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POSIX_SPAWN(3) Linux Programmer's Manual POSIX_SPAWN(3)
posix_spawn, posix_spawnp - spawn a process
#include <spawn.h>
int posix_spawn(pid_t *pid, const char *path,
const posix_spawn_file_actions_t *file_actions,
const posix_spawnattr_t *attrp,
char *const argv[], char *const envp[]);
int posix_spawnp(pid_t *pid, const char *file,
const posix_spawn_file_actions_t *file_actions,
const posix_spawnattr_t *attrp,
char *const argv[], char *const envp[]);
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
functionality 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 using 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:
* The pid argument points to a buffer that is used to return the
process ID of the new child process.
* The file_actions argument points to a spawn file actions object
that specifies file-related actions to be performed in the child
between the fork(2) and exec(3) steps. This object is initialized
and populated before the posix_spawn() call using
posix_spawn_file_actions_init(3) and the
posix_spawn_file_actions_*() functions.
* The attrp argument points to an attributes objects that specifies
various attributes of the created child process. This object is
initialized and populated before the posix_spawn() call using
posix_spawnattr_init(3) and the posix_spawnattr_*() functions.
* The argv and envp arguments 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
The posix_spawn() function commences by calling 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.
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_RESETIDS.
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(2) and the exec(3), 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
executable file. These operations are controlled by the attributes
object pointed to by attrp and the file actions object pointed to by
file_actions. 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
object pointed to by attrp.
2. File actions, as specified in the file_actions argument, are
performed 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
attributes specified in the object pointed to by attrp and the file
actions 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_spawnattr_setflags(3)) controls the general actions that occur,
and other attributes in the object specify values to be used during
those actions.
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 treatment 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
attrp.
* 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 override 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_SETPGROUP flag is not set, the child inherits the
parent's process group ID.
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
described 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
requests 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_adddup2(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).
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 before or during the fork(2), then no child is created, the
contents of *pid are unspecified, and these functions return an error
number as described 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()
initialization. In addition, the exec(3) may fail. In all of these
cases, the child process will exit with the exit value of 127.
The posix_spawn() and posix_spawnp() functions fail only in the case
where the underlying fork(2) or vfork(2) call fails; in these cases,
these functions return an error number, which will be one of the
errors described for fork(2) or vfork(2).
In addition, these functions fail if:
ENOSYS Function not supported on this system.
The posix_spawn() and posix_spawnp() functions are available since
glibc 2.2.
POSIX.1-2001, POSIX.1-2008.
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 unspecified whether fork handlers established
with pthread_atfork(3) are called when posix_spawn() is invoked. On
glibc, 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
functionality can be obtained by specifying the path argument as one
of the files in the caller's /proc/self/fd directory.
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.
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
attributes 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
signal 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 <spawn.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <wait.h>
#include <errno.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: %ld\n", (long) 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);
}
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
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2016-10-08 POSIX_SPAWN(3)
Pages that refer to this page: vfork(2)