SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)

       shmget - allocates a shared memory segment

       #include <sys/ipc.h>

       #include <sys/shm.h>

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

       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 cre-
       ated 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 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, 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.

       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).

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

       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 cre-
                   ated, 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

       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 suc-

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

       SHMALL     System  wide  maximum  of shared memory pages: policy depen-

       SHMMAX     Maximum size in bytes for a shared memory segment: implemen-
                  tation dependent (currently 4M).

       SHMMIN     Minimum size in bytes for a shared memory segment: implemen-
                  tation dependent (currently 1 byte, though PAGE_SIZE is  the
                  effective minimum size).

       SHMMNI     System wide maximum number of shared memory segments: imple-
                  mentation dependent (currently 4096, was  128  before  Linux

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

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

       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  seg-

       SVr4, SVID.  SVr4 documents an additional error condition EEXIST.  Nei-
       ther SVr4 nor SVID documents an EIDRM condition.

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

Linux 0.99.11                     1993-11-28                         SHMGET(2)