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. 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.
SEE ALSO
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
svipc(7)
COLOPHON
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