posix_spawn(3) Library Functions Manual posix_spawn(3)
NAME
posix_spawn, posix_spawnp - spawn a process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <spawn.h>
int posix_spawn(pid_t *restrict pid, const char *restrict path,
const posix_spawn_file_actions_t *restrict 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 *restrict file_actions,
const posix_spawnattr_t *restrict attrp,
char *const argv[restrict],
char *const envp[restrict]);
DESCRIPTION
The posix_spawn() and posix_spawnp() functions are used to create a new
child process that executes a specified file. These functions were
specified by POSIX to provide a standardized method of creating new
processes on machines that lack the capability to support the fork(2)
system call. These machines are generally small, embedded systems
lacking MMU support.
The posix_spawn() and posix_spawnp() functions provide the functional-
ity of a combined fork(2) and exec(3), with some optional housekeeping
steps in the child process before the exec(3). These functions are not
meant to replace the fork(2) and execve(2) system calls. In fact, they
provide only a subset of the functionality that can be achieved by us-
ing the system calls.
The only difference between posix_spawn() and posix_spawnp() is the
manner in which they specify the file to be executed by the child
process. With posix_spawn(), the executable file is specified as a
pathname (which can be absolute or relative). With posix_spawnp(), the
executable file is specified as a simple filename; the system searches
for this file in the list of directories specified by PATH (in the same
way as for execvp(3)). For the remainder of this page, the discussion
is phrased in terms of posix_spawn(), with the understanding that
posix_spawnp() differs only on the point just described.
The remaining arguments to these two functions are as follows:
pid points to a buffer that is used to return the process ID of the
new child process.
file_actions
points to a spawn file actions object that specifies file-re-
lated actions to be performed in the child between the fork(2)
and exec(3) steps. This object is initialized and populated be-
fore the posix_spawn() call using posix_spawn_file_ac-
tions_init(3) and the posix_spawn_file_actions_*() functions.
attrp points to an attributes objects that specifies various at-
tributes of the created child process. This object is initial-
ized and populated before the posix_spawn() call using
posix_spawnattr_init(3) and the posix_spawnattr_*() functions.
argv
envp specify the argument list and environment for the program that
is executed in the child process, as for execve(2).
Below, the functions are described in terms of a three-step process:
the fork() step, the pre-exec() step (executed in the child), and the
exec() step (executed in the child).
fork() step
Since glibc 2.24, the posix_spawn() function commences by calling
clone(2) with CLONE_VM and CLONE_VFORK flags. Older implementations
use fork(2), or possibly vfork(2) (see below).
The PID of the new child process is placed in *pid. The posix_spawn()
function then returns control to the parent process.
Subsequently, the parent can use one of the system calls described in
wait(2) to check the status of the child process. If the child fails
in any of the housekeeping steps described below, or fails to execute
the desired file, it exits with a status of 127.
Before glibc 2.24, the child process is created using vfork(2) instead
of fork(2) when either of the following is true:
• the spawn-flags element of the attributes object pointed to by attrp
contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or
• file_actions is NULL and the spawn-flags element of the attributes
object pointed to by attrp does not contain POSIX_SPAWN_SETSIGMASK,
POSIX_SPAWN_SETSIGDEF, POSIX_SPAWN_SETSCHEDPARAM,
POSIX_SPAWN_SETSCHEDULER, POSIX_SPAWN_SETPGROUP, or POSIX_SPAWN_RE-
SETIDS.
In other words, vfork(2) is used if the caller requests it, or if there
is no cleanup expected in the child before it exec(3)s the requested
file.
pre-exec() step: housekeeping
In between the fork() and the exec() steps, a child process may need to
perform a set of housekeeping actions. The posix_spawn() and
posix_spawnp() functions support a small, well-defined set of system
tasks that the child process can accomplish before it executes the exe-
cutable file. These operations are controlled by the attributes object
pointed to by attrp and the file actions object pointed to by file_ac-
tions. In the child, processing is done in the following sequence:
(1) Process attribute actions: signal mask, signal default handlers,
scheduling algorithm and parameters, process group, and effective
user and group IDs are changed as specified by the attributes ob-
ject pointed to by attrp.
(2) File actions, as specified in the file_actions argument, are per-
formed in the order that they were specified using calls to the
posix_spawn_file_actions_add*() functions.
(3) File descriptors with the FD_CLOEXEC flag set are closed.
All process attributes in the child, other than those affected by at-
tributes specified in the object pointed to by attrp and the file ac-
tions in the object pointed to by file_actions, will be affected as
though the child was created with fork(2) and it executed the program
with execve(2).
The process attributes actions are defined by the attributes object
pointed to by attrp. The spawn-flags attribute (set using posix_spaw-
nattr_setflags(3)) controls the general actions that occur, and other
attributes in the object specify values to be used during those ac-
tions.
The effects of the flags that may be specified in spawn-flags are as
follows:
POSIX_SPAWN_SETSIGMASK
Set the signal mask to the signal set specified in the spawn-
sigmask attribute of the object pointed to by attrp. If the
POSIX_SPAWN_SETSIGMASK flag is not set, then the child inherits
the parent's signal mask.
POSIX_SPAWN_SETSIGDEF
Reset the disposition of all signals in the set specified in the
spawn-sigdefault attribute of the object pointed to by attrp to
the default. For the treatment of the dispositions of signals
not specified in the spawn-sigdefault attribute, or the treat-
ment when POSIX_SPAWN_SETSIGDEF is not specified, see execve(2).
POSIX_SPAWN_SETSCHEDPARAM
If this flag is set, and the POSIX_SPAWN_SETSCHEDULER flag is
not set, then set the scheduling parameters to the parameters
specified in the spawn-schedparam attribute of the object
pointed to by attrp.
POSIX_SPAWN_SETSCHEDULER
Set the scheduling policy algorithm and parameters of the child,
as follows:
• The scheduling policy is set to the value specified in the
spawn-schedpolicy attribute of the object pointed to by at-
trp.
• The scheduling parameters are set to the value specified in
the spawn-schedparam attribute of the object pointed to by
attrp (but see BUGS).
If the POSIX_SPAWN_SETSCHEDPARAM and POSIX_SPAWN_SETSCHEDPOLICY
flags are not specified, the child inherits the corresponding
scheduling attributes from the parent.
POSIX_SPAWN_RESETIDS
If this flag is set, reset the effective UID and GID to the real
UID and GID of the parent process. If this flag is not set,
then the child retains the effective UID and GID of the parent.
In either case, if the set-user-ID and set-group-ID permission
bits are enabled on the executable file, their effect will over-
ride the setting of the effective UID and GID (se execve(2)).
POSIX_SPAWN_SETPGROUP
Set the process group to the value specified in the spawn-pgroup
attribute of the object pointed to by attrp. If the spawn-
pgroup attribute has the value 0, the child's process group ID
is made the same as its process ID. If the POSIX_SPAWN_SETP-
GROUP flag is not set, the child inherits the parent's process
group ID.
POSIX_SPAWN_USEVFORK
Since glibc 2.24, this flag has no effect. On older implementa-
tions, setting this flag forces the fork() step to use vfork(2)
instead of fork(2). The _GNU_SOURCE feature test macro must be
defined to obtain the definition of this constant.
POSIX_SPAWN_SETSID (since glibc 2.26)
If this flag is set, the child process shall create a new ses-
sion and become the session leader. The child process shall
also become the process group leader of the new process group in
the session (see setsid(2)). The _GNU_SOURCE feature test macro
must be defined to obtain the definition of this constant.
If attrp is NULL, then the default behaviors described above for each
flag apply.
The file_actions argument specifies a sequence of file operations that
are performed in the child process after the general processing de-
scribed above, and before it performs the exec(3). If file_actions is
NULL, then no special action is taken, and standard exec(3) semantics
apply—file descriptors open before the exec remain open in the new
process, except those for which the FD_CLOEXEC flag has been set. File
locks remain in place.
If file_actions is not NULL, then it contains an ordered set of re-
quests to open(2), close(2), and dup2(2) files. These requests are
added to the file_actions by posix_spawn_file_actions_addopen(3),
posix_spawn_file_actions_addclose(3), and posix_spawn_file_actions_ad-
ddup2(3). The requested operations are performed in the order they
were added to file_actions.
If any of the housekeeping actions fails (due to bogus values being
passed or other reasons why signal handling, process scheduling,
process group ID functions, and file descriptor operations might fail),
the child process exits with exit value 127.
exec() step
Once the child has successfully forked and performed all requested pre-
exec steps, the child runs the requested executable.
The child process takes its environment from the envp argument, which
is interpreted as if it had been passed to execve(2). The arguments to
the created process come from the argv argument, which is processed as
for execve(2).
RETURN VALUE
Upon successful completion, posix_spawn() and posix_spawnp() place the
PID of the child process in pid, and return 0. If there is an error
during the fork() step, then no child is created, the contents of *pid
are unspecified, and these functions return an error number as de-
scribed below.
Even when these functions return a success status, the child process
may still fail for a plethora of reasons related to its pre-exec() ini-
tialization. In addition, the exec(3) may fail. In all of these
cases, the child process will exit with the exit value of 127.
ERRORS
The posix_spawn() and posix_spawnp() functions fail only in the case
where the underlying fork(2), vfork(2), or clone(2) call fails; in
these cases, these functions return an error number, which will be one
of the errors described for fork(2), vfork(2), or clone(2).
In addition, these functions fail if:
ENOSYS Function not supported on this system.
VERSIONS
The posix_spawn() and posix_spawnp() functions are available since
glibc 2.2.
STANDARDS
POSIX.1-2001, POSIX.1-2008.
NOTES
The housekeeping activities in the child are controlled by the objects
pointed to by attrp (for non-file actions) and file_actions In POSIX
parlance, the posix_spawnattr_t and posix_spawn_file_actions_t data
types are referred to as objects, and their elements are not specified
by name. Portable programs should initialize these objects using only
the POSIX-specified functions. (In other words, although these objects
may be implemented as structures containing fields, portable programs
must avoid dependence on such implementation details.)
According to POSIX, it is unspecified whether fork handlers established
with pthread_atfork(3) are called when posix_spawn() is invoked. Since
glibc 2.24, the fork handlers are not executed in any case. On older
implementations, fork handlers are called only if the child is created
using fork(2).
There is no "posix_fspawn" function (i.e., a function that is to
posix_spawn() as fexecve(3) is to execve(2)). However, this function-
ality can be obtained by specifying the path argument as one of the
files in the caller's /proc/self/fd directory.
BUGS
POSIX.1 says that when POSIX_SPAWN_SETSCHEDULER is specified in spawn-
flags, then the POSIX_SPAWN_SETSCHEDPARAM (if present) is ignored.
However, before glibc 2.14, calls to posix_spawn() failed with an error
if POSIX_SPAWN_SETSCHEDULER was specified without also specifying
POSIX_SPAWN_SETSCHEDPARAM.
EXAMPLES
The program below demonstrates the use of various functions in the
POSIX spawn API. The program accepts command-line attributes that can
be used to create file actions and attributes objects. The remaining
command-line arguments are used as the executable name and command-line
arguments of the program that is executed in the child.
In the first run, the date(1) command is executed in the child, and the
posix_spawn() call employs no file actions or attributes objects.
$ ./a.out date
PID of child: 7634
Tue Feb 1 19:47:50 CEST 2011
Child status: exited, status=0
In the next run, the -c command-line option is used to create a file
actions object that closes standard output in the child. Consequently,
date(1) fails when trying to perform output and exits with a status of
1.
$ ./a.out -c date
PID of child: 7636
date: write error: Bad file descriptor
Child status: exited, status=1
In the next run, the -s command-line option is used to create an at-
tributes object that specifies that all (blockable) signals in the
child should be blocked. Consequently, trying to kill child with the
default signal sent by kill(1) (i.e., SIGTERM) fails, because that sig-
nal is blocked. Therefore, to kill the child, SIGKILL is necessary
(SIGKILL can't be blocked).
$ ./a.out -s sleep 60 &
[1] 7637
$ PID of child: 7638
$ kill 7638
$ kill -KILL 7638
$ Child status: killed by signal 9
[1]+ Done ./a.out -s sleep 60
When we try to execute a nonexistent command in the child, the exec(3)
fails and the child exits with a status of 127.
$ ./a.out xxxxx
PID of child: 10190
Child status: exited, status=127
Program source
#include <errno.h>
#include <spawn.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <wait.h>
#define errExit(msg) do { perror(msg); \
exit(EXIT_FAILURE); } while (0)
#define errExitEN(en, msg) \
do { errno = en; perror(msg); \
exit(EXIT_FAILURE); } while (0)
char **environ;
int
main(int argc, char *argv[])
{
pid_t child_pid;
int s, opt, status;
sigset_t mask;
posix_spawnattr_t attr;
posix_spawnattr_t *attrp;
posix_spawn_file_actions_t file_actions;
posix_spawn_file_actions_t *file_actionsp;
/* Parse command-line options, which can be used to specify an
attributes object and file actions object for the child. */
attrp = NULL;
file_actionsp = NULL;
while ((opt = getopt(argc, argv, "sc")) != -1) {
switch (opt) {
case 'c': /* -c: close standard output in child */
/* Create a file actions object and add a "close"
action to it. */
s = posix_spawn_file_actions_init(&file_actions);
if (s != 0)
errExitEN(s, "posix_spawn_file_actions_init");
s = posix_spawn_file_actions_addclose(&file_actions,
STDOUT_FILENO);
if (s != 0)
errExitEN(s, "posix_spawn_file_actions_addclose");
file_actionsp = &file_actions;
break;
case 's': /* -s: block all signals in child */
/* Create an attributes object and add a "set signal mask"
action to it. */
s = posix_spawnattr_init(&attr);
if (s != 0)
errExitEN(s, "posix_spawnattr_init");
s = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
if (s != 0)
errExitEN(s, "posix_spawnattr_setflags");
sigfillset(&mask);
s = posix_spawnattr_setsigmask(&attr, &mask);
if (s != 0)
errExitEN(s, "posix_spawnattr_setsigmask");
attrp = &attr;
break;
}
}
/* Spawn the child. The name of the program to execute and the
command-line arguments are taken from the command-line arguments
of this program. The environment of the program execed in the
child is made the same as the parent's environment. */
s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,
&argv[optind], environ);
if (s != 0)
errExitEN(s, "posix_spawn");
/* Destroy any objects that we created earlier. */
if (attrp != NULL) {
s = posix_spawnattr_destroy(attrp);
if (s != 0)
errExitEN(s, "posix_spawnattr_destroy");
}
if (file_actionsp != NULL) {
s = posix_spawn_file_actions_destroy(file_actionsp);
if (s != 0)
errExitEN(s, "posix_spawn_file_actions_destroy");
}
printf("PID of child: %jd\n", (intmax_t) child_pid);
/* Monitor status of the child until it terminates. */
do {
s = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
if (s == -1)
errExit("waitpid");
printf("Child status: ");
if (WIFEXITED(status)) {
printf("exited, status=%d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("killed by signal %d\n", WTERMSIG(status));
} else if (WIFSTOPPED(status)) {
printf("stopped by signal %d\n", WSTOPSIG(status));
} else if (WIFCONTINUED(status)) {
printf("continued\n");
}
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
exit(EXIT_SUCCESS);
}
SEE ALSO
close(2), dup2(2), execl(2), execlp(2), fork(2), open(2),
sched_setparam(2), sched_setscheduler(2), setpgid(2), setuid(2),
sigaction(2), sigprocmask(2), posix_spawn_file_actions_addclose(3),
posix_spawn_file_actions_adddup2(3),
posix_spawn_file_actions_addopen(3),
posix_spawn_file_actions_destroy(3), posix_spawn_file_actions_init(3),
posix_spawnattr_destroy(3), posix_spawnattr_getflags(3),
posix_spawnattr_getpgroup(3), posix_spawnattr_getschedparam(3),
posix_spawnattr_getschedpolicy(3), posix_spawnattr_getsigdefault(3),
posix_spawnattr_getsigmask(3), posix_spawnattr_init(3),
posix_spawnattr_setflags(3), posix_spawnattr_setpgroup(3),
posix_spawnattr_setschedparam(3), posix_spawnattr_setschedpolicy(3),
posix_spawnattr_setsigdefault(3), posix_spawnattr_setsigmask(3),
pthread_atfork(3), <spawn.h>, Base Definitions volume of POSIX.1-2001,
http://www.opengroup.org/unix/online.html
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