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    Command:

    semget

    
    
    

    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 System V 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  mean-
           ingful  for  semaphores, and write permissions mean permission 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 initial-
           ize the semaphores to the desired values.
    
           When creating a new semaphore set, semget() initializes the set's asso-
           ciated data structure, semid_ds (see semctl(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).
    
           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 corre-
                  sponding  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

           The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on  Linux
           or by any version of POSIX.  However, some old implementations required
           the inclusion of these header files, and the SVID also documented their
           inclusion.   Applications  intended  to be portable to such old systems
           may need to include these header files.
    
           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 sig-
           nificant 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 modified via the second field
                  of /proc/sys/kernel/sem).  Values greater than SEMMSL  *  SEMMNI
                  makes it irrelevant.
    
    
    

    BUGS

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

    Linux 2012-05-31 SEMGET(2)

    
    
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