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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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 shall be allocated as described in Section 2.14,
File Descriptor Allocation, and can be 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. 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 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.1‐2017 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.1‐2017 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.
Section 2.14, File Descriptor Allocation, 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.1‐2017, fcntl.h(0p),
sys_mman.h(0p)
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 .
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 2017 SHM_OPEN(3P)
Pages that refer to this page: sys_mman.h(0p), shmat(3p), shmctl(3p), shmdt(3p), shmget(3p), shm_unlink(3p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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