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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SEMGET(3P) POSIX Programmer's Manual SEMGET(3P)
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.
semget — get set of XSI semaphores
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
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.
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.
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.
Refer to semop(3p).
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.
None.
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.
Section 2.7, XSI Interprocess Communication, Section 2.8,
Realtime, ftok(3p), semctl(3p), semop(3p), sem_close(3p),
sem_destroy(3p), sem_getvalue(3p), sem_init(3p), sem_open(3p),
sem_post(3p), sem_trywait(3p), sem_unlink(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.17,
Semaphore, sys_sem.h(0p)
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 .
IEEE/The Open Group 2017 SEMGET(3P)
Pages that refer to this page: sys_sem.h(0p), ipcs(1p), ftok(3p), sem_close(3p), semctl(3p), sem_destroy(3p), sem_getvalue(3p), semop(3p), sem_open(3p), sem_post(3p), sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p), sigaction(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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