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posix_spawn_file_actions_addclose.3p (11024B)

'\" et
.TH POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE "3P" 2017 "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
system ("myprog <file1 3<file2");
.fi
.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\(hy2017,
.IR "\fB<spawn.h>\fP"
.\"
.SH 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 .
.PP
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 .