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/.