PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       semget — get set of XSI semaphores

SYNOPSIS

       #include <sys/sem.h>

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

DESCRIPTION

       The  semget()  function  operates  on  XSI  semaphores (see the Base Definitions volume of
       POSIX.1‐2017,  Section  4.17,  Semaphore).   It  is  unspecified  whether  this   function
       interoperates  with  the realtime interprocess communication facilities defined in Section
       2.8, Realtime.

       The semget() function shall return the semaphore identifier associated with key.

       A semaphore identifier with its associated semid_ds data structure and its associated  set
       of nsems semaphores (see <sys/sem.h>) is created for key if one of the following is true:

        *  The argument key is equal to IPC_PRIVATE.

        *  The  argument  key does not already have a semaphore identifier associated with it and
           (semflg &IPC_CREAT) is non-zero.

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

        *  In the operation permissions structure sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and
           sem_perm.gid  shall  be  set  to  the  effective  user  ID  and  effective  group  ID,
           respectively, of the calling process.

        *  The low-order 9 bits of sem_perm.mode shall be set to the low-order 9 bits of semflg.

        *  The variable sem_nsems shall be set to the value of nsems.

        *  The  variable  sem_otime  shall  be set to 0 and sem_ctime shall be set to the current
           time, as described in Section 2.7.1, IPC General Description.

        *  The data structure associated with each semaphore in the set need not be  initialized.
           The semctl() function with the command SETVAL or SETALL can be used to initialize each
           semaphore.

RETURN VALUE

       Upon successful completion,  semget()  shall  return  a  non-negative  integer,  namely  a
       semaphore identifier; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

       The semget() function shall fail if:

       EACCES A semaphore identifier exists for key, but operation permission as specified by the
              low-order 9 bits of semflg would not be granted; see Section 2.7, XSI  Interprocess
              Communication.

       EEXIST A  semaphore  identifier  exists  for  the  argument  key  but ((semflg &IPC_CREAT)
              &&(semflg &IPC_EXCL)) is non-zero.

       EINVAL The value of nsems is either less than or equal to 0 or greater  than  the  system-
              imposed  limit,  or  a  semaphore  identifier  exists for the argument key, but the
              number of semaphores in the set associated with it is less than nsems and nsems  is
              not equal to 0.

       ENOENT A  semaphore identifier does not exist for the argument key and (semflg &IPC_CREAT)
              is equal to 0.

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

       The following sections are informative.

EXAMPLES

       Refer to semop().

APPLICATION USAGE

       The   POSIX   Realtime   Extension   defines   alternative   interfaces  for  interprocess
       communication. Application developers who need to use IPC should design their applications
       so  that  modules  using  the  IPC  routines  described  in  Section 2.7, XSI Interprocess
       Communication can be easily modified to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       A future version may require that the value of the semval, sempid,  semncnt,  and  semzcnt
       members  of  all  semaphores  in  a  semaphore  set  be initialized to zero when a call to
       semget() creates a semaphore set. Many semaphore implementations already do  this  and  it
       greatly simplifies what an application must do to initialize a semaphore set.

SEE ALSO

       Section  2.7,  XSI  Interprocess  Communication,  Section 2.8, Realtime, ftok(), semctl(),
       semop(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(),  sem_open(),  sem_post(),
       sem_trywait(), sem_unlink()

       The Base Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore, <sys_sem.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
       the Institute of Electrical and Electronics Engineers, Inc and The  Open  Group.   In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .