'\" et

.TH POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH PROLOG

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.

.SH NAME

posix_spawn_file_actions_addclose,

posix_spawn_file_actions_addopen

\(em add close or open action to spawn file actions object

(\fBADVANCED REALTIME\fP)

.SH SYNOPSIS

.LP

.nf

#include <spawn.h>

.P

int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t

    *\fIfile_actions\fP, int \fIfildes\fP);

int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t

    *restrict \fIfile_actions\fP, int \fIfildes\fP,

    const char *restrict \fIpath\fP, int \fIoflag\fP, mode_t \fImode\fP);

.fi

.SH DESCRIPTION

These functions shall add or delete a close or open action to a

spawn file actions object.

.P

A spawn file actions object is of type

.BR posix_spawn_file_actions_t

(defined in

.IR <spawn.h> )

and is used to specify a series of actions to be performed by a

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

operation in order to arrive at the set of open file descriptors for

the child process given the set of open file descriptors of the parent.

POSIX.1\(hy2008 does not define comparison or assignment operators for the type

.BR posix_spawn_file_actions_t .

.P

A spawn file actions object, when passed to

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR(),

shall specify how the set of open file descriptors in the calling

process is transformed into a set of potentially open file descriptors

for the spawned process. This transformation shall be as if the

specified sequence of actions was performed exactly once, in the

context of the spawned process (prior to execution of the new process

image), in the order in which the actions were added to the object;

additionally, when the new process image is executed, any file

descriptor (from this new set) which has its FD_CLOEXEC

flag set shall be closed (see

.IR "\fIposix_spawn\fR\^(\|)").

.P

The

\fIposix_spawn_file_actions_addclose\fR()

function shall add a

.IR close

action to the object referenced by

.IR file_actions

that shall cause the file descriptor

.IR fildes

to be closed (as if

.IR close (\c

.IR fildes )

had been called) when a new process is spawned using this file actions

object.

.P

The

\fIposix_spawn_file_actions_addopen\fR()

function shall add an

.IR open

action to the object referenced by

.IR file_actions

that shall cause the file named by

.IR path

to be opened (as if

.IR open (\c

.IR path ,

.IR oflag ,

.IR mode )

had been called, and the returned file descriptor, if not

.IR fildes ,

had been changed to

.IR fildes )

when a new process is spawned using this file actions object. If

.IR fildes

was already an open file descriptor, it shall be closed before the new

file is opened.

.P

The string described by

.IR path

shall be copied by the

\fIposix_spawn_file_actions_addopen\fR()

function.

.SH "RETURN VALUE"

Upon successful completion, these functions shall return zero;

otherwise, an error number shall be returned to indicate the error.

.SH ERRORS

The

\fIposix_spawn_file_actions_addopen\fR()

function shall fail if:

.TP

.BR EBADF

The value specified by

.IR fildes

is negative or greater than or equal to

{OPEN_MAX}.

.P

The

\fIposix_spawn_file_actions_addclose\fR()

function shall fail if:

.TP

.BR EBADF

The value specified by

.IR fildes

is negative.

.br

.P

These functions may fail if:

.TP

.BR EINVAL

The value specified by

.IR file_actions

is invalid.

.TP

.BR ENOMEM

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

.P

It shall not be considered an error for the

.IR fildes

argument passed to these functions to specify a file descriptor for

which the specified operation could not be performed at the time of the

call. Any such error will be detected when the associated file actions

object is later used during a

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

operation.

.LP

.IR "The following sections are informative."

.SH EXAMPLES

None.

.SH "APPLICATION USAGE"

These functions are part of the Spawn option and need not be provided

on all implementations.

.P

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

\fIposix_spawn_file_actions_addclose\fR()

with an arbitrary integer risks non-conforming behavior, and this

function can only portably be used to close 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, an application should consider opening all file

descriptors with the FD_CLOEXEC bit set unless the file descriptor is

intended to be inherited across

.IR exec .

.SH RATIONALE

A spawn file actions object may be initialized to contain an ordered

sequence of

\fIclose\fR(),

\fIdup2\fR(),

and

\fIopen\fR()

operations to be used by

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

to arrive at the set of open file descriptors inherited by the spawned

process from the set of open file descriptors in the parent at the time

of the

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

call. It had been suggested that the

\fIclose\fR()

and

\fIdup2\fR()

operations alone are sufficient to rearrange file descriptors, and that

files which need to be opened for use by the spawned process can be

handled either by having the calling process open them before the

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

call (and close them after), or by passing pathnames to the spawned

process (in

.IR argv )

so that it may open them itself. The standard developers recommend that

applications use one of these two methods when practical, since

detailed error status on a failed open operation is always available to

the application this way. However, the standard developers feel that

allowing a spawn file actions object to specify open operations is

still appropriate because:

.IP " 1." 4

It is consistent with equivalent POSIX.5 (Ada) functionality.

.IP " 2." 4

It supports the I/O redirection paradigm commonly employed by POSIX

programs designed to be invoked from a shell. When such a program is

the child process, it may not be designed to open files on its own.

.IP " 3." 4

It allows file opens that might otherwise fail or violate file

ownership/access rights if executed by the parent process.

.P

Regarding 2. above, note that the spawn open file action provides to

\fIposix_spawn\fR()

and

\fIposix_spawnp\fR()

the same capability that the shell redirection operators provide to

\fIsystem\fR(),

only without the intervening execution of a shell; for example:

.sp

.RS 4

.nf

\fB

system ("myprog 

.fi \fR

.P

.RE

.P

Regarding 3. above, note that if the calling process needs to open one

or more files for access by the spawned process, but has insufficient

spare file descriptors, then the open action is necessary to allow the

\fIopen\fR()

to occur in the context of the child process after other file

descriptors have been closed (that must remain open in the parent).

.P

Additionally, if a parent is executed from a file having a

``set-user-id'' mode bit set and the POSIX_SPAWN_RESETIDS flag is set

in the spawn attributes, a file created within the parent process will

(possibly incorrectly) have the parent's effective user ID as its

owner, whereas a file created via an

\fIopen\fR()

action during

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

will have the parent's real ID as its owner; and an open by the parent

process may successfully open a file to which the real user should not

have access or fail to open a file to which the real user should have

access.

.SS "File Descriptor Mapping"

.P

The standard developers had originally proposed using an array which

specified the mapping of child file descriptors back to those of the

parent. It was pointed out by the ballot group that it is not possible

to reshuffle file descriptors arbitrarily in a library implementation

of

\fIposix_spawn\fR()

or

\fIposix_spawnp\fR()

without provision for one or more spare file descriptor entries (which

simply may not be available). Such an array requires that an

implementation develop a complex strategy to achieve the desired

mapping without inadvertently closing the wrong file descriptor at the

wrong time.

.P

It was noted by a member of the Ada Language Bindings working group

that the approved Ada Language

.IR Start_Process

family of POSIX process primitives use a caller-specified set of file

actions to alter the normal

\fIfork\fR()/\c

.IR exec

semantics for inheritance of file descriptors in a very flexible way,

yet no such problems exist because the burden of determining how to

achieve the final file descriptor mapping is completely on the

application. Furthermore, although the file actions interface appears

frightening at first glance, it is actually quite simple to implement

in either a library or the kernel.

.P

The

\fIposix_spawn_file_actions_addclose\fR()

function is not required to check whether the file descriptor is less than

{OPEN_MAX}

because on some implementations

{OPEN_MAX}

reflects the RLIMIT_NOFILE soft limit and therefore calling

\fIsetrlimit\fR()

to reduce this limit can result in an

{OPEN_MAX}

value less than or equal to an already open file descriptor.

Applications need to be able to close such file descriptors on spawn.

On implementations where

{OPEN_MAX}

does not change, it is recommended that

\fIposix_spawn_file_actions_addclose\fR()

should return

.BR [EBADF] 

if

.IR fildes

is greater than or equal to

{OPEN_MAX}.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.ad l

.IR "\fIclose\fR\^(\|)",

.IR "\fIdup\fR\^(\|)",

.IR "\fIopen\fR\^(\|)",

.IR "\fIposix_spawn\fR\^(\|)",

.IR "\fIposix_spawn_file_actions_adddup2\fR\^(\|)",

.IR "\fIposix_spawn_file_actions_destroy\fR\^(\|)"

.ad b

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "\fB<spawn.h>\fP"

.SH COPYRIGHT

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 .