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SHM_OPEN(3) Linux Programmer’s Manual SHM_OPEN(3)
NAME
shm_open, shm_unlink - Create/open or unlink POSIX shared memory objects
SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t mode);
int shm_unlink(const char *name);
DESCRIPTION
shm_open creates and opens a new, or opens an existing, POSIX shared memory object.
A POSIX shared memory object is in effect a handle which can be used by unrelated
processes to mmap(2) the same region of shared memory. The shm_unlink function
performs the converse operation, removing an object previously created by shm_open.
The operation of shm_open is analogous to that of open(2). name specifies the
shared memory object to be created or opened. For portable use, name should have
an initial slash (/) and contain no embedded slashes.
oflag is a bit mask created by ORing together exactly one of O_RDONLY or O_RWDR and
any of the other flags listed here:
O_RDONLY Open the object for read access. A shared memory object opened in this
way can only be mmap(2)ed for read (PROT_READ) access.
O_RDWR Open the object for read-write access.
O_CREAT Create the shared memory object if it does not exist. The user and
group ownership of the object are set as for open(2), and the object’s
permission bits are set according to the low-order 9 bits of mode,
except that those bits set in the process file mode creation mask (see
umask(2)) are cleared for the new object. (A set of macro constants
which can be used to define mode is listed in open(2).)
A new shared memory object initially has zero length - the size of the
object can be set using ftruncate(2). (The newly-allocated bytes of a
shared memory object are automatically initialised to 0.)
O_EXCL If O_CREAT was also specified, and a share memory object with the given
name already exists, return an error. The check for the existence of
the object, and its creation if it does not exist, are performed atomi-
cally.
O_TRUNC If the shared memory object already exists, truncate it to zero bytes.
On successful completion shm_open returns a new file descriptor referring to the
shared memory object. This file descriptor is guaranteed to be the lowest-numbered
file descriptor not previously opened within the process. The FD_CLOEXEC flag (see
fcntl(2)) is set for the file descriptor.
The file descriptor is normally used in subsequent calls to ftruncate(2) (for a
newly-created object) and mmap(2). After a call to mmap(2) the file descriptor may
be closed without affecting the memory mapping.
The operation of shm_unlink is analogous to unlink(2): it removes a shared memory
object name, and, once all processes have unmapped the object, de-allocates and
destroys the contents of the associated memory region. After a successful
shm_unlink, attempts to shm_open an object with the same name will fail (unless
O_CREAT was specified, in which case a new, distinct object is created).
RETURN VALUE
On success, shm_open returns a non-negative file descriptor. On failure, shm_open
returns -1. shm_unlink returns 0 on success, or -1 on error.
ERRORS
On failure, errno is set to indicate the cause of the error. Values which may
appear in errno include the following:
EACCES Permission was denied to shm_open name in the specified mode, or O_TRUNC was
specified and the caller does not have write permission on the object.
EACCES Permission to shm_unlink the shared memory object was denied.
EEXIST Both O_CREAT and O_EXCL were specified to shm_open and the shared memory
object specified by name already exists.
EINVAL The name argument to shm_open was invalid.
EMFILE The process already has the maximum number of files open.
ENAMETOOLONG
The length of name exceeds PATH_MAX.
ENFILE The limit on the total number of files open on the system has been reached.
ENOENT An attempt was made to shm_open a name that did not exist, and O_CREAT was
not specified.
ENOENT An attempt was to made to shm_unlink a name that does not exist.
NOTES
These functions are provided in glibc 2.2 and later. Programs using these func-
tions must specify the -lrt flag to cc in order to link against the required
("realtime") library.
POSIX leaves the behavior of the combination of O_RDONLY and O_TRUNC unspecified.
On Linux, this will successfully truncate an existing shared memory object - this
may not be so on other Unices.
The POSIX shared memory object implementation on Linux 2.4 makes use of a dedicated
file system, which is normally mounted under /dev/shm.
CONFORMING TO
POSIX 1003.1 (2001).
SEE ALSO
mmap(2), open(2), close(2), ftruncate(2), fstat(2), fchown(2), fchmod(2), umask(2),
fcntl(2)
Linux 2.4 2002-02-22 SHM_OPEN(3)
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