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NAME    [Toc]    [Back]

       shmget - allocates a shared memory segment

SYNOPSIS    [Toc]    [Back]

       #include <sys/ipc.h>

       #include <sys/shm.h>

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

DESCRIPTION    [Toc]    [Back]

       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 shm-
       flg (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

       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    [Toc]    [Back]

       fork() After a fork() the child inherits  the  attached	shared	memory

       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    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

       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

       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

NOTES    [Toc]    [Back]

       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

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

BUGS    [Toc]    [Back]

       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    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

       ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)

Linux 0.99.11			  1993-11-28			     SHMGET(2)
[ Back ]
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