SEMCTL(3POSIX) POSIX Programmer's Manual SEMCTL(3POSIX)

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.

semctl — XSI semaphore control operations

#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);

The semctl() 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 semctl() function provides a variety of semaphore control operations as specified by cmd. The fourth argument is optional and depends upon the operation requested. If required, it is of type union semun, which the application shall explicitly declare:

union semun {
    int val;
    struct semid_ds *buf;
    unsigned short  *array;
} arg;

Each operation shall be performed atomically.

The following semaphore control operations as specified by cmd are executed with respect to the semaphore specified by semid and semnum. The level of permission required for each operation is shown with each command; see Section 2.7, XSI Interprocess Communication. The symbolic names for the values of cmd are defined in the <sys/sem.h> header:

Return the value of semval; see <sys/sem.h>. Requires read permission.
Set the value of semval to arg.val, where arg is the value of the fourth argument to semctl(). When this command is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared. Also, the sem_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description. Requires alter permission; see Section 2.7, XSI Interprocess Communication.
Return the value of sempid. Requires read permission.
Return the value of semncnt. Requires read permission.
Return the value of semzcnt. Requires read permission.

The following values of cmd operate on each semval in the set of semaphores:

Return the value of semval for each semaphore in the semaphore set and place into the array pointed to by arg.array, where arg is the fourth argument to semctl(). Requires read permission.
Set the value of semval for each semaphore in the semaphore set according to the array pointed to by arg.array, where arg is the fourth argument to semctl(). When this command is successfully executed, the semadj values corresponding to each specified semaphore in all processes are cleared. Also, the sem_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description. Requires alter permission.

The following values of cmd are also available:

Place the current value of each member of the semid_ds data structure associated with semid into the structure pointed to by arg.buf, where arg is the fourth argument to semctl(). The contents of this structure are defined in <sys/sem.h>. Requires read permission.
Set the value of the following members of the semid_ds data structure associated with semid to the corresponding value found in the structure pointed to by arg.buf, where arg is the fourth argument to semctl():

sem_perm.uid
sem_perm.gid
sem_perm.mode

The mode bits specified in Section 2.7.1, IPC General Description are copied into the corresponding bits of the sem_perm.mode associated with semid. The stored values of any other bits are unspecified. The sem_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

This command can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of sem_perm.cuid or sem_perm.uid in the semid_ds data structure associated with semid.

Remove the semaphore identifier specified by semid from the system and destroy the set of semaphores and semid_ds data structure associated with it. This command can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of sem_perm.cuid or sem_perm.uid in the semid_ds data structure associated with semid.

If successful, the value returned by semctl() depends on cmd as follows:

The value of semval.
The value of sempid.
The value of semncnt.
The value of semzcnt.
0.

Otherwise, semctl() shall return -1 and set errno to indicate the error.

The semctl() function shall fail if:

Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication.
The value of semid is not a valid semaphore identifier, or the value of semnum is less than 0 or greater than or equal to sem_nsems, or the value of cmd is not a valid command.
The argument cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.
The argument cmd is equal to SETVAL or SETALL and the value to which semval is to be set is greater than the system-imposed maximum.

The following sections are informative.

Refer to semop().

The fourth parameter in the SYNOPSIS section is now specified as "..." in order to avoid a clash with the ISO C standard when referring to the union semun (as defined in Issue 3) and for backwards-compatibility.

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.

None.

Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, semget(), semop(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()

The Base Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore, <sys_sem.h>

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 .

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