Ubuntu Manpage: shmget - allocates a System V shared memory segment

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NAME

       shmget - allocates a System V 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 System V 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:

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 Linux 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 initialized to zero values, and its
associated data structure, shmid_ds (see shmctl(2)), is initialized as follows:

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.

RETURN VALUE

       On  success, a valid shared memory identifier is returned.  On errir, -1 is returned, and errno is set to
       indicate the error.

ERRORS

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

       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).

CONFORMING TO

       SVr4, POSIX.1-2001.

       SHM_HUGETLB is a nonportable Linux extension.

NOTES

The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version of POSIX.
However, some old implementations required the inclusion of these header files, and the SVID also
documented their inclusion. Applications intended to be portable to such old systems may need to include
these header files.

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:

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).

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

BUGS

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2013-04-19                                          SHMGET(2)