POSIX_SPAWN(3P)

💫 POSIX_SPAWN(3P) 💫

okay bestie, so this is POSIX_SPAWN(3P), from the POSIX Programmer's Manual. Buckle up, it's a long one, but we're getting through it together. 🫶

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🌸 PROLOG 🌸

so like, this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go peek at the corresponding Linux manual page for the real tea ☕ — or honestly the interface might just not be implemented on Linux at all. rip. 💀

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✨ NAME ✨

posix_spawn, posix_spawnp — spawn a process (ADVANCED REALTIME) 👑

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🎀 SYNOPSIS 🎀

#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]);

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🦋 DESCRIPTION 🦋

so the way posix_spawn() and posix_spawnp() work is: they shall create a new process (the child process, our little baby 💖) from the specified process image. And that new process image shall be constructed from a regular executable file, which we call the new process image file.

When a C program gets executed as the result of this call, it shall be entered as a C-language function call, like this:

int main(int argc, char *argv[]);

where argc is the argument count and argv is an array of character pointers to the arguments themselves. And on top of that, this variable:

extern char **environ;

shall be initialized as a pointer to an array of character pointers to the environment strings. periodt. 💅

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 it is not counted in argc. These strings are the argument list that's available to the new process image. The value in argv[0] should point to a filename string that's associated with the process image being started by posix_spawn() or posix_spawnp(). (iykyk — argv[0] is basically the child's little name tag 🏷️)

The argument envp is an array of character pointers to null-terminated strings too. These strings constitute the environment for the new process image. And 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.1‐2017, Chapter 2, Conformance) whether any list overhead — like length words, null terminators, pointers, or alignment bytes — is included in this total. so yeah, check the docs, it's giving "implementation-defined, ask nicely" 🙈

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. Here's the vibe check on that:

• If the file parameter contains a <slash> character, then file shall be used as the pathname for the new process image file.
• Otherwise, the path prefix for this file shall be obtained by searching the directories passed as the environment variable PATH (see the Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables).

And if that environment variable is not defined? The results of the search are implementation-defined. (mysterious. we don't know her. 🧚)

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okay so now the file descriptors part, and this is where it gets juicy:

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()). And for those file descriptors that do remain open, the child process shall not inherit any file locks, but all the remaining attributes of the corresponding open file descriptions (see fcntl()) 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. The child process shall not inherit any file locks, but all remaining attributes of the corresponding open file descriptions (see fcntl()) 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()) shall be closed.

⚠️ okay so listen: 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 that file descriptor in the new process image. And 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. so like. don't do that. 😭

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The posix_spawnattr_t spawn attributes object type is defined in <spawn.h>. It shall contain at least the attributes defined below. (the lore deepens ✨)

🎀 process group stuff 🎀

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 that 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 that 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. (main character energy: the child gets its own group named after itself 💅)

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.

🧚 scheduling stuff 🧚

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.

👑 user and group ID stuff 👑

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.

🌈 signal stuff 🌈

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. And 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. (unspecified?? the ambiguity is lowkey iconic ngl 🫠)

🍓 and the rest 🍓

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's giving "fork+exec but make it a single call" 💖)

It is implementation-defined whether the fork handlers are run when posix_spawn() or posix_spawnp() is called.

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💜 RETURN VALUE 💜

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. slay. ✨

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. 😭

And if the pid argument is a null pointer, the process ID of the child is not returned to the caller. (you simply don't get to know her name 💀)

💖 ERRORS 💖

okay bestie, so these functions may fail if:

• EINVAL — the value you passed in for file_actions or attrp is invalid. that's it, that's the one listed here. 🚩

If this error happens after the calling process has already successfully returned from posix_spawn() or posix_spawnp(), then the child process may exit with exit status 127. 😭 like, the parent is out here living its best life and the kid just... bails.

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. same energy, same 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. (yes, the 127 is literally everywhere. it's giving "recurring character." 💀)

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 it 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. and get this: 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(). yeah. one action, multiple possible problem sources. multi-talented. ✨

the following sections are informative. (not me flagging this so you don't go looking for normative requirements in there 🙈)

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🦋 EXAMPLES 🦋

None.

...that's it. that's the whole section. iconic of them honestly. 💅

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🌸 APPLICATION USAGE 🌸

These functions are part of the Spawn option and need not be provided on all implementations. so like, vibe check your platform first, bestie — it might just not be there. 🫶

See also the APPLICATION USAGE section for exec. (yes, another APPLICATION USAGE. they're nesting. it's a whole thing.)

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👑 RATIONALE 👑

okay so here's the tea on why these functions exist at all. ☕

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. 🚨 read that again. not a drop-in replacement. it said what it said.

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. 😭 literally a whole programming language as arguments. 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. six. count them. 💅

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. so, easy. allegedly. 🎀

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. 💅 (design decision, defended, periodt.)

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. (only two. they chose peace. 🕊️)

The parameter list does not differ between posix_spawn() and posix_spawnp(); posix_spawnp() interprets the second parameter more elaborately than posix_spawn(). that's the whole difference, bestie. one parameter, read fancier. 💅

💜 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. three things. that's the whole gap. 🔥

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 🫂

okay so the process group inheritance field is your girl's way of yeeting the child process into a process group that already exists. And if you set the spawn-pgroup attribute of the object that attrp points at to zero? Then the setpgid() mechanism swoops in and drops the child into a brand new process group. Fresh group, who dis. 💅

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Threads 🧵✨

ngl, here's the tea: even if your system has no address translation, you can still fake a little concurrency with threads, no posix_spawn() or posix_spawnp() required. And honestly, in many cases just making a thread is enough — but it is not always a good substitute, bestie. Not even close.

The way that posix_spawn() and posix_spawnp() are considerably ``heavier'' than thread creation? Yeah, that's real. Processes have several important attributes that threads simply do not have:

• even without address translation, a process can have base-and-bound memory protection 🛡️
• every process has a process environment, including security attributes and file capabilities, plus powerful scheduling attributes
• processes abstract the behavior of non-uniform-memory-architecture multi-processors better than threads do
• and they're just more convenient for activities that aren't closely linked

Also, lowkey important: posix_spawn() and posix_spawnp() may not bring support for multiple processes to every configuration. Process creation isn't the only piece of OS support you need for multiple processes — the total cost of that support can be quite high in some circumstances. Existing practice shows support for multiple processes is uncommon and threads are common among ``tiny kernels''. So there should, therefore, probably continue to be AEPs for operating systems with only one process. Periodt.

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Asynchronous Error Notification 😭📣

A library implementation of posix_spawn() or posix_spawnp() might not be able to detect every possible error before it forks the child process. So POSIX.1‐2008 provides a way for the child to signal back: an error indication returned from a child process that couldn't successfully complete the spawn operation, delivered via a special exit status that you can detect 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. So 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 indicating 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() just to embed special info in stat_val to indicate a spawn failure. Not the vibe.

The 8 bits of child process exit status that POSIX.1‐2008 guarantees are accessible to the waiting parent process are insufficient to disambiguate a spawn error from any other kind of error an arbitrary process image might return. 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 how system() and popen() use that value — to signal failures in those operations that occur after the function has returned but before a shell is able to execute. But heads up: an exit status of 127 does not uniquely identify this class of error, and it does not 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, which gives the parent process more detailed failure information.

So, 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. iykyk 🧚

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🔮 FUTURE DIRECTIONS 🔮

None.

(yeah that's it. nothing coming. the future is a blank canvas, bestie 🎨)

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👀 SEE ALSO 👀

alarm(), chmod(), close(), dup(), exec, exit(), fcntl(), fork(), fstatat(), kill(), open(), posix_spawn_file_actions_addclose(), posix_spawn_file_actions_adddup2(), posix_spawn_file_actions_destroy(), posix_spawnattr_destroy(), posix_spawnattr_getsigdefault(), posix_spawnattr_getflags(), posix_spawnattr_getpgroup(), posix_spawnattr_getschedparam(), posix_spawnattr_getschedpolicy(), posix_spawnattr_getsigmask(), sched_setparam(), sched_setscheduler(), setpgid(), setuid(), times(), wait(), waitid()

The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, <spawn.h>

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💖 COPYRIGHT 💖

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 http://www.opengroup.org/unix/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 .

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and that's a wrap on the whole page, bestie!! 🎀 this edition was served up by IEEE/The Open Group with a 2017 date stamp, filed under POSIX_SPAWN(3P) — go forth and spawn responsibly, you absolute icon 👑✨