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POSIX_SPAWN(3P) POSIX Programmer's Manual POSIX_SPAWN(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
posix_spawn, posix_spawnp — spawn a process (ADVANCED REALTIME)
#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]);
The posix_spawn() and posix_spawnp() functions shall create a new
process (child process) from the specified process image. The new
process image shall be 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 shall be
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;
shall be 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 shall be 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 string 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 combined argument and
environment lists of the child process is {ARG_MAX}. The
implementation shall specify in the system documentation (see the
Base Definitions volume of POSIX.1‐2008, Chapter 2, Conformance)
whether any list overhead, such as length words, null terminators,
pointers, or alignment bytes, is 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() shall be used to construct a
pathname that identifies the new process image file. If the file
parameter contains a <slash> character, the file parameter shall be
used as the pathname for the new process image file. Otherwise, the
path prefix for this file shall be obtained by a search of the
directories passed as the environment variable PATH (see the Base
Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables). 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 shall remain open in the child process, except for
those whose close-on-exec flag FD_CLOEXEC is set (see fcntl(3p)).
For those file descriptors that remain open, all attributes of the
corresponding open file descriptions, including file locks (see
fcntl(3p)), shall remain unchanged.
If file_actions is not NULL, then the file descriptors open in the
child process shall be 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 shall be:
1. The set of open file descriptors for the child process shall
initially be the same set as is open for the calling process. All
attributes of the corresponding open file descriptions, including
file locks (see fcntl(3p)), shall remain unchanged.
2. The signal mask, signal default actions, and the effective user
and group IDs for the child process shall be changed as specified
in the attributes object referenced by attrp.
3. The file actions specified by the spawn file actions object shall
be 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(3p)) shall be closed.
If file descriptor 0, 1, or 2 would otherwise be closed in the new
process image created by posix_spawn() or posix_spawnp(),
implementations may open an unspecified file for the file descriptor
in the new process image. If a standard utility or a conforming
application is executed with file descriptor 0 not open for reading
or with file descriptor 1 or 2 not open for writing, the environment
in which the utility or application is executed shall be deemed non-
conforming, and consequently the utility or application might not
behave as described in this standard.
The posix_spawnattr_t spawn attributes object type is defined in
<spawn.h>. It shall contain at least the attributes defined below.
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 shall be
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 shall be in a new process group with a process group ID equal
to its process ID.
If the POSIX_SPAWN_SETPGROUP flag is not set in the spawn-flags
attribute of the object referenced by attrp, the new child process
shall inherit the parent's process group.
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 shall
initially have 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 the spawn-flags
attribute of the object referenced by attrp (regardless of the
setting of the POSIX_SPAWN_SETSCHEDPARAM flag), the new process image
shall initially have 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 shall inherit the
effective user ID of the parent process. If this flag is set, the
effective user ID of the child process shall be 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
shall become 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 shall
inherit the effective group ID of the parent process. If this flag is
set, the effective group ID of the child process shall be 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 shall become 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 shall
initially have 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 shall be set to their
default actions in the child process. Signals set to the default
action in the parent process shall be set to the default action in
the child process.
Signals set to be caught by the calling process shall be set to the
default action in the child process.
Except for SIGCHLD, signals set to be ignored by the calling process
image shall be 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, shall 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.
It is implementation-defined whether the fork handlers are run when
posix_spawn() or posix_spawnp() is called.
Upon successful completion, posix_spawn() and posix_spawnp() shall
return the process ID of the child process to the parent process, in
the variable pointed to by a non-NULL pid argument, and shall return
zero as the function return value. Otherwise, no child process shall
be created, the value stored into the variable pointed to by a non-
NULL pid is unspecified, and an error number shall be 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.
These functions may fail if:
EINVAL The value specified by file_actions or attrp is invalid.
If this error occurs after the calling process successfully returns
from the posix_spawn() or posix_spawnp() function, the child process
may exit with exit status 127.
If posix_spawn() or posix_spawnp() fail for any of the reasons that
would cause fork() or one of the exec family of functions to fail, an
error value shall be returned as described by fork() and exec,
respectively (or, if the error occurs after the calling process
successfully returns, the child process shall exit with exit status
127).
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 shall be
returned as described by setpgid() (or, if the error occurs after the
calling process successfully returns, the child process shall exit
with exit status 127).
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
shall be returned as described by sched_setparam() (or, if the error
occurs after the calling process successfully returns, the child
process shall exit with exit status 127).
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 shall be returned as
described by sched_setscheduler() (or, if the error occurs after the
calling process successfully returns, the child process shall exit
with exit status 127).
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 shall be returned as
described by close(), dup2(), and open(), respectively (or, if the
error occurs after the calling process successfully returns, the
child process shall exit with exit status 127). An open file action
may, by itself, result in any of the errors described by close() or
dup2(), in addition to those described by open().
The following sections are informative.
None.
These functions are part of the Spawn option and need not be provided
on all implementations.
See also the APPLICATION USAGE section for exec(1p).
The posix_spawn() function and its close relation posix_spawnp() have
been introduced to overcome the following perceived difficulties with
fork(): the fork() function is difficult or impossible to implement
without swapping or dynamic address translation.
* Swapping is generally too slow for a realtime environment.
* Dynamic address translation is not available everywhere that
POSIX might be useful.
* Processes are too useful to simply option out of POSIX whenever
it must run without address translation or other MMU services.
Thus, POSIX needs process creation and file execution primitives that
can be efficiently implemented without address translation or other
MMU services.
The posix_spawn() function is implementable as a library routine, but
both posix_spawn() and posix_spawnp() are designed as kernel
operations. Also, although they may be an efficient replacement for
many fork()/exec pairs, their goal is to provide useful process
creation primitives for systems that have difficulty with fork(), not
to provide drop-in replacements for fork()/exec.
This view of the role of posix_spawn() and posix_spawnp() influenced
the design of their API. It does not attempt to provide the full
functionality of fork()/exec in which arbitrary user-specified
operations of any sort are permitted between the creation of the
child process and the execution of the new process image; any attempt
to reach that level would need to provide a programming language as
parameters. Instead, posix_spawn() and posix_spawnp() are process
creation primitives like the Start_Process and Start_Process_Search
Ada language bindings package POSIX_Process_Primitives and also like
those in many operating systems that are not UNIX systems, but with
some POSIX-specific additions.
To achieve its coverage goals, posix_spawn() and posix_spawnp() have
control of six types of inheritance: file descriptors, process group
ID, user and group ID, signal mask, scheduling, and whether each
signal ignored in the parent will remain ignored in the child, or be
reset to its default action in the child.
Control of file descriptors is required to allow an independently
written child process image to access data streams opened by and even
generated or read by the parent process without being specifically
coded to know which parent files and file descriptors are to be used.
Control of the process group ID is required to control how the job
control of the child process relates to that of the parent.
Control of the signal mask and signal defaulting is sufficient to
support the implementation of system(). Although support for
system() is not explicitly one of the goals for posix_spawn() and
posix_spawnp(), it is covered under the ``at least 50%'' coverage
goal.
The intention is that the normal file descriptor inheritance across
fork(), the subsequent effect of the specified spawn file actions,
and the normal file descriptor inheritance across one of the exec
family of functions should fully specify open file inheritance. The
implementation need make no decisions regarding the set of open file
descriptors when the child process image begins execution, those
decisions having already been made by the caller and expressed as the
set of open file descriptors and their FD_CLOEXEC flags at the time
of the call and the spawn file actions object specified in the call.
We have been assured that in cases where the POSIX Start_Process Ada
primitives have been implemented in a library, this method of
controlling file descriptor inheritance may be implemented very
easily.
We can identify several problems with posix_spawn() and
posix_spawnp(), but there does not appear to be a solution that
introduces fewer problems. Environment modification for child process
attributes not specifiable via the attrp or file_actions arguments
must be done in the parent process, and since the parent generally
wants to save its context, it is more costly than similar
functionality with fork()/exec. It is also complicated to modify the
environment of a multi-threaded process temporarily, since all
threads must agree when it is safe for the environment to be changed.
However, this cost is only borne by those invocations of
posix_spawn() and posix_spawnp() that use the additional
functionality. Since extensive modifications are not the usual case,
and are particularly unlikely in time-critical code, keeping much of
the environment control out of posix_spawn() and posix_spawnp() is
appropriate design.
The posix_spawn() and posix_spawnp() functions do not have all the
power of fork()/exec. This is to be expected. The fork() function is
a wonderfully powerful operation. We do not expect to duplicate its
functionality in a simple, fast function with no special hardware
requirements. It is worth noting that posix_spawn() and
posix_spawnp() are very similar to the process creation operations on
many operating systems that are not UNIX systems.
Requirements
The requirements for posix_spawn() and posix_spawnp() are:
* They must be implementable without an MMU or unusual hardware.
* They must be compatible with existing POSIX standards.
Additional goals are:
* They should be efficiently implementable.
* They should be able to replace at least 50% of typical executions
of fork().
* A system with posix_spawn() and posix_spawnp() and without fork()
should be useful, at least for realtime applications.
* A system with fork() and the exec family should be able to
implement posix_spawn() and posix_spawnp() as library routines.
Two-Syntax
POSIX exec has several calling sequences with approximately the same
functionality. These appear to be required for compatibility with
existing practice. Since the existing practice for the posix_spawn*()
functions is otherwise substantially unlike POSIX, we feel that
simplicity outweighs compatibility. There are, therefore, only two
names for the posix_spawn*() functions.
The parameter list does not differ between posix_spawn() and
posix_spawnp(); posix_spawnp() interprets the second parameter more
elaborately than posix_spawn().
Compatibility with POSIX.5 (Ada)
The Start_Process and Start_Process_Search procedures from the
POSIX_Process_Primitives package from the Ada language binding to
POSIX.1 encapsulate fork() and exec functionality in a manner similar
to that of posix_spawn() and posix_spawnp(). Originally, in keeping
with our simplicity goal, the standard developers had limited the
capabilities of posix_spawn() and posix_spawnp() to a subset of the
capabilities of Start_Process and Start_Process_Search; certain non-
default capabilities were not supported. However, based on
suggestions by the ballot group to improve file descriptor mapping or
drop it, and on the advice of an Ada Language Bindings working group
member, the standard developers decided that posix_spawn() and
posix_spawnp() should be sufficiently powerful to implement
Start_Process and Start_Process_Search. The rationale is that if the
Ada language binding to such a primitive had already been approved as
an IEEE standard, there can be little justification for not approving
the functionally-equivalent parts of a C binding. The only three
capabilities provided by posix_spawn() and posix_spawnp() that are
not provided by Start_Process and Start_Process_Search are optionally
specifying the child's process group ID, the set of signals to be
reset to default signal handling in the child process, and the
child's scheduling policy and parameters.
For the Ada language binding for Start_Process to be implemented with
posix_spawn(), that binding would need to explicitly pass an empty
signal mask and the parent's environment to posix_spawn() whenever
the caller of Start_Process allowed these arguments to default, since
posix_spawn() does not provide such defaults. The ability of
Start_Process to mask user-specified signals during its execution is
functionally unique to the Ada language binding and must be dealt
with in the binding separately from the call to posix_spawn().
Process Group
The process group inheritance field can be used to join the child
process with an existing process group. By assigning a value of zero
to the spawn-pgroup attribute of the object referenced by attrp, the
setpgid() mechanism will place the child process in a new process
group.
Threads
Without the posix_spawn() and posix_spawnp() functions, systems
without address translation can still use threads to give an
abstraction of concurrency. In many cases, thread creation suffices,
but it is not always a good substitute. The posix_spawn() and
posix_spawnp() functions are considerably ``heavier'' than thread
creation. Processes have several important attributes that threads do
not. Even without address translation, a process may have base-and-
bound memory protection. Each process has a process environment
including security attributes and file capabilities, and powerful
scheduling attributes. Processes abstract the behavior of non-
uniform-memory-architecture multi-processors better than threads, and
they are more convenient to use for activities that are not closely
linked.
The posix_spawn() and posix_spawnp() functions may not bring support
for multiple processes to every configuration. Process creation is
not the only piece of operating system support required to support
multiple processes. The total cost of support for multiple processes
may be quite high in some circumstances. Existing practice shows that
support for multiple processes is uncommon and threads are common
among ``tiny kernels''. There should, therefore, probably continue
to be AEPs for operating systems with only one process.
Asynchronous Error Notification
A library implementation of posix_spawn() or posix_spawnp() may not
be able to detect all possible errors before it forks the child
process. POSIX.1‐2008 provides for an error indication returned from
a child process which could not successfully complete the spawn
operation via a special exit status which may be detected using the
status value returned by wait(), waitid(), and waitpid().
The stat_val interface and the macros used to interpret it are not
well suited to the purpose of returning API errors, but they are the
only path available to a library implementation. Thus, an
implementation may cause the child process to exit with exit status
127 for any error detected during the spawn process after the
posix_spawn() or posix_spawnp() function has successfully returned.
The standard developers had proposed using two additional macros to
interpret stat_val. The first, WIFSPAWNFAIL, would have detected a
status that indicated that the child exited because of an error
detected during the posix_spawn() or posix_spawnp() operations rather
than during actual execution of the child process image; the second,
WSPAWNERRNO, would have extracted the error value if WIFSPAWNFAIL
indicated a failure. Unfortunately, the ballot group strongly opposed
this because it would make a library implementation of posix_spawn()
or posix_spawnp() dependent on kernel modifications to waitpid() to
be able to embed special information in stat_val to indicate a spawn
failure.
The 8 bits of child process exit status that are guaranteed by
POSIX.1‐2008 to be accessible to the waiting parent process are
insufficient to disambiguate a spawn error from any other kind of
error that may be returned by an arbitrary process image. No other
bits of the exit status are required to be visible in stat_val, so
these macros could not be strictly implemented at the library level.
Reserving an exit status of 127 for such spawn errors is consistent
with the use of this value by system() and popen() to signal failures
in these operations that occur after the function has returned but
before a shell is able to execute. The exit status of 127 does not
uniquely identify this class of error, nor does it provide any
detailed information on the nature of the failure. Note that a kernel
implementation of posix_spawn() or posix_spawnp() is permitted (and
encouraged) to return any possible error as the function value, thus
providing more detailed failure information to the parent process.
Thus, no special macros are available to isolate asynchronous
posix_spawn() or posix_spawnp() errors. Instead, errors detected by
the posix_spawn() or posix_spawnp() operations in the context of the
child process before the new process image executes are reported by
setting the child's exit status to 127. The calling process may use
the WIFEXITED and WEXITSTATUS macros on the stat_val stored by the
wait() or waitpid() functions to detect spawn failures to the extent
that other status values with which the child process image may exit
(before the parent can conclusively determine that the child process
image has begun execution) are distinct from exit status 127.
None.
alarm(3p), chmod(3p), close(3p), dup(3p), exec(1p), exit(3p),
fcntl(3p), fork(3p), fstatat(3p), kill(3p), open(3p),
posix_spawn_file_actions_addclose(3p),
posix_spawn_file_actions_adddup2(3p),
posix_spawn_file_actions_destroy(3p), posix_spawnattr_destroy(3p),
posix_spawnattr_getsigdefault(3p), posix_spawnattr_getflags(3p),
posix_spawnattr_getpgroup(3p), posix_spawnattr_getschedparam(3p),
posix_spawnattr_getschedpolicy(3p), posix_spawnattr_getsigmask(3p),
sched_setparam(3p), sched_setscheduler(3p), setpgid(3p), setuid(3p),
times(3p), wait(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, spawn.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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 http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 POSIX_SPAWN(3P)
Pages that refer to this page: spawn.h(0p), exec(3p), fdopen(3p), posix_spawnattr_destroy(3p), posix_spawnattr_getflags(3p), posix_spawnattr_getpgroup(3p), posix_spawnattr_getschedparam(3p), posix_spawnattr_getschedpolicy(3p), posix_spawnattr_getsigdefault(3p), posix_spawnattr_getsigmask(3p), posix_spawn_file_actions_addclose(3p), posix_spawn_file_actions_adddup2(3p), posix_spawn_file_actions_destroy(3p), posix_spawnp(3p)
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