SHMGET

Section: Linux Programmer's Manual (2)
Updated: 1993-11-28
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NAME

shmget - allocates a shared memory segment

SYNOPSIS

#include <sys/ipc.h>

#include <sys/shm.h>

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

DESCRIPTION

shmget() returns the identifier of the shared memory segment associated to the value of the argument key. A new shared memory segment, with size equal to the round up of size to a multiple of PAGE_SIZE, is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE, no shared memory segment is associated to key, and IPC_CREAT is asserted in shmflg (i.e. shmflg&IPC_CREAT isn't zero). The presence in

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, check to see if the user has permission to receive the shmid associated with the segment, and ensure the segment is not marked for destruction.
IPC_EXCL: used with IPC_CREAT to ensure failure if the segment exists.
mode_flags (lowest 9 bits): specifying the permissions granted to the owner, group, and world. Presently, the execute permissions are not used by the system.

If a new segment is created, the access permissions from shmflg are copied into the shm_perm member of the shmid_ds structure that defines the segment. The shmid_ds structure:

struct shmid_ds {
        struct  ipc_perm shm_perm;      /* operation perms */
        int     shm_segsz;              /* size of segment (bytes) */
        time_t  shm_atime;              /* last attach time */
        time_t  shm_dtime;              /* last detach time */
        time_t  shm_ctime;              /* last change time */
        unsigned short  shm_cpid;       /* pid of creator */
        unsigned short  shm_lpid;       /* pid of last operator */
        short   shm_nattch;             /* no. of current attaches */
};

struct ipc_perm {
  key_t  key;
  ushort uid;   /* owner euid and egid */
  ushort gid;
  ushort cuid;  /* creator euid and egid */
  ushort cgid;
  ushort mode;  /* lower 9 bits of shmflg */
  ushort seq;   /* sequence number */
};

Furthermore, while creating, the system call initializes the system shared memory segment data structure shmid_ds 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 lowest order 9 bits of shm_perm.mode are set to the lowest order 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 access permissions are verified, and a check is made to see if it is marked for destruction.

SYSTEM CALLS

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:

EINVAL: is returned if 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.
EEXIST: is returned if IPC_CREAT | IPC_EXCL was specified and the segment exists.
EIDRM: is returned if the segment is marked as destroyed, or was removed.
ENOSPC: is returned if all possible shared memory id's have been taken (SHMMNI) or if allocating a segment of the requested size would cause the system to exceed the system-wide limit on shared memory (SHMALL).
ENOENT: is returned if no segment exists for the given key, and IPC_CREAT was not specified.
EACCES: is returned if the user does not have permission to access the shared memory segment.
ENOMEM: is returned if no memory could be allocated for segment overhead.

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 lowest order 9 bits of shmflg and creates a new shared memory segment (on success).

The followings are limits on shared memory segment resources affecting a shmget call:

SHMALL: System wide maximum of shared memory pages: policy dependent.
SHMMAX: Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M).
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 in Linux 2.2 kernels).

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

BUGS

Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated shared memory segment.

As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a shared memory segment. Asserting both IPC_CREAT and IPC_EXCL in shmflg only ensures (on success) that a new shared memory segment will be created, it doesn't imply exclusive access to the segment.

CONFORMING TO

SVr4, SVID. SVr4 documents an additional error condition EEXIST. Neither SVr4 nor SVID documents an EIDRM condition.

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Time: 01:58:18 GMT, January 21, 2025