shall specify in the system documentation (see
the Base Definitions volume of POSIX.1‐2008, Chapter 2, Conformance) whether any list
overhead, such as length words, null terminators, pointers, or alignment bytes, is
included in this total.
The path argument to posix_spawn() is a pathname that identifies the new process image
file to execute.
The file parameter to posix_spawnp() shall be used to construct a pathname that identifies
the new process image file. If the file parameter contains a <slash> character, the file
parameter shall be used as the pathname for the new process image file. Otherwise, the
path prefix for this file shall be obtained by a search of the directories passed as the
environment variable PATH (see the Base Definitions volume of POSIX.1‐2008, Chapter 8,
Environment Variables). If this environment variable is not defined, the results of the
search are implementation-defined.
If file_actions is a null pointer, then file descriptors open in the calling process shall
remain open in the child process, except for those whose close-on-exec flag FD_CLOEXEC is
set (see fcntl()). For those file descriptors that remain open, all attributes of the
corresponding open file descriptions, including file locks (see fcntl()), shall remain
unchanged.
If file_actions is not NULL, then the file descriptors open in the child process shall be
those open in the calling process as modified by the spawn file actions object pointed to
by file_actions and the FD_CLOEXEC flag of each remaining open file descriptor after the
spawn file actions have been processed. The effective order of processing the spawn file
actions shall be:
1. The set of open file descriptors for the child process shall initially be the same set
as is open for the calling process. All attributes of the corresponding open file
descriptions, including file locks (see fcntl()), shall remain unchanged.
2. The signal mask, signal default actions, and the effective user and group IDs for the
child process shall be changed as specified in the attributes object referenced by
attrp.
3. The file actions specified by the spawn file actions object shall be performed in the
order in which they were added to the spawn file actions object.
4. Any file descriptor that has its FD_CLOEXEC flag set (see fcntl()) shall be closed.
If file descriptor 0, 1, or 2 would otherwise be closed in the new process image created
by posix_spawn() or posix_spawnp(), implementations may open an unspecified file for the
file descriptor in the new process image. If a standard utility or a conforming
application is executed with file descriptor 0 not open for reading or with file
descriptor 1 or 2 not open for writing, the environment in which the utility or
application is executed shall be deemed non-conforming, and consequently the utility or
application might not behave as described in this standard.
The posix_spawnattr_t spawn attributes object type is defined in <spawn.h>. It shall
contain at least the attributes defined below.
If the POSIX_SPAWN_SETPGROUP flag is set in the spawn-flags attribute of the object
referenced by attrp, and the spawn-pgroup attribute of the same object is non-zero, then
the child's process group shall be as specified in the spawn-pgroup attribute of the
object referenced by attrp.
As a special case, if the POSIX_SPAWN_SETPGROUP flag is set in the spawn-flags attribute
of the object referenced by attrp, and the spawn-pgroup attribute of the same object is
set to zero, then the child shall be in a new process group with a process group ID equal
to its process ID.
If the POSIX_SPAWN_SETPGROUP flag is not set in the spawn-flags attribute of the object
referenced by attrp, the new child process shall inherit the parent's process group.
If the POSIX_SPAWN_SETSCHEDPARAM flag is set in the spawn-flags attribute of the object
referenced by attrp, but POSIX_SPAWN_SETSCHEDULER is not set, the new process image shall
initially have the scheduling policy of the calling process with the scheduling parameters
specified in the spawn-schedparam attribute of the object referenced by attrp.
If the POSIX_SPAWN_SETSCHEDULER flag is set in the spawn-flags attribute of the object
referenced by attrp (regardless of the setting of the POSIX_SPAWN_SETSCHEDPARAM flag), the
new process image shall initially have the scheduling policy specified in the spawn-
schedpolicy attribute of the object referenced by attrp and the scheduling parameters
specified in the spawn-schedparam attribute of the same object.
The POSIX_SPAWN_RESETIDS flag in the spawn-flags attribute of the object referenced by
attrp governs the effective user ID of the child process. If this flag is not set, the
child process shall inherit the effective user ID of the parent process. If this flag is
set, the effective user ID of the child process shall be reset to the parent's real user
ID. In either case, if the set-user-ID mode bit of the new process image file is set, the
effective user ID of the child process shall become that file's owner ID before the new
process image begins execution.
The POSIX_SPAWN_RESETIDS flag in the spawn-flags attribute of the object referenced by
attrp also governs the effective group ID of the child process. If this flag is not set,
the child process shall inherit the effective group ID of the parent process. If this flag
is set, the effective group ID of the child process shall be reset to the parent's real
group ID. In either case, if the set-group-ID mode bit of the new process image file is
set, the effective group ID of the child process shall become that file's group ID before
the new process image begins execution.
If the POSIX_SPAWN_SETSIGMASK flag is set in the spawn-flags attribute of the object
referenced by attrp, the child process shall initially have the signal mask specified in
the spawn-sigmask attribute of the object referenced by attrp.
If the POSIX_SPAWN_SETSIGDEF flag is set in the spawn-flags attribute of the object
referenced by attrp, the signals specified in the spawn-sigdefault attribute of the same
object shall be set to their default actions in the child process. Signals set to the
default action in the parent process shall be set to the default action in the child
process.
Signals set to be caught by the calling process shall be set to the default action in the
child process.
Except for SIGCHLD, signals set to be ignored by the calling process image shall be set to
be ignored by the child process, unless otherwise specified by the POSIX_SPAWN_SETSIGDEF
flag being set in the spawn-flags attribute of the object referenced by attrp and the
signals being indicated in the spawn-sigdefault attribute of the object referenced by
attrp.
If the SIGCHLD signal is set to be ignored by the calling process, it is unspecified
whether the SIGCHLD signal is set to be ignored or to the default action in the child
process, unless otherwise specified by the POSIX_SPAWN_SETSIGDEF flag being set in the
spawn_flags attribute of the object referenced by attrp and the SIGCHLD signal being
indicated in the spawn_sigdefault attribute of the object referenced by attrp.
If the value of the attrp pointer is NULL, then the default values are used.
All process attributes, other than those influenced by the attributes set in the object
referenced by attrp as specified above or by the file descriptor manipulations specified
in file_actions, shall appear in the new process image as though fork() had been called to
create a child process and then a member of the exec family of functions had been called
by the child process to execute the new process image.
It is implementation-defined whether the fork handlers are run when posix_spawn() or
posix_spawnp() is called.
RETURN VALUE
Upon successful completion, posix_spawn() and posix_spawnp() shall return the process ID
of the child process to the parent process, in the variable pointed to by a non-NULL pid
argument, and shall return zero as the function return value. Otherwise, no child process
shall be created, the value stored into the variable pointed to by a non-NULL pid is
unspecified, and an error number shall be returned as the function return value to
indicate the error. If the pid argument is a null pointer, the process ID of the child is
not returned to the caller.
ERRORS
These functions may fail if:
EINVAL The value specified by file_actions or attrp is invalid.
If this error occurs after the calling process successfully returns from the posix_spawn()
or posix_spawnp() function, the child process may exit with exit status 127.
If posix_spawn() or posix_spawnp() fail for any of the reasons that would cause fork() or
one of the exec family of functions to fail, an error value shall be returned as described
by fork() and exec, respectively (or, if the error occurs after the calling process
successfully returns, the child process shall exit with exit status 127).
If POSIX_SPAWN_SETPGROUP is set in the spawn-flags attribute of the object referenced by
attrp, and posix_spawn() or posix_spawnp() fails while changing the child's process group,
an error value shall be returned as described by setpgid() (or, if the error occurs after
the calling process successfully returns, the child process shall exit with exit status
127).
If POSIX_SPAWN_SETSCHEDPARAM is set and POSIX_SPAWN_SETSCHEDULER is not set in the spawn-
flags attribute of the object referenced by attrp, then if posix_spawn() or posix_spawnp()
fails for any of the reasons that would cause sched_setparam() to fail, an error value
shall be returned as described by sched_setparam() (or, if the error occurs after the
calling process successfully returns, the child process shall exit with exit status 127).
If POSIX_SPAWN_SETSCHEDULER is set in the spawn-flags attribute of the object referenced
by attrp, and if posix_spawn() or posix_spawnp() fails for any of the reasons that would
cause sched_setscheduler() to fail, an error value shall be returned as described by
sched_setscheduler() (or, if the error occurs after the calling process successfully
returns, the child process shall exit with exit status 127).
If the file_actions argument is not NULL, and specifies any close, dup2, or open actions
to be performed, and if posix_spawn() or posix_spawnp() fails for any of the reasons that
would cause close(), dup2(), or open() to fail, an error value shall be returned as
described by close(), dup2(), and open(), respectively (or, if the error occurs after the
calling process successfully returns, the child process shall exit with exit status 127).
An open file action may, by itself, result in any of the errors described by close() or
dup2(), in addition to those described by open().
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
These functions are part of the Spawn option and need not be provided on all
implementations.
See also the APPLICATION USAGE section for exec.
RATIONALE
The posix_spawn() function and its close relation posix_spawnp() have been introduced to
overcome the following perceived difficulties with fork(): the fork() function is
difficult or impossible to implement without swapping or dynamic address translation.
* Swapping is generally too slow for a realtime environment.
* Dynamic address translation is not available everywhere that POSIX might be useful.
* Processes are too useful to simply option out of POSIX whenever it must run without
address translation or other MMU services.
Thus, POSIX needs process creation and file execution primitives that can be efficiently
implemented without address translation or other MMU services.
The posix_spawn() function is implementable as a library routine, but both posix_spawn()
and posix_spawnp() are designed as kernel operations. Also, although they may be an
efficient replacement for many fork()/exec pairs, their goal is to provide useful process
creation primitives for systems that have difficulty with fork(), not to provide drop-in
replacements for fork()/exec.
This view of the role of posix_spawn() and posix_spawnp() influenced the design of their
API. It does not attempt to provide the full functionality of fork()/exec in which
arbitrary user-specified operations of any sort are permitted between the creation of the
child process and the execution of the new process image; any attempt to reach that level
would need to provide a programming language as parameters. Instead, posix_spawn() and
posix_spawnp() are process creation primitives like the Start_Process and
Start_Process_Search Ada language bindings package POSIX_Process_Primitives and also like
those in many operating systems that are not UNIX systems, but with some POSIX-specific
additions.
To achieve its coverage goals, posix_spawn() and posix_spawnp() have control of six types
of inheritance: file descriptors, process group ID, user and group ID, signal mask,
scheduling, and whether each signal ignored in the parent will remain ignored in the
child, or be reset to its default action in the child.
Control of file descriptors is required to allow an independently written child process
image to access data streams opened by and even generated or read by the parent process
without being specifically coded to know which parent files and file descriptors are to be
used. Control of the process group ID is required to control how the job control of the
child process relates to that of the parent.
Control of the signal mask and signal defaulting is sufficient to support the
implementation of system(). Although support for system() is not explicitly one of the
goals for posix_spawn() and posix_spawnp(), it is covered under the ``at least 50%''
coverage goal.
The intention is that the normal file descriptor inheritance across fork(), the subsequent
effect of the specified spawn file actions, and the normal file descriptor inheritance
across one of the exec family of functions should fully specify open file inheritance. The
implementation need make no decisions regarding the set of open file descriptors when the
child process image begins execution, those decisions having already been made by the
caller and expressed as the set of open file descriptors and their FD_CLOEXEC flags at the
time of the call and the spawn file actions object specified in the call. We have been
assured that in cases where the POSIX Start_Process Ada primitives have been implemented
in a library, this method of controlling file descriptor inheritance may be implemented
very easily.
We can identify several problems with posix_spawn() and posix_spawnp(), but there does not
appear to be a solution that introduces fewer problems. Environment modification for child
process attributes not specifiable via the attrp or file_actions arguments must be done in
the parent process, and since the parent generally wants to save its context, it is more
costly than similar functionality with fork()/exec. It is also complicated to modify the
environment of a multi-threaded process temporarily, since all threads must agree when it
is safe for the environment to be changed. However, this cost is only borne by those
invocations of posix_spawn() and posix_spawnp() that use the additional functionality.
Since extensive modifications are not the usual case, and are particularly unlikely in
time-critical code, keeping much of the environment control out of posix_spawn() and
posix_spawnp() is appropriate design.
The posix_spawn() and posix_spawnp() functions do not have all the power of fork()/exec.
This is to be expected. The fork() function is a wonderfully powerful operation. We do not
expect to duplicate its functionality in a simple, fast function with no special hardware
requirements. It is worth noting that posix_spawn() and posix_spawnp() are very similar to
the process creation operations on many operating systems that are not UNIX systems.
Requirements
The requirements for posix_spawn() and posix_spawnp() are:
* They must be implementable without an MMU or unusual hardware.
* They must be compatible with existing POSIX standards.
Additional goals are:
* They should be efficiently implementable.
* They should be able to replace at least 50% of typical executions of fork().
* A system with posix_spawn() and posix_spawnp() and without fork() should be useful, at
least for realtime applications.
* A system with fork() and the exec family should be able to implement posix_spawn() and
posix_spawnp() as library routines.
Two-Syntax
POSIX exec has several calling sequences with approximately the same functionality. These
appear to be required for compatibility with existing practice. Since the existing
practice for the posix_spawn*() functions is otherwise substantially unlike POSIX, we feel
that simplicity outweighs compatibility. There are, therefore, only two names for the
posix_spawn*() functions.
The parameter list does not differ between posix_spawn() and posix_spawnp();
posix_spawnp() interprets the second parameter more elaborately than posix_spawn().
Compatibility with POSIX.5 (Ada)
The Start_Process and Start_Process_Search procedures from the POSIX_Process_Primitives
package from the Ada language binding to POSIX.1 encapsulate fork() and exec functionality
in a manner similar to that of posix_spawn() and posix_spawnp(). Originally, in keeping
with our simplicity goal, the standard developers had limited the capabilities of
posix_spawn() and posix_spawnp() to a subset of the capabilities of Start_Process and
Start_Process_Search; certain non-default capabilities were not supported. However, based
on suggestions by the ballot group to improve file descriptor mapping or drop it, and on
the advice of an Ada Language Bindings working group member, the standard developers
decided that posix_spawn() and posix_spawnp() should be sufficiently powerful to implement
Start_Process and Start_Process_Search. The rationale is that if the Ada language binding
to such a primitive had already been approved as an IEEE standard, there can be little
justification for not approving the functionally-equivalent parts of a C binding. The only
three capabilities provided by posix_spawn() and posix_spawnp() that are not provided by
Start_Process and Start_Process_Search are optionally specifying the child's process group
ID, the set of signals to be reset to default signal handling in the child process, and
the child's scheduling policy and parameters.
For the Ada language binding for Start_Process to be implemented with posix_spawn(), that
binding would need to explicitly pass an empty signal mask and the parent's environment to
posix_spawn() whenever the caller of Start_Process allowed these arguments to default,
since posix_spawn() does not provide such defaults. The ability of Start_Process to mask
user-specified signals during its execution is functionally unique to the Ada language
binding and must be dealt with in the binding separately from the call to posix_spawn().
Process Group
The process group inheritance field can be used to join the child process with an existing
process group. By assigning a value of zero to the spawn-pgroup attribute of the object
referenced by attrp, the setpgid() mechanism will place the child process in a new process
group.
Threads
Without the posix_spawn() and posix_spawnp() functions, systems without address
translation can still use threads to give an abstraction of concurrency. In many cases,
thread creation suffices, but it is not always a good substitute. The posix_spawn() and
posix_spawnp() functions are considerably ``heavier'' than thread creation. Processes have
several important attributes that threads do not. Even without address translation, a
process may have base-and-bound memory protection. Each process has a process environment
including security attributes and file capabilities, and powerful scheduling attributes.
Processes abstract the behavior of non-uniform-memory-architecture multi-processors better
than threads, and they are more convenient to use for activities that are not closely
linked.
The posix_spawn() and posix_spawnp() functions may not bring support for multiple
processes to every configuration. Process creation is not the only piece of operating
system support required to support multiple processes. The total cost of support for
multiple processes may be quite high in some circumstances. Existing practice shows that
support for multiple processes is uncommon and threads are common among ``tiny kernels''.
There should, therefore, probably continue to be AEPs for operating systems with only one
process.
Asynchronous Error Notification
A library implementation of posix_spawn() or posix_spawnp() may not be able to detect all
possible errors before it forks the child process. POSIX.1‐2008 provides for an error
indication returned from a child process which could not successfully complete the spawn
operation via a special exit status which may be detected using the status value returned
by wait(), waitid(), and waitpid().
The stat_val interface and the macros used to interpret it are not well suited to the
purpose of returning API errors, but they are the only path available to a library
implementation. Thus, an implementation may cause the child process to exit with exit
status 127 for any error detected during the spawn process after the posix_spawn() or
posix_spawnp() function has successfully returned.
The standard developers had proposed using two additional macros to interpret stat_val.
The first, WIFSPAWNFAIL, would have detected a status that indicated that the child exited
because of an error detected during the posix_spawn() or posix_spawnp() operations rather
than during actual execution of the child process image; the second, WSPAWNERRNO, would
have extracted the error value if WIFSPAWNFAIL indicated a failure. Unfortunately, the
ballot group strongly opposed this because it would make a library implementation of
posix_spawn() or posix_spawnp() dependent on kernel modifications to waitpid() to be able
to embed special information in stat_val to indicate a spawn failure.
The 8 bits of child process exit status that are guaranteed by POSIX.1‐2008 to be
accessible to the waiting parent process are insufficient to disambiguate a spawn error
from any other kind of error that may be returned by an arbitrary process image. No other
bits of the exit status are required to be visible in stat_val, so these macros could not
be strictly implemented at the library level. Reserving an exit status of 127 for such
spawn errors is consistent with the use of this value by system() and popen() to signal
failures in these operations that occur after the function has returned but before a shell
is able to execute. The exit status of 127 does not uniquely identify this class of error,
nor does it provide any detailed information on the nature of the failure. Note that a
kernel implementation of posix_spawn() or posix_spawnp() is permitted (and encouraged) to
return any possible error as the function value, thus providing more detailed failure
information to the parent process.
Thus, no special macros are available to isolate asynchronous posix_spawn() or
posix_spawnp() errors. Instead, errors detected by the posix_spawn() or posix_spawnp()
operations in the context of the child process before the new process image executes are
reported by setting the child's exit status to 127. The calling process may use the
WIFEXITED and WEXITSTATUS macros on the stat_val stored by the wait() or waitpid()
functions to detect spawn failures to the extent that other status values with which the
child process image may exit (before the parent can conclusively determine that the child
process image has begun execution) are distinct from exit status 127.
FUTURE DIRECTIONS
None.
SEE ALSO
alarm(), chmod(), close(), dup(), exec, exit(), fcntl(), fork(), fstatat(), kill(),
open(), posix_spawn_file_actions_addclose(), posix_spawn_file_actions_adddup2(),
posix_spawn_file_actions_destroy(), posix_spawnattr_destroy(),
posix_spawnattr_getsigdefault(), posix_spawnattr_getflags(), posix_spawnattr_getpgroup(),
posix_spawnattr_getschedparam(), posix_spawnattr_getschedpolicy(),
posix_spawnattr_getsigmask(), sched_setparam(), sched_setscheduler(), setpgid(), setuid(),
times(), wait(), waitid()
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables, <spawn.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any
discrepancy between this version and the original IEEE and The Open Group Standard, the
original IEEE and The Open Group Standard is the referee document. The original Standard
can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .