shmget(2)                                                            shmget(2)


NAME
     shmget - get shared memory segment identifier

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

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

DESCRIPTION
     shmget returns the shared memory identifier associated with key.

     A shared memory identifier and associated data structure and shared
     memory segment of at least size bytes [see intro(2)] are created for key
     if one of the following are true:

          key is equal to IPC_PRIVATE.

          key does not already have a shared memory identifier associated with
          it, and (shmflg&IPC_CREAT) is true.

     Upon creation, the contents of the shared memory segment are initialized
     to zero, and the data structure associated with the new shared memory
     identifier is initialized as follows:

          shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set
          equal to the effective user ID and effective group ID, respectively,
          of the calling process.

          The access permission bits of shm_perm.mode are set equal to the
          access permission bits of shmflg.  shm_segsz is set equal to the
          value of size.

          shm_lpid, shm_nattch shm_atime, and shm_dtime are set equal to 0.

          shm_ctime is set equal to the current time.


          If the shmflg IPC_AUTORESRV is true, logical swap space is
          automatically reserved as each page is first referenced with a load
          or store operation instead of when the shared memory segment is
          created.  When using this option, no logical swap space is reserved
          when the shared memory segment is created.  Therefore, the system
          cannot guarantee that space will be available when needed.  If all
          the logical swap space has been taken by other processes when a page
          in an IPC_AUTORESRV segment is first stored to, then the process
          will be sent a SIGBUS.  (See swap(1m) for a discussion of logical
          swap space.)


     shmget fails if one or more of the following are true:

     EINVAL         size is less than the system-imposed minimum or greater
                    than the system-imposed maximum.

     EACCES         A shared memory identifier exists for key but operation
                    permission [see intro(2)] as specified by the low-order 9
                    bits of shmflg would not be granted.

     EINVAL         A shared memory identifier exists for key but the size of
                    the segment associated with it is less than size and size
                    is not equal to zero.

     ENOENT         A shared memory identifier does not exist for key and
                    (shmflg&IPC_CREAT) is false.

     ENOSPC         A shared memory identifier is to be created but the
                    system-imposed limit on the maximum number of allowed
                    shared memory identifiers system wide would be exceeded.

     ENOMEM         A shared memory identifier and associated shared memory
                    segment are to be created but the amount of available
                    memory is not sufficient to fill the request.

     EEXIST         A shared memory identifier exists for key but both
                    (shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.

SEE ALSO
     intro(2), shmctl(2), shmop(2), stdipc(3C), ipcs(1), ipcrm(1), swap(1m).

DIAGNOSTICS
     Upon successful completion, a non-negative integer, namely a shared
     memory identifier is returned.  Otherwise, a value of -1 is returned and
     errno is set to indicate the error.

NOTES
     The user must explicitly remove shared memory segments after the last
     reference to them has been removed.


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