libc/gen/posix_spawn.3

.\" Copyright (c) 2008 Ed Schouten <ed@FreeBSD.org>
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.\" Portions of this text are reprinted and reproduced in electronic form
.\" from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
.\" Portable Operating System Interface (POSIX), The Open Group Base
.\" Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
.\" Electrical and Electronics Engineers, Inc and The Open Group.  In the
.\" event of any discrepancy between this version and the original IEEE and
.\" The Open Group Standard, the original IEEE and The Open Group Standard is
.\" the referee document.  The original Standard can be obtained online at
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.Dd November 28, 2021
.Dt POSIX_SPAWN 3
.Os
.Sh NAME
.Nm posix_spawn ,
.Nm posix_spawnp
.Nd "spawn a process"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In spawn.h
.Ft int
.Fo posix_spawn
.Fa "pid_t *restrict pid"
.Fa "const char *restrict path"
.Fa "const posix_spawn_file_actions_t *file_actions"
.Fa "const posix_spawnattr_t *restrict attrp"
.Fa "char *const argv[restrict]"
.Fa "char *const envp[restrict]"
.Fc
.Ft int
.Fo posix_spawnp
.Fa "pid_t *restrict pid"
.Fa "const char *restrict file"
.Fa "const posix_spawn_file_actions_t *file_actions"
.Fa "const posix_spawnattr_t *restrict attrp"
.Fa "char *const argv[restrict]"
.Fa "char *const envp[restrict]"
.Fc
.Sh DESCRIPTION
The
.Fn posix_spawn
and
.Fn 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.
.Pp
When a C program is executed as the result of this call, it is
entered as a C-language function call as follows:
.Bd -literal -offset indent
int main(int argc, char *argv[]);
.Ed
.Pp
where
.Fa argc
is the argument count and
.Fa argv
is an array of character pointers to the arguments themselves.
In addition, the variable:
.Bd -literal -offset indent
extern char **environ;
.Ed
.Pp
points to an array of character pointers to
the environment strings.
.Pp
The argument
.Fa 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
.Fa argc .
These strings constitute the argument list available to the new process
image.
The value in
.Fa argv Ns [0]
should point to
a filename that is associated with the process image being started by
the
.Fn posix_spawn
or
.Fn posix_spawnp
function.
.Pp
The argument
.Fa 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.
.Pp
The
.Fa path
argument to
.Fn posix_spawn
is a pathname that identifies the new process image file to execute.
.Pp
The
.Fa file
parameter to
.Fn 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
.Dq Ev PATH .
If this variable is not specified,
the default path is set according to the
.Dv _PATH_DEFPATH
definition in
.In paths.h ,
which is set to
.Dq Ev /sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin .
.Pp
If
.Fa 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
.Dv FD_CLOEXEC
is set (see
.Fn fcntl ) .
For those
file descriptors that remain open, all attributes of the corresponding
open file descriptions, including file locks (see
.Fn fcntl ) ,
remain unchanged.
.Pp
If
.Fa 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
.Fa file_actions
and the
.Dv 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:
.Bl -enum
.It
The set of open file descriptors for the child process initially
are the same set as is open for the calling process.
All attributes of the corresponding open file descriptions, including
file locks (see
.Fn fcntl ) ,
remain unchanged.
.It
The signal mask, signal default actions, and the effective user and
group IDs for the child process are changed as specified in the
attributes object referenced by
.Fa attrp .
.It
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.
.It
Any file descriptor that has its
.Dv FD_CLOEXEC
flag set (see
.Fn fcntl )
is closed.
.El
.Pp
The
.Vt posix_spawnattr_t
spawn attributes object type is defined in
.In spawn.h .
It contains the attributes defined below.
.Pp
If the
.Dv POSIX_SPAWN_SETPGROUP
flag is set in the spawn-flags attribute of the object referenced by
.Fa 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
.Fa attrp .
.Pp
As a special case, if the
.Dv POSIX_SPAWN_SETPGROUP
flag is set in the spawn-flags attribute of the object referenced by
.Fa attrp ,
and the spawn-pgroup attribute of the same object is set to zero, then
the child is in a new process group with a process group ID equal
to its process ID.
.Pp
If the
.Dv POSIX_SPAWN_SETPGROUP
flag is not set in the spawn-flags attribute of the object referenced by
.Fa attrp ,
the new child process inherits the parent's process group.
.Pp
If the
.Dv POSIX_SPAWN_SETSCHEDPARAM
flag is set in the spawn-flags attribute of the object referenced by
.Fa attrp ,
but
.Dv 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
.Fa attrp .
.Pp
If the
.Dv POSIX_SPAWN_SETSCHEDULER
flag is set in the spawn-flags attribute of the object referenced by
.Fa attrp
(regardless of the setting of the
.Dv POSIX_SPAWN_SETSCHEDPARAM
flag), the new process image initially has the scheduling policy
specified in the spawn-schedpolicy attribute of the object referenced by
.Fa attrp
and the scheduling parameters specified in the spawn-schedparam
attribute of the same object.
.Pp
The
.Dv POSIX_SPAWN_RESETIDS
flag in the spawn-flags attribute of the object referenced by
.Fa attrp
governs the effective user ID of the child process.
If this flag is not set, the child process inherits the parent
process' effective user ID.
If this flag is set, the child process' 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.
.Pp
The
.Dv POSIX_SPAWN_RESETIDS
flag in the spawn-flags attribute of the object referenced by
.Fa attrp
also governs the effective group ID of the child process.
If this flag is not set, the child process inherits the parent
process' effective group ID.
If this flag is set, the child process' 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.
.Pp
If the
.Dv POSIX_SPAWN_SETSIGMASK
flag is set in the spawn-flags attribute of the object referenced by
.Fa attrp ,
the child process initially has the signal mask specified in the
spawn-sigmask attribute of the object referenced by
.Fa attrp .
.Pp
If the
.Dv POSIX_SPAWN_SETSIGDEF
flag is set in the spawn-flags attribute of the object referenced by
.Fa attrp ,
the signals specified in the spawn-sigdefault attribute of the same
object are set to their default actions in the child process.
Signals set to the default action in the parent process are set to
the default action in the child process.
.Pp
Signals set to be caught by the calling process are set to the
default action in the child process.
.Pp
Signals set to be ignored by the calling process image are set to
be ignored by the child process, unless otherwise specified by the
.Dv POSIX_SPAWN_SETSIGDEF
flag being set in the spawn-flags attribute of the object referenced by
.Fa attrp
and the signals being indicated in the spawn-sigdefault attribute
of the object referenced by
.Fa attrp .
.Pp
If the value of the
.Fa attrp
pointer is NULL, then the default values are used.
.Pp
All process attributes, other than those influenced by the attributes
set in the object referenced by
.Fa attrp
as specified above or by the file descriptor manipulations specified in
.Fa file_actions ,
appear in the new process image as though
.Fn vfork
had been called to create a child process and then
.Fn execve
had been called by the child process to execute the new process image.
.Pp
The implementation uses
.Fn vfork ,
thus the fork handlers are not run when
.Fn posix_spawn
or
.Fn posix_spawnp
is called.
.Sh RETURN VALUES
Upon successful completion,
.Fn posix_spawn
and
.Fn posix_spawnp
return the process ID of the child process to the parent process,
in the variable pointed to by a non-NULL
.Fa pid
argument, and return zero as the function return value.
Otherwise, no child process is created, no value is stored into
the variable pointed to by
.Fa pid ,
and an error number is returned as the function return value to
indicate the error.
If the
.Fa pid
argument is a null pointer, the process ID of the child is not returned
to the caller.
.Sh ERRORS
.Bl -enum
.It
If
.Fn posix_spawn
and
.Fn posix_spawnp
fail for any of the reasons that would cause
.Fn vfork
or one of the
.Nm exec
to fail, an error value is returned as described by
.Fn vfork
and
.Nm exec ,
respectively (or, if the error occurs after the calling process successfully
returns, the child process exits with exit status 127).
.It
If
.Nm POSIX_SPAWN_SETPGROUP
is set in the spawn-flags attribute of the object referenced by attrp, and
.Fn posix_spawn
or
.Fn posix_spawnp
fails while changing the child's process group, an error value is returned as
described by
.Fn setpgid
(or, if the error occurs after the calling process successfully returns,
the child process exits with exit status 127).
.It
If
.Nm POSIX_SPAWN_SETSCHEDPARAM
is set and
.Nm POSIX_SPAWN_SETSCHEDULER
is not set in the spawn-flags attribute of the object referenced by attrp, then
if
.Fn posix_spawn
or
.Fn posix_spawnp
fails for any of the reasons that would cause
.Fn sched_setparam
to fail, an error value is returned as described by
.Fn sched_setparam
(or, if the error occurs after the calling process successfully returns, the
child process exits with exit status 127).
.It
If
.Nm POSIX_SPAWN_SETSCHEDULER
is set in the spawn-flags attribute of the object referenced by attrp, and if
.Fn posix_spawn
or
.Fn posix_spawnp
fails for any of the reasons that would cause
.Fn sched_setscheduler
to fail, an error value is returned as described by
.Fn sched_setscheduler
(or, if the error occurs after the calling process successfully returns,
the child process exits with exit status 127).
.It
If the
.Fa file_actions
argument is not NULL, and specifies any dup2 or open actions to be
performed, and if
.Fn posix_spawn
or
.Fn posix_spawnp
fails for any of the reasons that would cause
.Fn dup2
or
.Fn open
to fail, an error value is returned as described by
.Fn dup2
and
.Fn open ,
respectively (or, if the error occurs after the calling process successfully
returns, the child process exits with exit status 127). An open file action
may, by itself, result in any of the errors described by
.Fn dup2 ,
in addition to those described by
.Fn open .
This implementation ignores any errors from
.Fn close ,
including trying to close a descriptor that is not open.
The ignore extends to any errors from individual file descriptors
.Fn close
executed as part of the
.Fn closefrom
action.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup2 2 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr open 2 ,
.Xr sched_setparam 2 ,
.Xr sched_setscheduler 2 ,
.Xr setpgid 2 ,
.Xr vfork 2 ,
.Xr posix_spawn_file_actions_addclose 3 ,
.Xr posix_spawn_file_actions_addclosefrom_np 3 ,
.Xr posix_spawn_file_actions_adddup2 3 ,
.Xr posix_spawn_file_actions_addopen 3 ,
.Xr posix_spawn_file_actions_addchdir_np 3 ,
.Xr posix_spawn_file_actions_addfchdir_np 3 ,
.Xr posix_spawn_file_actions_destroy 3 ,
.Xr posix_spawn_file_actions_init 3 ,
.Xr posix_spawnattr_destroy 3 ,
.Xr posix_spawnattr_getflags 3 ,
.Xr posix_spawnattr_getpgroup 3 ,
.Xr posix_spawnattr_getschedparam 3 ,
.Xr posix_spawnattr_getschedpolicy 3 ,
.Xr posix_spawnattr_getsigdefault 3 ,
.Xr posix_spawnattr_getsigmask 3 ,
.Xr posix_spawnattr_init 3 ,
.Xr posix_spawnattr_setflags 3 ,
.Xr posix_spawnattr_setpgroup 3 ,
.Xr posix_spawnattr_setschedparam 3 ,
.Xr posix_spawnattr_setschedpolicy 3 ,
.Xr posix_spawnattr_setsigdefault 3 ,
.Xr posix_spawnattr_setsigmask 3
.Sh STANDARDS
The
.Fn posix_spawn
and
.Fn posix_spawnp
functions conform to
.St -p1003.1-2001 ,
except that they ignore all errors from
.Fn close .
A future update of the Standard is expected to require that these functions
not fail because a file descriptor to be closed (via
.Fn posix_spawn_file_actions_addclose )
is not open.
.Sh HISTORY
The
.Fn posix_spawn
and
.Fn posix_spawnp
functions first appeared in
.Fx 8.0 .
.Sh AUTHORS
.An \&Ed Schouten Aq Mt ed@FreeBSD.org