Manpages - 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 ( )/). 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 ( )/), 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 ( )/), 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.

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 /.

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.

None.

/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.1‐2017, Chapter 8, Environment Variables, <spawn.h>

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 .

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