POSIX_SPAWN(3)             POSIX Programmer's Manual            POSIX_SPAWN(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       posix_spawn, posix_spawnp -- spawn a process (ADVANCED REALTIME)

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]);

DESCRIPTION
       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.1-2017, 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.1-2017, 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()).  For
       those file descriptors that remain open, the child process shall not
       inherit any file locks, but all 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.

       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.

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.  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.

ERRORS
       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.

EXAMPLES
       None.

APPLICATION USAGE
       These functions are part of the Spawn option and need not be provided
       on all implementations.

       See also the APPLICATION USAGE section for exec.

RATIONALE
       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.

FUTURE DIRECTIONS
       None.

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>

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 .



IEEE/The Open Group                  2017                       POSIX_SPAWN(3)
