posix_spawn or posix_spawnp Subroutine

Edit online

Purpose

Spawns a process.

Syntax

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 subroutines 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.

When a C program is executed as the result of this call, the program is 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;
is initialized as a pointer to an array of character pointers to the environment strings.

The argv parameter 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 argc. These strings constitute the argument list available to the new process image. The value in argv[0] should point to a file name 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 child process' combined argument and environment lists is {ARG_MAX}. The implementation specifies in the system documentation 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 path name that identifies the new process image file to execute.

The file parameter to posix_spawnp is used to construct a path name that identifies the new process image file. If the file parameter contains a slash character (/), the file parameter is used as the path name 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 PATH. If this environment variable is not defined, the results of the search are implementation-defined.

If file_actions is a null pointer, file descriptors that are open in the calling process remain open in the child process, except for those whose FD_CLOEXEC flag is set. For those file descriptors that remain open, all attributes of the corresponding open file descriptions, including file locks, remain unchanged.

If file_actions is not a null pointer, 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 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 is as follows:
  1. The set of open file descriptors for the child process is initially the same set as is open for the calling process. All attributes of the corresponding open file descriptions, including file locks, remain unchanged.
  2. 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 attrp.
  3. 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.
  4. Any file descriptor that has its FD_CLOEXEC flag set is closed.
The posix_spawnattr_t spawn attributes object type is defined in the spawn.h header file. Its attributes are defined as follows:
  • 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, the child's process group is 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 0, then the child is 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 inherits 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 initially has 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 initially has 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 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.
  • 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 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.
  • If the POSIX_SPAWN_SETSIGMASK flag is set in the spawn-flags attribute of the object referenced by attrp, the child process initially has 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 is 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. Signals set to be caught by the calling process are set to the default action in the child process.
  • Except for SIGCHLD, signals set to be ignored by the calling process image are 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 set to the default action in the child process. This is true 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 in the preceding list or by the file descriptor manipulations specified in file_actions, are displayed 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.

By default, fork handlers are not run in posix_spawn or posix_spawnp routines. To enable fork handlers, set the POSIX_SPAWN_FORK_HANDLERS flag in the attribute.

Return Values

Upon successful completion, posix_spawn and posix_spawnp return the process ID of the child process to the parent process, in the variable pointed to by a non-NULL pid argument, and return 0 as the function return value. Otherwise, no child process is created, the value stored into the variable pointed to by a non-NULL pid is unspecified, and an error number is 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.

Error Codes

The posix_spawn and posix_spawnp subroutines will fail if the following is true:
Item Description
EINVAL The value specified by file_actions or attrp is invalid.
The error codes for the posix_spawn and posix_spawnp subroutines are affected by the following conditions:
  • If this error occurs after the calling process successfully returns from the posix_spawn or posix_spawnp function, the child process might 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 is returned as described by fork and exec, respectively (or, if the error occurs after the calling process successfully returns, the child process exits 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 is 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 is returned as described by sched_setparam (or, if the error occurs after the calling process successfully returns, the child process sexit 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 is returned as described by sched_setscheduler (or, if the error occurs after the calling process successfully returns, the child process exits 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 is returned as described by close, dup2, and 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 might, by itself, result in any of the errors described by close or dup2, in addition to those described by open.