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# shmget() - Unix, Linux System Call

# NAME

shmget - allocates a shared memory segment

# SYNOPSIS

#include <sys/ipc.h>

#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);

# DESCRIPTION

shmget() returns the identifier of the shared memory segment associated with the value of the argument key. A new shared memory segment, with size equal to the value of size rounded up to a multiple of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn’t IPC_PRIVATE, no shared memory segment corresponding to key exists, and IPC_CREAT is specified in shmflg.

If shmflg specifies both IPC_CREAT and IPC_EXCL and a shared memory segment already exists for key, then shmget() fails with errno set to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

The value shmflg is composed of:

Tag | Description
--- | ---
SHM_HUGETLB | used for allocating HUGETLB pages for shared memory. IPC_CREAT to create a new segment. If this flag is not used, then shmget() will find the segment associated with key and check to see if the user has permission to access the segment.
IPC_EXCL | used with IPC_CREAT to ensure failure if the segment already exists.
mode_flags | (least significant 9 bits) specifying the permissions granted to the owner, group, and world. These bits have the same format, and the same meaning, as the mode argument of open(2). Presently, the execute permissions are not used by the system.
SHM_HUGETLB (since Linux 2.6)
| Allocate the segment using "huge pages." See the kernel source file Documentation/vm/hugetlbpage.txt for further information.
SHM_NORESERVE (since Linux 2.6.15)
| This flag serves the same purpose as the mmap(2) MAP_NORESERVE flag. Do not reserve swap space for this segment. When swap space is reserved, one has the guarantee that it is possible to modify the segment. When swap space is not reserved one might get SIGSEGV upon a write if no physical memory is available. See also the discussion of the file /proc/sys/vm/overcommit_memory in proc(5).

When a new shared memory segment is created, its contents are initialised to zero values, and its associated data structure, shmid_ds (see shmctl(2)), is initialised as follows:

Tag | Description
--- | ---
| shm_perm.cuid and shm_perm.uid are set to the effective user ID of the calling process.
| shm_perm.cgid and shm_perm.gid are set to the effective group ID of the calling process.
| The least significant 9 bits of shm_perm.mode are set to the least significant 9 bit of shmflg.
| shm_segsz is set to the value of size.
| shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
| shm_ctime is set to the current time.

If the shared memory segment already exists, the permissions are verified, and a check is made to see if it is marked for destruction.

# SYSTEM CALLS

Tag | Description
--- | ---
fork() | After a fork() the child inherits the attached shared memory segments.
exec() | After an exec() all attached shared memory segments are detached (not destroyed).
exit() | Upon exit() all attached shared memory segments are detached (not destroyed).

# RETURN VALUE

A valid segment identifier, shmid, is returned on success, -1 on error.

# ERRORS

On failure, errno is set to one of the following:

Tag | Description
--- | ---
EACCES | The user does not have permission to access the shared memory segment, and does not have the CAP_IPC_OWNER capability.
EEXIST | IPC_CREAT | IPC_EXCL was specified and the segment exists.
EINVAL | A new segment was to be created and size < SHMMIN or size > SHMMAX, or no new segment was to be created, a segment with given key existed, but size is greater than the size of that segment.
ENFILE | The system limit on the total number of open files has been reached.
ENOENT | No segment exists for the given key, and IPC_CREAT was not specified.
ENOMEM | No memory could be allocated for segment overhead.
ENOSPC | All possible shared memory IDs have been taken (SHMMNI), or allocating a segment of the requested size would cause the system to exceed the system-wide limit on shared memory (SHMALL).
EPERM | The SHM_HUGETLB flag was specified, but the caller was not privileged (did not have the CAP_IPC_LOCK capability).

# NOTES

IPC_PRIVATE isn’t a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the least significant 9 bits of shmflg and creates a new shared memory segment (on success).

The following limits on shared memory segment resources affect the shmget() call:

Tag | Description
--- | ---
SHMALL | System wide maximum of shared memory pages (on Linux, this limit can be read and modified via /proc/sys/kernel/shmall).
SHMMAX | Maximum size in bytes for a shared memory segment: policy dependent (on Linux, this limit can be read and modified via /proc/sys/kernel/shmmax).
SHMMIN | Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though PAGE_SIZE is the effective minimum size).
SHMMNI | System wide maximum number of shared memory segments: implementation dependent (currently 4096, was 128 before Linux 2.3.99; on Linux, this limit can be read and modified via /proc/sys/kernel/shmmni).

The implementation has no specific limits for the per process maximum number of shared memory segments (SHMSEG).

# BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

# CONFORMING TO

SVr4, POSIX.1-2001.

# LINUX NOTES

Until version 2.3.30 Linux would return EIDRM for a shmget() on a shared memory segment scheduled for deletion.

SHM_HUGETLB is a non-portable Linux extension.

# SEE ALSO

- shmat (2)

- shmctl (2)

- shmdt (2)

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