path: root/man3p/shm_open.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SHM_OPEN" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" shm_open 
.SH NAME
shm_open \- open a shared memory object (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <sys/mman.h>
.br
.sp
int shm_open(const char *\fP\fIname\fP\fB, int\fP \fIoflag\fP\fB,
mode_t\fP \fImode\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIshm_open\fP() function shall establish a connection between
a shared memory object and a file descriptor. It shall
create an open file description that refers to the shared memory object
and a file descriptor that refers to that open file
description. The file descriptor is used by other functions to refer
to that shared memory object. The \fIname\fP argument points
to a string naming a shared memory object. It is unspecified whether
the name appears in the file system and is visible to other
functions that take pathnames as arguments. The \fIname\fP argument
conforms to the construction rules for a pathname. If
\fIname\fP begins with the slash character, then processes calling
\fIshm_open\fP() with the same value of \fIname\fP refer to
the same shared memory object, as long as that name has not been removed.
If \fIname\fP does not begin with the slash character,
the effect is implementation-defined. The interpretation of slash
characters other than the leading slash character in \fIname\fP
is implementation-defined.
.LP
If successful, \fIshm_open\fP() shall return a file descriptor for
the shared memory object that is the lowest numbered file
descriptor not currently open for that process. The open file description
is new, and therefore the file descriptor does not share
it with any other processes. It is unspecified whether the file offset
is set. The FD_CLOEXEC file descriptor flag associated with
the new file descriptor is set.
.LP
The file status flags and file access modes of the open file description
are according to the value of \fIoflag\fP. The
\fIoflag\fP argument is the bitwise-inclusive OR of the following
flags defined in the \fI<fcntl.h>\fP header. Applications specify
exactly one of the first two values (access
modes) below in the value of \fIoflag\fP:
.TP 7
O_RDONLY
Open for read access only.
.TP 7
O_RDWR
Open for read or write access.
.sp
.LP
Any combination of the remaining flags may be specified in the value
of \fIoflag\fP:
.TP 7
O_CREAT
If the shared memory object exists, this flag has no effect, except
as noted under O_EXCL below. Otherwise, the shared memory
object is created; the user ID of the shared memory object shall be
set to the effective user ID of the process; the group ID of
the shared memory object is set to a system default group ID or to
the effective group ID of the process. The permission bits of
the shared memory object shall be set to the value of the \fImode\fP
argument except those set in the file mode creation mask of
the process. When bits in \fImode\fP other than the file permission
bits are set, the effect is unspecified. The \fImode\fP
argument does not affect whether the shared memory object is opened
for reading, for writing, or for both. The shared memory object
has a size of zero.
.TP 7
O_EXCL
If O_EXCL and O_CREAT are set, \fIshm_open\fP() fails if the shared
memory object exists. The check for the existence of the
shared memory object and the creation of the object if it does not
exist is atomic with respect to other processes executing
\fIshm_open\fP() naming the same shared memory object with O_EXCL
and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
result is undefined.
.TP 7
O_TRUNC
If the shared memory object exists, and it is successfully opened
O_RDWR, the object shall be truncated to zero length and the
mode and owner shall be unchanged by this function call. The result
of using O_TRUNC with O_RDONLY is undefined.
.sp
.LP
When a shared memory object is created, the state of the shared memory
object, including all data associated with the shared
memory object, persists until the shared memory object is unlinked
and all other references are gone. It is unspecified whether the
name and shared memory object state remain valid after a system reboot.
.SH RETURN VALUE
.LP
Upon successful completion, the \fIshm_open\fP() function shall return
a non-negative integer representing the lowest numbered
unused file descriptor. Otherwise, it shall return -1 and set \fIerrno\fP
to indicate the error.
.SH ERRORS
.LP
The \fIshm_open\fP() function shall fail if:
.TP 7
.B EACCES
The shared memory object exists and the permissions specified by \fIoflag\fP
are denied, or the shared memory object does not
exist and permission to create the shared memory object is denied,
or O_TRUNC is specified and write permission is denied.
.TP 7
.B EEXIST
O_CREAT and O_EXCL are set and the named shared memory object already
exists.
.TP 7
.B EINTR
The \fIshm_open\fP() operation was interrupted by a signal.
.TP 7
.B EINVAL
The \fIshm_open\fP() operation is not supported for the given name.
.TP 7
.B EMFILE
Too many file descriptors are currently in use by this process.
.TP 7
.B ENAMETOOLONG
The length of the \fIname\fP argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
.TP 7
.B ENFILE
Too many shared memory objects are currently open in the system.
.TP 7
.B ENOENT
O_CREAT is not set and the named shared memory object does not exist.
.TP 7
.B ENOSPC
There is insufficient space for the creation of the new shared memory
object.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
When the Memory Mapped Files option is supported, the normal \fIopen\fP()
call is used to
obtain a descriptor to a file to be mapped according to existing practice
with \fImmap\fP().
When the Shared Memory Objects option is supported, the \fIshm_open\fP()
function shall obtain a descriptor to the shared memory
object to be mapped.
.LP
There is ample precedent for having a file descriptor represent several
types of objects. In the POSIX.1-1990 standard, a file
descriptor can represent a file, a pipe, a FIFO, a tty, or a directory.
Many implementations simply have an operations vector,
which is indexed by the file descriptor type and does very different
operations. Note that in some cases the file descriptor passed
to generic operations on file descriptors is returned by \fIopen\fP()
or \fIcreat\fP() and in some cases returned by alternate functions,
such as \fIpipe\fP(). The latter technique is used by \fIshm_open\fP().
.LP
Note that such shared memory objects can actually be implemented as
mapped files. In both cases, the size can be set after the
open using \fIftruncate\fP(). The \fIshm_open\fP() function itself
does not create a
shared object of a specified size because this would duplicate an
extant function that set the size of an object referenced by a
file descriptor.
.LP
On implementations where memory objects are implemented using the
existing file system, the \fIshm_open\fP() function may be
implemented using a macro that invokes \fIopen\fP(), and the \fIshm_unlink\fP()
function may be implemented using a macro that invokes \fIunlink\fP().
.LP
For implementations without a permanent file system, the definition
of the name of the memory objects is allowed not to survive
a system reboot. Note that this allows systems with a permanent file
system to implement memory objects as data structures internal
to the implementation as well.
.LP
On implementations that choose to implement memory objects using memory
directly, a \fIshm_open\fP() followed by an \fIftruncate\fP() and
\fIclose\fP() can be used to
preallocate a shared memory area and to set the size of that preallocation.
This may be necessary for systems without virtual
memory hardware support in order to ensure that the memory is contiguous.
.LP
The set of valid open flags to \fIshm_open\fP() was restricted to
O_RDONLY, O_RDWR, O_CREAT, and O_TRUNC because these could be
easily implemented on most memory mapping systems. This volume of
IEEE\ Std\ 1003.1-2001 is silent on the results if the
implementation cannot supply the requested file access because of
implementation-defined reasons, including hardware ones.
.LP
The error conditions [EACCES] and [ENOTSUP] are provided to inform
the application that the implementation cannot complete a
request.
.LP
[EACCES] indicates for implementation-defined reasons, probably hardware-related,
that the implementation cannot comply with a
requested mode because it conflicts with another requested mode. An
example might be that an application desires to open a memory
object two times, mapping different areas with different access modes.
If the implementation cannot map a single area into a
process space in two places, which would be required if different
access modes were required for the two areas, then the
implementation may inform the application at the time of the second
open.
.LP
[ENOTSUP] indicates for implementation-defined reasons, probably hardware-related,
that the implementation cannot comply with a
requested mode at all. An example would be that the hardware of the
implementation cannot support write-only shared memory
areas.
.LP
On all implementations, it may be desirable to restrict the location
of the memory objects to specific file systems for
performance (such as a RAM disk) or implementation-defined reasons
(shared memory supported directly only on certain file systems).
The \fIshm_open\fP() function may be used to enforce these restrictions.
There are a number of methods available to the
application to determine an appropriate name of the file or the location
of an appropriate directory. One way is from the
environment via \fIgetenv\fP(). Another would be from a configuration
file.
.LP
This volume of IEEE\ Std\ 1003.1-2001 specifies that memory objects
have initial contents of zero when created. This is
consistent with current behavior for both files and newly allocated
memory. For those implementations that use physical memory, it
would be possible that such implementations could simply use available
memory and give it to the process uninitialized. This,
however, is not consistent with standard behavior for the uninitialized
data area, the stack, and of course, files. Finally, it is
highly desirable to set the allocated memory to zero for security
reasons. Thus, initializing memory objects to zero is
required.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIclose\fP() , \fIdup\fP() , \fIexec\fP() , \fIfcntl\fP() , \fImmap\fP()
, \fIshmat\fP() , \fIshmctl\fP() , \fIshmdt\fP() , \fIshm_unlink\fP()
, \fIumask\fP() , the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<fcntl.h>\fP, \fI<sys/mman.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .