SHM_OPEN(3) Linux Programmer's Manual SHM_OPEN(3)
shm_open, shm_unlink - Create/open or unlink POSIX shared memory objects
void * shm_open(const char *name, int oflag, mode_t mode);
int shm_inlink(const char *name);
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 con-
verse 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 mem-
ory 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 own-
ership 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
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 atomically.
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 descrip-
tor 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-cre-
ated object) and mmap(2). After a call to mmap(2) the file descriptor may be closed with-
out 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 con-
tents 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).
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.
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 speci-
fied 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.
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
ENOENT An attempt was to made to shm_unlink a name that does not exist.
These functions are provided in glibc 2.2 and later. Programs using these functions 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.
POSIX 1003.1 (2001).
mmap(2), open(2), close(2), ftruncate(2), fstat(2), fchown(2), fchmod(2), umask(2),
Linux 2.4 2002-02-22 SHM_OPEN(3)