man3/posix_spawn.3

.\" Copyright (c) 2009 Bill O. Gallmeister (bgallmeister@gmail.com)
.\" and Copyright 2010 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" References consulted:
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.TH POSIX_SPAWN 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
posix_spawn, posix_spawnp \- spawn a process
.SH SYNOPSIS
.nf
.B #include <spawn.h>
.PP
.BI "int posix_spawn(pid_t *" pid ", const char *" path ,
.BI "                const posix_spawn_file_actions_t *" file_actions ,
.BI "                const posix_spawnattr_t *" attrp ,
.BI "                char *const " argv[] ", char *const " envp[] );
.PP
.BI "int posix_spawnp(pid_t *" pid ", const char *" file ,
.BI "                const posix_spawn_file_actions_t *" file_actions ,
.BI "                const posix_spawnattr_t *" attrp ,
.BI "                char *const " argv[] ", char *const " envp[] );
.fi
.SH DESCRIPTION
The
.BR posix_spawn ()
and
.BR 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
.BR fork (2)
system call.
These machines are generally small, embedded systems lacking MMU support.
.PP
The
.BR posix_spawn ()
and
.BR posix_spawnp ()
functions provide the functionality of a combined
.BR fork (2)
and
.BR exec (3),
with some optional housekeeping steps in the child process before the
.BR exec (3).
These functions are not meant to replace the
.BR fork (2)
and
.BR execve (2)
system calls.
In fact, they provide only a subset of the functionality
that can be achieved by using the system calls.
.PP
The only difference between
.BR posix_spawn ()
and
.BR posix_spawnp ()
is the manner in which they specify the file to be executed by
the child process.
With
.BR posix_spawn (),
the executable file is specified as a pathname
(which can be absolute or relative).
With
.BR posix_spawnp (),
the executable file is specified as a simple filename;
the system searches for this file in the list of directories specified by
.BR PATH
(in the same way as for
.BR execvp (3)).
For the remainder of this page, the discussion is phrased in terms of
.BR posix_spawn (),
with the understanding that
.BR posix_spawnp ()
differs only on the point just described.
.PP
The remaining arguments to these two functions are as follows:
.IP * 3
The
.I pid
argument points to a buffer that is used to return the process ID
of the new child process.
.IP *
The
.I file_actions
argument points to a
.I "spawn file actions object"
that specifies file-related actions to be performed in the child
between the
.BR fork (2)
and
.BR exec (3)
steps.
This object is initialized and populated before the
.BR posix_spawn ()
call using
.BR posix_spawn_file_actions_init (3)
and the
.BR posix_spawn_file_actions_* ()
functions.
.IP *
The
.I attrp
argument points to an
.I attributes objects
that specifies various attributes of the created child process.
This object is initialized and populated before the
.BR posix_spawn ()
call using
.BR posix_spawnattr_init (3)
and the
.BR posix_spawnattr_* ()
functions.
.IP *
The
.I argv
and
.I envp
arguments specify the argument list and environment for the program
that is executed in the child process, as for
.BR execve (2).
.PP
Below, the functions are described in terms of a three-step process: the
.BR fork ()
step, the
.RB pre- exec ()
step (executed in the child),
and the
.BR exec ()
step (executed in the child).
.SS fork() step
The
.BR posix_spawn ()
function commences by calling
.BR fork (2),
or possibly
.BR vfork (2)
(see below).
.PP
The PID of the new child process is placed in
.IR *pid .
The
.BR posix_spawn ()
function then returns control to the parent process.
.PP
Subsequently, the parent can use one of the system calls described in
.BR 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.
.PP
The child process is created using
.BR vfork (2)
instead of
.BR fork (2)
when either of the following is true:
.IP * 3
the
.I spawn-flags
element of the attributes object pointed to by
.I attrp
contains the GNU-specific flag
.BR POSIX_SPAWN_USEVFORK ;
or
.IP *
.I file_actions
is NULL and the
.I spawn-flags
element of the attributes object pointed to by
.I attrp
does \fInot\fP contain
.BR POSIX_SPAWN_SETSIGMASK ,
.BR POSIX_SPAWN_SETSIGDEF ,
.BR POSIX_SPAWN_SETSCHEDPARAM ,
.BR POSIX_SPAWN_SETSCHEDULER ,
.BR POSIX_SPAWN_SETPGROUP ,
or
.BR POSIX_SPAWN_RESETIDS .
.PP
In other words,
.BR vfork (2)
is used if the caller requests it,
or if there is no cleanup expected in the child before it
.BR exec (3)s
the requested file.
.PP
.SS pre-exec() step: housekeeping
In between the
.BR fork (2)
and the
.BR exec (3),
a child process may need to perform a set of housekeeping actions.
The
.BR posix_spawn ()
and
.BR 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
.IR attrp
and the file actions object pointed to by
.IR file_actions .
In the child, processing is done in the following sequence:
.IP 1. 3
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
.IR attrp .
.IP 2.
File actions, as specified in the
.I file_actions
argument,
are performed in the order that they were specified using calls to the
.BR posix_spawn_file_actions_add* ()
functions.
.IP 3.
File descriptors with the
.B FD_CLOEXEC
flag set are closed.
.PP
All process attributes in the child,
other than those affected by attributes specified in the
object pointed to by
.IR attrp
and the file actions in the object pointed to by
.IR file_actions ,
will be affected as though the child was created with
.BR fork (2)
and it executed the program with
.BR execve (2).
.PP
The process attributes actions are defined by the attributes object
pointed to by
.IR attrp .
The
.I spawn-flags
attribute (set using
.BR posix_spawnattr_setflags (3))
controls the general actions that occur,
and other attributes in the object specify values
to be used during those actions.
.PP
The effects of the flags that may be specified in
.IR spawn-flags
are as follows:
.TP 8
.B POSIX_SPAWN_SETSIGMASK
Set the signal mask to the signal set specified in the
.I spawn-sigmask
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setsigmask (3))
of the object pointed to by
.IR attrp .
If the
.B POSIX_SPAWN_SETSIGMASK
flag is not set, then the child inherits the parent's signal mask.
.TP
.B POSIX_SPAWN_SETSIGDEF
Reset the disposition of all signals in the set specified in the
.I spawn-sigdefault
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setsigdefault (3))
of the object pointed to by
.IR attrp
to the default.
For the treatment of the dispositions of signals not specified in the
.I spawn-sigdefault
attribute, or the treatment when
.B POSIX_SPAWN_SETSIGDEF
is not specified, see
.BR execve (2).
.TP
.B POSIX_SPAWN_SETSCHEDPARAM
.\" (POSIX_PRIORITY_SCHEDULING only)
If this flag is set, and the
.B POSIX_SPAWN_SETSCHEDULER
flag is not set, then set the scheduling parameters
to the parameters specified in the
.I spawn-schedparam
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setschedparam (3))
of the object pointed to by
.IR attrp .
.TP
.B POSIX_SPAWN_SETSCHEDULER
Set the scheduling policy algorithm and parameters of the child,
as follows:
.RS
.IP * 3
The scheduling policy is set to the value specified in the
.I spawn-schedpolicy
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setpolicy (3))
of the object pointed to by
.IR attrp .
.IP *
The scheduling parameters are set to the value specified in the
.I spawn-schedparam
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setschedparam (3))
of the object pointed to by
.IR attrp
(but see BUGS).
.PP
If the
.B POSIX_SPAWN_SETSCHEDPARAM
and
.B POSIX_SPAWN_SETSCHEDPOLICY
flags are not specified,
the child inherits the corresponding scheduling attributes from the parent.
.RE
.TP
.B 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
.BR execve (2)).
.TP
.B POSIX_SPAWN_SETPGROUP
Set the process group to the value specified in the
.I spawn-pgroup
attribute
.\" FIXME .
.\" (see
.\" .BR posix_spawnattr_setpgroup (3))
of the object pointed to by
.IR attrp .
If the
.I spawn-pgroup
attribute has the value 0,
the child's process group ID is made the same as its process ID.
If the
.B POSIX_SPAWN_SETPGROUP
flag is not set, the child inherits the parent's process group ID.
.PP
If
.I attrp
is NULL, then the default behaviors described above for each flag apply.
.\" mtk: I think we probably don't want to say the following, since it
.\"      could lead people to do the wrong thing
.\" The POSIX standard tells you to call
.\" this function to de-initialize the attributes object pointed to by
.\" .I attrp
.\" when you are done with it;
.\" however, on Linux systems this operation is a no-op.
.PP
The
.I 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
.BR exec (3).
If
.I file_actions
is NULL, then no special action is taken, and standard
.BR exec (3)
semantics apply--file descriptors open before the exec
remain open in the new process,
except those for which the
.B FD_CLOEXEC
flag has been set.
File locks remain in place.
.PP
If
.I file_actions
is not NULL, then it contains an ordered set of requests to
.BR open (2),
.BR close (2),
and
.BR dup2 (2)
files.
These requests are added to the
.I file_actions
by
.BR posix_spawn_file_actions_addopen (3),
.BR posix_spawn_file_actions_addclose (3),
and
.BR posix_spawn_file_actions_adddup2 (3).
The requested operations are performed in the order they were added to
.IR file_actions .
.\" FIXME . I think the following is best placed in the
.\" posix_spawn_file_actions_adddup2(3) page, and a similar statement is
.\" also needed in posix_spawn_file_actions_addclose(3)
.\" Note that you can specify file descriptors in
.\" .I posix_spawn_file_actions_adddup2 (3)
.\" which would not be usable if you called
.\" .BR dup2 (2)
.\" at that time--i.e., file descriptors that are opened or
.\" closed by the earlier operations
.\" added to
.\" .I file_actions .
.PP
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.
.SS exec() step
Once the child has successfully forked and performed
all requested pre-exec steps,
the child runs the requested executable.
.PP
The child process takes its environment from the
.I envp
argument, which is interpreted as if it had been passed to
.BR execve (2).
The arguments to the created process come from the
.I argv
argument, which is processed as for
.BR execve (2).
.SH RETURN VALUE
Upon successful completion,
.BR posix_spawn ()
and
.BR posix_spawnp ()
place the PID of the child process in
.IR pid ,
and return 0.
If there is an error before or during the
.BR fork (2),
then no child is created,
the contents of
.IR *pid
are unspecified,
and these functions return an error number as described below.
.PP
Even when these functions return a success status,
the child process may still fail for a plethora of reasons related to its
pre-\fBexec\fR() initialization.
In addition, the
.BR exec (3)
may fail.
In all of these cases, the child process will exit with the exit value of 127.
.SH ERRORS
The
.BR posix_spawn ()
and
.BR posix_spawnp ()
functions fail only in the case where the underlying
.BR fork (2)
or
.BR vfork (2)
call fails;  in these cases, these functions return an error number,
which will be one of the errors described for
.BR fork (2)
or
.BR vfork (2).
.PP
In addition, these functions fail if:
.TP
.B ENOSYS
Function not supported on this system.
.SH VERSIONS
The
.BR posix_spawn ()
and
.BR posix_spawnp ()
functions are available since glibc 2.2.
.SH CONFORMING TO
.PP
POSIX.1-2001, POSIX.1-2008.
.\" FIXME . This piece belongs in spawnattr_setflags(3)
.\" The
.\" .B POSIX_SPAWN_USEVFORK
.\" flag is a GNU extension; the
.\" .B _GNU_SOURCE
.\" feature test macro must be defined (before including any header files)
.\" to obtain the definition of this constant.
.SH NOTES
The housekeeping activities in the child are controlled by
the objects pointed to by
.I attrp
(for non-file actions) and
.I file_actions
In POSIX parlance, the
.I posix_spawnattr_t
and
.I 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.)
.PP
According to POSIX, it unspecified whether fork handlers established with
.BR pthread_atfork (3)
are called when
.BR posix_spawn ()
is invoked.
On glibc,
.\" Tested on glibc 2.12
fork handlers are called only if the child is created using
.BR fork (2).
.PP
There is no "posix_fspawn" function (i.e., a function that is to
.BR posix_spawn ()
as
.BR fexecve (3)
is to
.BR execve (2)).
However, this functionality can be obtained by specifying the
.I path
argument as one of the files in the caller's
.IR /proc/self/fd
directory.
.SH BUGS
POSIX.1 says that when
.B POSIX_SPAWN_SETSCHEDULER
is specified in
.IR spawn-flags ,
then the
.B POSIX_SPAWN_SETSCHEDPARAM
(if present) is ignored.
However, before glibc 2.14, calls to
.BR posix_spawn ()
failed with an error if
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=12052
.BR POSIX_SPAWN_SETSCHEDULER
was specified without also specifying
.BR POSIX_SPAWN_SETSCHEDPARAM .
.SH EXAMPLE
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.
.PP
In the first run, the
.BR date (1)
command is executed in the child, and the
.BR posix_spawn ()
call employs no file actions or attributes objects.
.PP
.in +4
.EX
$ \fB./a.out date\fP
PID of child: 7634
Tue Feb  1 19:47:50 CEST 2011
Child status: exited, status=0
.EE
.in
.PP
In the next run, the
.I \-c
command-line option is used to create a file actions object that closes
standard output in the child.
Consequently,
.BR date (1)
fails when trying to perform output and exits with a status of 1.
.PP
.in +4
.EX
$ \fB./a.out -c date\fP
PID of child: 7636
date: write error: Bad file descriptor
Child status: exited, status=1
.EE
.in
.PP
In the next run, the
.I \-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
.BR kill (1)
(i.e.,
.BR SIGTERM )
fails, because that signal is blocked.
Therefore, to kill the child,
.BR SIGKILL
is necessary
.RB ( SIGKILL
can't be blocked).
.PP
.in +4
.EX
$ \fB./a.out -s sleep 60 &\fP
[1] 7637
$ PID of child: 7638

$ \fBkill 7638\fP
$ \fBkill -KILL 7638\fP
$ Child status: killed by signal 9
[1]+  Done                    ./a.out -s sleep 60
.EE
.in
.PP
When we try to execute a nonexistent command in the child, the
.BR exec (3)
fails and the child exits with a status of 127.
.PP
.in +4
.EX
$ \fB./a.out xxxxx
PID of child: 10190
Child status: exited, status=127
.EE
.in
.SS Program source
\&
.EX
#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); \e
                             exit(EXIT_FAILURE); } while (0)

#define errExitEN(en, msg) \e
                        do { errno = en; perror(msg); \e
                             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 \(aqc\(aq:       /* \-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 \(aqs\(aq:       /* \-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\(aqs 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\en", (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\en", WEXITSTATUS(status));
        } else if (WIFSIGNALED(status)) {
            printf("killed by signal %d\en", WTERMSIG(status));
        } else if (WIFSTOPPED(status)) {
            printf("stopped by signal %d\en", WSTOPSIG(status));
        } else if (WIFCONTINUED(status)) {
            printf("continued\en");
        }
    } while (!WIFEXITED(status) && !WIFSIGNALED(status));

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.nh \" Disable hyphenation
.ad l
.BR close (2),
.BR dup2 (2),
.BR execl (2),
.BR execlp (2),
.BR fork (2),
.BR open (2),
.BR sched_setparam (2),
.BR sched_setscheduler (2),
.BR setpgid (2),
.BR setuid (2),
.BR sigaction (2),
.BR sigprocmask (2),
.BR posix_spawn_file_actions_addclose (3),
.BR posix_spawn_file_actions_adddup2 (3),
.BR posix_spawn_file_actions_addopen (3),
.BR posix_spawn_file_actions_destroy (3),
.BR posix_spawn_file_actions_init (3),
.BR posix_spawnattr_destroy (3),
.BR posix_spawnattr_getflags (3),
.BR posix_spawnattr_getpgroup (3),
.BR posix_spawnattr_getschedparam (3),
.BR posix_spawnattr_getschedpolicy (3),
.BR posix_spawnattr_getsigdefault (3),
.BR posix_spawnattr_getsigmask (3),
.BR posix_spawnattr_init (3),
.BR posix_spawnattr_setflags (3),
.BR posix_spawnattr_setpgroup (3),
.BR posix_spawnattr_setschedparam (3),
.BR posix_spawnattr_setschedpolicy (3),
.BR posix_spawnattr_setsigdefault (3),
.BR posix_spawnattr_setsigmask (3),
.BR pthread_atfork (3),
.IR <spawn.h> ,
Base Definitions volume of POSIX.1-2001,
.I http://www.opengroup.org/unix/online.html