|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
POSIX_SPAWN(3P) POSIX Programmer's Manual POSIX_SPAWN(3P)
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.
posix_spawn, posix_spawnp — spawn a process (ADVANCED REALTIME)
#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]);
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.1‐2017, 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.1‐2017, 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(3p)). For those file descriptors that remain open, the
child process shall not inherit any file locks, but all remaining
attributes of the corresponding open file descriptions (see
fcntl(3p)), 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.
The child process shall not inherit any file locks, but all
remaining attributes of the corresponding open file
descriptions (see fcntl(3p)), 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(3p)) 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.
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.
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.
None.
These functions are part of the Spawn option and need not be
provided on all implementations.
See also the APPLICATION USAGE section for exec(1p).
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.
None.
alarm(3p), chmod(3p), close(3p), dup(3p), exec(1p), exit(3p),
fcntl(3p), fork(3p), fstatat(3p), kill(3p), open(3p),
posix_spawn_file_actions_addclose(3p),
posix_spawn_file_actions_adddup2(3p),
posix_spawn_file_actions_destroy(3p), posix_spawnattr_destroy(3p),
posix_spawnattr_getsigdefault(3p), posix_spawnattr_getflags(3p),
posix_spawnattr_getpgroup(3p), posix_spawnattr_getschedparam(3p),
posix_spawnattr_getschedpolicy(3p),
posix_spawnattr_getsigmask(3p), sched_setparam(3p),
sched_setscheduler(3p), setpgid(3p), setuid(3p), times(3p),
wait(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, spawn.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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 .
IEEE/The Open Group 2017 POSIX_SPAWN(3P)
Pages that refer to this page: spawn.h(0p), exec(3p), fdopen(3p), posix_spawnattr_destroy(3p), posix_spawnattr_getflags(3p), posix_spawnattr_getpgroup(3p), posix_spawnattr_getschedparam(3p), posix_spawnattr_getschedpolicy(3p), posix_spawnattr_getsigdefault(3p), posix_spawnattr_getsigmask(3p), posix_spawn_file_actions_addclose(3p), posix_spawn_file_actions_adddup2(3p), posix_spawn_file_actions_destroy(3p), posix_spawnp(3p)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|