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SEMGET(2)                     System Calls Manual                    SEMGET(2)



NAME
       semget - get set of semaphores

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/sem.h>&gt;

       int semget(key, nsems, semflg)
       key_t key;
       int nsems, semflg;

DESCRIPTION
       semget() returns the semaphore identifier associated with key.

       A semaphore identifier and associated data structure and set containing
       nsems semaphores (see intro(2)) are created for key if one of the  fol-
       lowing are true:

       o  key is equal to IPC_PRIVATE.

       o  key does not already have a semaphore identifier associated with it,
          and (semflg & IPC_CREAT) is ``true''.

       Upon creation, the data structure associated  with  the  new  semaphore
       identifier is initialized as follows:

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

       o  The low-order 9 bits of sem_perm.mode are set equal to the low-order
          9 bits of semflg.

       o  sem_nsems is set equal to the value of nsems.

       o  sem_otime is set equal to 0 and sem_ctime is set equal to  the  cur-
          rent time.

       A  semaphore identifier (semid) is a unique positive integer created by
       a semget(2) system call.  Each semid has a set of semaphores and a data
       structure  associated  with  it.   The data structure is referred to as
       semid_ds and contains the following members:

              struct  ipc_perm sem_perm;  /* operation permission struct */
              ushort  sem_nsems;          /* number of sems in set */
              time_t  sem_otime;          /* last operation time */
              time_t  sem_ctime;          /* last change time */
                                          /* Times measured in secs since */
                                          /* 00:00:00 GMT, Jan. 1, 1970 */

       sem_perm is an ipc_perm structure that specifies the  semaphore  opera-
       tion  permission  (see  below).   This structure includes the following
       members:

              ushort  cuid;               /* creator user id */
              ushort  cgid;               /* creator group id */
              ushort  uid;                /* user id */
              ushort  gid;                /* group id */
              ushort  mode;               /* r/a permission */

       The value of sem_nsems is equal to the number of semaphores in the set.
       Each  semaphore in the set is referenced by a positive integer referred
       to as a sem_num.  sem_num values run sequentially from 0 to  the  value
       of sem_nsems minus 1.  sem_otime is the time of the last semop(2) oper-
       ation, and sem_ctime is the time of the last semctl(2)  operation  that
       changed a member of the above structure.

       A semaphore is a data structure that contains the following members:

              ushort  semval;             /* semaphore value */
              short   sempid;             /* pid of last operation  */
              ushort  semncnt;            /* # awaiting semval >&gt; cval */
              ushort  semzcnt;            /* # awaiting semval = 0 */

       semval is a non-negative integer.  sempid is equal to the process ID of
       the last process that performed a semaphore  operation  on  this  sema-
       phore.   semncnt  is  a  count of the number of processes that are cur-
       rently suspended awaiting this semaphore's  semval  to  become  greater
       than  its current value.  semzcnt is a count of the number of processes
       that are currently suspended awaiting this semaphore's semval to become
       zero.

RETURN VALUES
       semget()  returns  a  non-negative semaphore identifier on success.  On
       failure, it returns -1 and sets errno to indicate the error.

ERRORS
       EACCES         A semaphore identifier exists  for  key,  but  operation
                      permission  (see intro(2)) as specified by the low-order
                      9 bits of semflg would not be granted.

       EEXIST         A semaphore identifier exists for key but  (  (semflg  &
                      IPC_CREAT) and (semflg & IPC_EXCL)) is ``true''.

       EINVAL         nsems  is  either  less than or equal to zero or greater
                      than the system-imposed limit.

                      A semaphore identifier exists for key, but the number of
                      semaphores  in  the  set associated with it is less than
                      nsems and nsems is not equal to zero.

       ENOENT         A semaphore identifier does not exist for key and  (sem-
                      flg & IPC_CREAT) is ``false''.

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

                      A  semaphore identifier is to be created but the system-
                      imposed limit on the maximum  number  of  allowed  sema-
                      phores system wide would be exceeded.

SEE ALSO
       ipcrm(1), ipcs(1), intro(2), semctl(2), semop(2)



                                21 January 1990                      SEMGET(2)