SEMCTL(3) Library Functions Manual SEMCTL(3)

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 XBD V1_chap04(7)). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in V2_chap02(3).

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 , 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 . The level of permission required for each operation is shown with each command; see V2_chap02(3). 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 2.7.1 IPC General Description . Requires alter permission; see 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 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 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 V2_chap02(3) are copied into the corresponding bits of the associated with semid. The stored values of any other bits are unspecified. The timestamp shall be set to the current time, as described in V2_chap02(3).

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 or in the 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 to indicate the error.

The semctl () function shall fail if:

Operation permission is denied to the calling process; see 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.

Refer to semop(3).

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 (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 V2_chap02(3) can be easily modified to use the alternative interfaces.

None.

None.

V2_chap02(3), V2_chap02(3), semget(3), semop(3), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_trywait(3), sem_unlink(3)

XBD V1_chap04(7), <sys/sem.h>

First released in Issue 2. Derived from Issue 2 of the SVID.

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to the APPLICATION USAGE section.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0532 [345], XSH/TC1-2008/0533 [345], XSH/TC1-2008/0534 [345], and XSH/TC1-2008/0535 [335] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0319 [532] is applied.

June 14, 2024 posix.fail