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



NAME
       semget - get a semaphore set identifier

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

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

DESCRIPTION
       The  semget()  system call returns the semaphore set identifier associated with the argument key.  A new set of
       nsems semaphores is created if key has the value IPC_PRIVATE or if no existing semaphore set is associated with
       key and IPC_CREAT is specified in semflg.

       If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key, then semget() fails
       with errno set to EEXIST.  (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

       Upon creation, the least significant 9 bits of the argument semflg define the permissions (for owner, group and
       others)  for the semaphore set.  These bits have the same format, and the same meaning, as the mode argument of
       open(2) (though the execute permissions are not meaningful for semaphores, and write permissions  mean  permis-
       sion to alter semaphore values).

       The  values  of  the  semaphores  in  a newly created set are indeterminate.  (POSIX.1-2001 is explicit on this
       point.)  Although Linux, like many other implementations, initializes the semaphore values  to  0,  a  portable
       application cannot rely on this: it should explicitly initialize the semaphores to the desired values.

       When creating a new semaphore set, semget() initializes the set's associated data structure, semid_ds (see sem-
       ctl(2)), as follows:

              sem_perm.cuid and sem_perm.uid are set to the effective user ID of the calling process.

              sem_perm.cgid and sem_perm.gid are set to the effective group ID of the calling process.

              The least significant 9 bits of sem_perm.mode are set to the least significant 9 bits of semflg.

              sem_nsems is set to the value of nsems.

              sem_otime is set to 0.

              sem_ctime is set to the current time.

       The argument nsems can be 0 (a don't care) when a semaphore set is not being created.  Otherwise nsems must  be
       greater than 0 and less than or equal to the maximum number of semaphores per semaphore set (SEMMSL).

       If the semaphore set already exists, the permissions are verified.

RETURN VALUE
       If  successful, the return value will be the semaphore set identifier (a non-negative integer), otherwise -1 is
       returned, with errno indicating the error.

ERRORS
       On failure errno will be set to one of the following:

       EACCES A semaphore set exists for key, but the calling process does not have permission to access the set,  and
              does not have the CAP_IPC_OWNER capability.

       EEXIST A semaphore set exists for key and semflg specified both IPC_CREAT and IPC_EXCL.

       EINVAL nsems  is  less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL),
              or a semaphore set corresponding to key  already  exists,  and  nsems  is  larger  than  the  number  of
              semaphores in that set.

       ENOENT No semaphore set exists for key and semflg did not specify IPC_CREAT.

       ENOMEM A semaphore set has to be created but the system does not have enough memory for the new data structure.

       ENOSPC A semaphore set has to be created but the  system  limit  for  the  maximum  number  of  semaphore  sets
              (SEMMNI), or the system wide maximum number of semaphores (SEMMNS), would be exceeded.

CONFORMING TO
       SVr4, POSIX.1-2001.

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 least significant 9 bits of semflg and creates a new semaphore set (on success).

       The following limits on semaphore set resources affect the semget() call:

       SEMMNI System wide maximum number of semaphore sets: policy dependent (on Linux, this limit  can  be  read  and
              modified via the fourth field of /proc/sys/kernel/sem).

       SEMMSL Maximum  number  of semaphores per semid: implementation dependent (on Linux, this limit can be read and
              modified via the first field of /proc/sys/kernel/sem).

       SEMMNS System wide maximum number of semaphores: policy dependent (on Linux, this limit can be read  and  modi-
              fied via the second field of /proc/sys/kernel/sem).  Values greater than SEMMSL * SEMMNI makes it irrel-
              evant.

BUGS
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

       The semaphores in a set are not initialized by semget().  In order to initialize the semaphores, semctl(2) must
       be  used to perform a SETVAL or a SETALL operation on the semaphore set.  (Where multiple peers do not know who
       will be the first to initialize the set, checking for a non-zero sem_otime in  the  associated  data  structure
       retrieved by a semctl(2) IPC_STAT operation can be used to avoid races.)

SEE ALSO
       semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), svipc(7)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2004-05-27                         SEMGET(2)