Tribblix: manual page: posix

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

NAME

posix_spawn, posix_spawnp - spawn a process

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 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 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 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 posix_spawn() is a pathname that identifies the
new process image file to execute.

The file parameter to posix_spawnp() 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
progress 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. 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 wait(), waitid(P_ALL), or
waitid(P_PGID), will succeed in reaping the child, and the child will
not be reaped automatically due 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 posix_spawn() 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 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.

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

RETURN VALUES

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.

ERRORS

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

EINVAL
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.

ATTRIBUTES

NOTES

The POSIX standard permits the posix_spawn() 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
the Solaris implementation. Any error that occurs before the new
process image is successfully constructed causes the posix_spawn()
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
posix_spawnattr_getsigignore_np() and
posix_spawnattr_setsigignore_np() functions are non-portable Solaris
extensions to the posix_spawn() and posix_spawnp() interfaces.

January 5, 2025 POSIX_SPAWN(3C)