unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Page:
Section:
Apropos / Subsearch:
optional field

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



NAME
       semget - get a semaphore set identifier

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

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

DESCRIPTION
       The  semget()  system call returns the semaphore set identifier associ-
       ated 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  porta-
       ble  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 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).

       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  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  depen-
                  dent  (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 sec-
                  ond 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.

       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 sem-
       ctl(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 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2004-05-27                         SEMGET(2)