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SHM_OPEN(3P) POSIX Programmer's Manual SHM_OPEN(3P)
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.
shm_open — open a shared memory object (REALTIME)
#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t mode);
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,
except that the interpretation of <slash> characters other than the
leading <slash> character in name is implementation-defined, and that
the length limits for the name argument are implementation-defined
and need not be the same as the pathname limits {PATH_MAX} and
{NAME_MAX}. 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.
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 shall be set to the effective group ID of the
process; however, if the name argument is visible in the
file system, the group ID may be set to the group ID of
the containing directory. 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.
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.
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 All file descriptors available to the process are currently
open.
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 shm_open() function may fail if:
ENAMETOOLONG
The length of the name argument exceeds {_POSIX_PATH_MAX} on
systems that do not support the XSI option or exceeds
{_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
that is longer than {_POSIX_NAME_MAX} on systems that do not
support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
systems.
The following sections are informative.
Creating and Mapping a Shared Memory Object
The following code segment demonstrates the use of shm_open() to
create a shared memory object which is then sized using ftruncate()
before being mapped into the process address space using mmap():
#include <unistd.h>
#include <sys/mman.h>
...
#define MAX_LEN 10000
struct region { /* Defines "structure" of shared memory */
int len;
char buf[MAX_LEN];
};
struct region *rptr;
int fd;
/* Create shared memory object and set its size */
fd = shm_open("/myregion", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
if (fd == −1)
/* Handle error */;
if (ftruncate(fd, sizeof(struct region)) == −1)
/* Handle error */;
/* Map shared memory object */
rptr = mmap(NULL, sizeof(struct region),
PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (rptr == MAP_FAILED)
/* Handle error */;
/* Now we can refer to mapped region using fields of rptr;
for example, rptr->len */
...
None.
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
POSIX.1‐2008 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 POSIX.1‐2008 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.
A future version might require the shm_open() and shm_unlink()
functions to have semantics similar to normal file system operations.
close(3p), dup(3p), exec(1p), fcntl(3p), mmap(3p), shmat(3p),
shmctl(3p), shmdt(3p), shm_unlink(3p), umask(3p)
The Base Definitions volume of POSIX.1‐2008, fcntl.h(0p),
sys_mman.h(0p)
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 .
IEEE/The Open Group 2013 SHM_OPEN(3P)
Pages that refer to this page: sys_mman.h(0p), shmat(3p), shmctl(3p), shmdt(3p), shmget(3p), shm_unlink(3p)