SHM_OPEN(3C) Standard C Library Functions SHM_OPEN(3C)


shm_open - open a shared memory object


#include <sys/mman.h>

int shm_open(const char *name, int oflag, mode_t mode);


The shm_open() function establishes a connection between a shared memory
object and a file descriptor. It creates 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. The first character of name must be a
slash (/) character and the remaining characters of name cannot include
any slash characters. For maximum portability, name should include no
more than 14 characters, but this limit is not enforced.

If successful, shm_open() returns 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 header <fcntl.h>.
Applications specify exactly one of the first two values (access modes)
below in the value of oflag:

Open for read access only.

Open for read or write access.

Any combination of the remaining flags may be specified in the value of

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 will be set to the effective user ID of the process;
the group ID of the shared memory object will be set to a
system default group ID or to the effective group ID of the
process. The permission bits of the shared memory object will
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.

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.

If the shared memory object exists, and it is successfully
opened O_RDWR, the object will be truncated to zero length
and the mode and owner will 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 returns a non-
negative integer representing the lowest numbered unused file descriptor.
Otherwise, it returns -1 and sets errno to indicate the error condition.


The shm_open() function will fail if:

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.

O_CREAT and O_EXCL are set and the named shared memory
object already exists.

The shm_open() operation was interrupted by a signal.

The shm_open() operation is not supported for the given

Too many file descriptors are currently in use by this

The length of the name string exceeds PATH_MAX, or a
pathname component is longer than NAME_MAX while
_POSIX_NO_TRUNC is in effect.

Too many shared memory objects are currently open in the

O_CREAT is not set and the named shared memory object
does not exist.

There is insufficient space for the creation of the new
shared memory object.

The shm_open() function is not supported by the system.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Committed |
|MT-Level | MT-Safe |
|Standard | See standards(7). |


close(2), dup(2), exec(2), fcntl(2), mmap(2), umask(2), shm_unlink(3C),
sysconf(3C), fcntl.h(3HEAD), attributes(7), standards(7)


Solaris 2.6 was the first release to support the Asynchronous Input and
Output option. Prior to this release, this function always returned -1
and set errno to ENOSYS.

February 5, 2008 SHM_OPEN(3C)