NAME

posix_spawn_file_actions_adddup2 — add dup2 action to spawn file actions object (ADVANCED REALTIME)

SYNOPSIS

[SPN] #include <spawn.h>

int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t
       *file_actions, int fildes, int newfildes);

DESCRIPTION

The posix_spawn_file_actions_adddup2() function shall add a dup2() action to the object referenced by file_actions that shall cause the file descriptor fildes to be duplicated as newfildes (as if dup2(fildes, newfildes) had been called) when a new process is spawned using this file actions object.

If fildes and newfildes are equal, then the action shall ensure that newfildes is inherited by the new process with FD_CLOEXEC clear, even if the FD_CLOEXEC flag of fildes is set at the time the new process is spawned, and even though dup2() would not make such a change.

A spawn file actions object is as defined in posix_spawn_file_actions_addclose.

RETURN VALUE

Upon successful completion, the posix_spawn_file_actions_adddup2() function shall return zero; otherwise, an error number shall be returned to indicate the error.

ERRORS

The posix_spawn_file_actions_adddup2() function shall fail if:

[EBADF]

The value specified by fildes or newfildes is negative or greater than or equal to {OPEN_MAX}.

[ENOMEM]

Insufficient memory exists to add to the spawn file actions object.

The posix_spawn_file_actions_adddup2() function may fail if:

[EINVAL]

The value specified by file_actions is invalid.

It shall not be considered an error for the fildes argument passed to the posix_spawn_file_actions_adddup2() function to specify a file descriptor for which the specified operation could not be performed at the time of the call. Any such error shall be detected when the associated file actions object is later used during a posix_spawn() or posix_spawnp() operation.

EXAMPLES

None.

APPLICATION USAGE

The posix_spawn_file_actions_adddup2() function is part of the Spawn option and need not be provided on all implementations.

Implementations may use file descriptors that must be inherited into child processes for the child process to remain conforming, such as for message catalog or tracing purposes. Therefore, an application that calls posix_spawn_file_actions_adddup2() with an arbitrary integer for newfildes risks non-conforming behavior, and this function can only portably be used to overwrite file descriptor values that the application has obtained through explicit actions, or for the three file descriptors corresponding to the standard file streams. In order to avoid a race condition of leaking an unintended file descriptor into a child process or executed program, an application should consider opening all file descriptors with the FD_CLOFORK or FD_CLOEXEC flag, or both flags, set unless the file descriptor is intended to be inherited by child processes or executed programs, respectively.

RATIONALE

Refer to the RATIONALE section in posix_spawn_file_actions_addclose.

Although dup2() is required to do nothing when fildes and newfildes are equal and fildes is an open descriptor, the use of posix_spawn_file_actions_adddup2() is required to cause fildes to be accessible in the child with FD_CLOEXEC clear. This is because there is no counterpart posix_spawn_file_actions_fcntl() that could be used for clearing the flag as an independent file action. It would also be possible to achieve this effect by using two calls to posix_spawn_file_actions_adddup2() and a temporary fildes value known not to conflict with any other file descriptors, coupled with a posix_spawn_file_actions_addclose() to avoid leaking the temporary, but this approach is complex, and risks [EMFILE] or [ENFILE] failure that can be avoided with the in-place removal of FD_CLOEXEC.

There is no need for posix_spawn_file_actions_adddup3(), since it makes no sense to create a file descriptor with FD_CLOEXEC set before spawning the child process, where that file descriptor would immediately be closed again.

FUTURE DIRECTIONS

None.

SEE ALSO

dup, posix_spawn, posix_spawn_file_actions_addclose, posix_spawn_file_actions_destroy

XBD <spawn.h>

CHANGE HISTORY

First released in Issue 6. Derived from IEEE Std 1003.1d-1999.

IEEE PASC Interpretation 1003.1 #104 is applied, noting that the [EBADF] error can apply to the newfildes argument in addition to fildes.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0440 [149] is applied.

Issue 8

Austin Group Defect 411 is applied, changing requirements relating to the FD_CLOEXEC flag when fildes and newfildes are equal.

Austin Group Defect 1318 is applied, adding FD_CLOFORK.