Ubuntu Manpage: semget - get a System V semaphore set identifier

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. 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 permission to alter semaphore
values).

When creating a new semaphore set, semget() initializes the set's associated 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 initialize 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 structure 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 semaphores 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.

COLOPHON

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