semget

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

NAME
       semget - get a System V 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 System V semaphore set identifier
       associated with the argument key.  It may be used either to obtain  the
       identifier  of  a previously created semaphore set (when semflg is zero
       and key does not have the value IPC_PRIVATE), or to create a new set.

       A new set of nsems semaphores is created if key has the value  IPC_PRI-
       VATE  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 al-
       ready  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 de-
       fine 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).

       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
       nonnegative 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 in the user namespace that governs its
              IPC namespace.

       EEXIST IPC_CREAT and IPC_EXCL were specified in semflg, but a semaphore
              set already exists for key.

       EINVAL nsems  is less than 0 or greater than the limit on the number of
              semaphores per semaphore set (SEMMSL).

       EINVAL A semaphore set corresponding to key already exists,  but  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 all but the least  significant
       9 bits of semflg and creates a new semaphore set (on success).

   Semaphore initialization
       The  values of the semaphores in a newly created set are indeterminate.
       (POSIX.1-2001 and POSIX.1-2008 are explicit  on  this  point,  although
       POSIX.1-2008 notes that a future version of the standard may require an
       implementation to initialize the semaphores  to  0.)   Although  Linux,
       like many other implementations, initializes the semaphore values to 0,
       a portable application cannot rely on this: it should  explicitly  ini-
       tialize the semaphores to the desired values.

       Initialization  can be done using semctl(2) SETVAL or SETALL operation.
       Where multiple peers do not know who will be the  first  to  initialize
       the set, checking for a nonzero sem_otime in the associated data struc-
       ture retrieved by a semctl(2) IPC_STAT operation can be used  to  avoid
       races.

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

       SEMMNI System-wide limit on the number of  semaphore  sets.   On  Linux
              systems  before  version  3.19, the default value for this limit
              was 128.  Since Linux 3.19, the default  value  is  32,000.   On
              Linux,  this limit can be read and modified via the fourth field
              of /proc/sys/kernel/sem.

       SEMMSL Maximum number of semaphores per semaphore ID.  On Linux systems
              before  version  3.19, the default value for this limit was 250.
              Since Linux 3.19, the default value is 32,000.  On  Linux,  this
              limit   can  be  read  and  modified  via  the  first  field  of
              /proc/sys/kernel/sem.

       SEMMNS System-wide limit on the number of semaphores: policy  dependent
              (on  Linux,  this  limit can be read and modified via the second
              field of /proc/sys/kernel/sem).  Note that the number  of  sema-
              phores  system-wide is also limited by the product of SEMMSL and
              SEMMNI.

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

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

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2019-08-02                         SEMGET(2)