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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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SEMGET(2) Linux Programmer's Manual SEMGET(2)
semget - get a System V semaphore set identifier
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
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.
If successful, the return value will be the semaphore set identifier
(a nonnegative integer), otherwise, -1 is returned, with errno
indicating the error.
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.
SVr4, POSIX.1-2001.
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.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would
more clearly show its function.
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
svipc(7)
This page is part of release 4.15 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 2017-09-15 SEMGET(2)
Pages that refer to this page: ipcrm(1), ipcs(1), lsipc(1), pcp-ipcs(1), ipc(2), semctl(2), semop(2), syscalls(2), umask(2), ftok(3), sem_overview(7), svipc(7)
Copyright and license for this manual page
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