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NAME
shm_open - open a shared memory object (REALTIME)
SYNOPSIS
#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t mode);
DESCRIPTION
The shm_open() 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 name 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 name argument conforms to the construction rules for a pathname. If name
begins with the slash character, then processes calling shm_open() with the same value of name refer to
the same shared memory object, as long as that name has not been removed. If name does not begin with
the slash character, the effect is implementation-defined. The interpretation of slash characters other
than the leading slash character in name is implementation-defined.
If successful, shm_open() 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.
The file status flags and file access modes of the open file description are according to the value of
oflag. The oflag argument is the bitwise-inclusive OR of the following flags defined in the <fcntl.h>
header. Applications specify exactly one of the first two values (access modes) below in the value of
oflag:
O_RDONLY
Open for read access only.
O_RDWR Open for read or write access.
Any combination of the remaining flags may be specified in the value of oflag:
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 mode argument except those set in the file
mode creation mask of the process. When bits in mode other than the file permission bits are set,
the effect is unspecified. The mode 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.
O_EXCL If O_EXCL and O_CREAT are set, shm_open() 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 shm_open() 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.
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.
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.
RETURN VALUE
Upon successful completion, the shm_open() function shall return a non-negative integer representing the
lowest numbered unused file descriptor. Otherwise, it shall return -1 and set errno to indicate the
error.
ERRORS
The shm_open() function shall fail if:
EACCES The shared memory object exists and the permissions specified by oflag 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.
EEXIST O_CREAT and O_EXCL are set and the named shared memory object already exists.
EINTR The shm_open() operation was interrupted by a signal.
EINVAL The shm_open() operation is not supported for the given name.
EMFILE Too many file descriptors are currently in use by this process.
ENAMETOOLONG
The length of the name argument exceeds {PATH_MAX} or a pathname component is longer than
{NAME_MAX}.
ENFILE Too many shared memory objects are currently open in the system.
ENOENT O_CREAT is not set and the named shared memory object does not exist.
ENOSPC There is insufficient space for the creation of the new shared memory object.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
When the Memory Mapped Files option is supported, the normal open() call is used to obtain a descriptor
to a file to be mapped according to existing practice with mmap(). When the Shared Memory Objects option
is supported, the shm_open() function shall obtain a descriptor to the shared memory object to be mapped.
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 open() or creat() and in some cases returned by alternate functions,
such as pipe(). The latter technique is used by shm_open().
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 ftruncate(). The shm_open() 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.
On implementations where memory objects are implemented using the existing file system, the shm_open()
function may be implemented using a macro that invokes open(), and the shm_unlink() function may be
implemented using a macro that invokes unlink().
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.
On implementations that choose to implement memory objects using memory directly, a shm_open() followed
by an ftruncate() and close() 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.
The set of valid open flags to shm_open() 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.
The error conditions [EACCES] and [ENOTSUP] are provided to inform the application that the
implementation cannot complete a request.
[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.
[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.
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 shm_open() 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
getenv(). Another would be from a configuration file.
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.
FUTURE DIRECTIONS
None.
SEE ALSO
close() , dup() , exec() , fcntl() , mmap() , shmat() , shmctl() , shmdt() , shm_unlink() , umask() , the
Base Definitions volume of IEEE Std 1003.1-2001, <fcntl.h>, <sys/mman.h>
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 .