SHMGET(2)

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 SHMMIN > size or size > SHMMAX, or size is greater than size of 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).

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

BUGS
Use of IPC_PRIVATE don't inhibits 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.

SEE ALSO
ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2).