POSIX_SPAWN(3C) Standard C Library Functions POSIX_SPAWN(3C)

posix_spawn, posix_spawnp — spawn a process

Standard C Library (libc, -lc)

#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 () and posix_spawnp() functions create a new process (child process) from the specified process image. The new process image is 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 is 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;

is 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 is 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 that is associated with the process image being started by the () 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 child process's combined argument and environment lists is {ARG_MAX}, counting all character pointers, the strings they point to, the trailing null bytes in the strings, and the list-terminating null pointers. There is no additional system overhead included in this total.

The path argument to () is a pathname that identifies the new process image file to execute.

The file parameter to () is used to construct a pathname that identifies the new process image file. If the file parameter contains a slash character, the file parameter is used as the pathname for the new process image file. Otherwise, the path prefix for this file is obtained by a search of the directories passed as the environment variable PATH. 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 remain open in the child process, except for those whose close-on-exec flag FD_CLOEXEC is set (see fcntl(2)). For those file descriptors that remain open, all attributes of the corresponding open file descriptions, including file locks (see fcntl(2)), remain unchanged.

If file_actions is not NULL, then the file descriptors open in the child process are 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 are:

  1. The set of open file descriptors for the child process are initially the same set as is open for the calling process. All attributes of the corresponding open file descriptions, including file locks (see fcntl(2)), remain unchanged.
  2. The signal mask, signal default or ignore actions, and the effective user and group IDs for the child process are changed as specified in the attributes object referenced by attrp.
  3. The file actions specified by the spawn file actions object are 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(2)) is closed.

The child's working directory is dependent on the file actions specified. By default, the child inherits the parent's current working directory. However, this may be changed by using the posix_spawn_file_actions_addchdir(3C) family of functions. These file actions are processed like any other file actions and are handled per the steps described above.

The posix_spawnattr_t spawn attributes object type is defined in <spawn.h>. It contains at least the attributes defined below.

By default, the child process inherits the existing session and process group of the parent process. This behavior can be changed by specifying one of the POSIX_SPAWN_SETSID or POSIX_SPAWN_SETPGROUP flags described below.

If the POSIX_SPAWN_SETSID flag is set in the spawn-flags attribute of the object referenced by attrp then the child will have all the effects of calling setsid(2) applied. Chiefly, it will have no controlling terminal, it will be the leader of a new process group, and it will be the leader of a new session. POSIX leaves undefined the behavior of setting both the POSIX_SPAWN_SETSID and POSIX_SPAWN_SETPGROUP (described below) flags. It is recommended to only set one or the other.

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 is 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 will be in a new process group with a process group ID equal to its process ID.

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 initially has 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 spawn-flags attribute of the object referenced by attrp (regardless of the setting of the POSIX_SPAWN_SETSCHEDPARAM flag), the new process image initially has 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 inherits the parent process's effective user ID. If this flag is set, the child process's effective user ID is 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 becomes 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 inherits the parent process's effective group ID. If this flag is set, the child process's effective group ID is 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 becomes 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 initially has 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 is set to their default actions in the child process.

If the POSIX_SPAWN_SETSIGIGN_NP flag is set in the spawn-flags attribute of the object referenced by attrp, the signals specified in the spawn-sigignore attribute of the same object are set to be ignored in the child process.

If both POSIX_SPAWN_SETSIGDEF and POSIX_SPAWN_SETSIGIGN_NP flags are set in the spawn-flags attribute of the object referenced by attrp, the actions for POSIX_SPAWN_SETSIGDEF take precedence over the actions for POSIX_SPAWN_SETSIGIGN_NP.

If the POSIX_SPAWN_NOSIGCHLD_NP flag is set in the spawn-flags attribute of the object referenced by attrp, no SIGCHLD signal will be posted to the parent process when the child process terminates, regardless of the disposition of the SIGCHLD signal in the parent. SIGCHLD signals are still possible for job control stop and continue actions if the parent has requested them.

If the POSIX_SPAWN_WAITPID_NP flag is set in the spawn-flags attribute of the object referenced by attrp, no wait-for-multiple-pids operation by the parent, as in (), (P_ALL), or waitid(P_PGID), will succeed in reaping the child, and the child will not be reaped automatically due to the disposition of the SIGCHLD signal being set to be ignored in the parent. Only a specific wait for the child, as in waitid(P_PID, pid), is allowed and it is required, else when the child exits it will remain a zombie until the parent exits.

If the POSIX_SPAWN_NOEXECERR_NP flag is set in the spawn-flags attribute of the object referenced by attrp, and if the specified process image file cannot be executed, then the () and posix_spawnp() functions do not fail with one of the exec(2) error codes, as is normal, but rather return successfully having created a child process that exits immediately with exit status 127. This flag permits system(3C) and popen(3C) to be implemented with posix_spawn() and still conform strictly to their POSIX specifications.

Signals set to be caught or set to the default action in the calling process are set to the default action in the child process, unless the POSIX_SPAWN_SETSIGIGN_NP flag is set in the spawn-flags attribute of the object referenced by attrp and the signals are specified in the spawn-sigignore attribute of the same object.

Except for SIGCHLD, signals set to be ignored by the calling process image are 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 appear in the new process image as though () 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.

The fork handlers are not run when () or posix_spawnp() is called.

Upon successful completion, posix_spawn() and posix_spawnp() return the process ID of the child process to the parent process in the variable pointed to by a non-null pid argument, and return zero as the function return value. Otherwise, no child process is created, the value stored into the variable pointed to by a non-null pid is unspecified, and an error number is 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.

The posix_spawn() and posix_spawnp() functions will fail if:

The value specified by file_actions or attrp is invalid.

If posix_spawn() or posix_spawnp() fails for any of the reasons that would cause fork() or one of the exec family of functions to fail, an error value is returned as described by fork(2) and exec(2), respectively.

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 is returned as described by setpgid(2).

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 is returned as described by sched_setparam(3C).

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 is returned as described by sched_setscheduler(3C).

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 is returned as described by close(2), dup2(3C), or open(2), respectively. An open file action might, by itself, result in any of the errors described by close() or dup2(), in addition to those described by open().

If a close(2) operation is specified to be performed for a file descriptor that is not open at the time of the call to posix_spawn() or posix_spawnp(), the action does not cause posix_spawn() or posix_spawnp() to fail.

alarm(2), chmod(2), close(2), dup(2), exec(2), exit(2), fcntl(2), fork(2), kill(2), open(2), setpgid(2), setsid(2), setuid(2), stat(2), times(2), dup2(3C), popen(3C), posix_spawn_file_actions_addchdir(3C), posix_spawn_file_actions_addclose(3C), posix_spawn_file_actions_adddup2(3C), posix_spawn_file_actions_addopen(3C), posix_spawn_file_actions_destroy(3C), posix_spawn_file_actions_init(3C), posix_spawn_pipe_np(3C), posix_spawnattr_destroy(3C), posix_spawnattr_getflags(3C), posix_spawnattr_getpgroup(3C), posix_spawnattr_getschedparam(3C), posix_spawnattr_getschedpolicy(3C), posix_spawnattr_getsigdefault(3C), posix_spawnattr_getsigignore_np(3C), posix_spawnattr_getsigmask(3C), posix_spawnattr_init(3C), posix_spawnattr_setflags(3C), posix_spawnattr_setpgroup(3C), posix_spawnattr_setschedparam(3C), posix_spawnattr_setschedpolicy(3C), posix_spawnattr_setsigdefault(3C), posix_spawnattr_setsigignore_np(3C), posix_spawnattr_setsigmask(3C), sched_setparam(3C), sched_setscheduler(3C), system(3C), wait(3C), attributes(7), standards(7)

The posix_spawn() and posix_spawnp() functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).

The POSIX standard permits the () and posix_spawnp() functions to return successfully before some of the above-described errors are detected, allowing the child process to fail instead:

... if the error occurs after the calling process
successfully returns, the child process exits with
exit status 127.

With the one exception of when the POSIX_SPAWN_NOEXECERR_NP flag is passed in the attributes structure, this behavior is not present in this implementation. Any error that occurs before the new process image is successfully constructed causes the () and posix_spawnp() functions to return the corresponding non-zero error value without creating a child process.

The POSIX_SPAWN_NOSIGCHLD_NP, POSIX_SPAWN_WAITPID_NP, POSIX_SPAWN_NOEXECERR_NP, and POSIX_SPAWN_SETSIGIGN_NP flags and the () and () functions are non-portable illumos extensions to the posix_spawn() and posix_spawnp() interfaces.

July 4, 2026 OmniOS