SEMCTL(2)

NAME
semctl - semaphore control operations

SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
/* union semun is defined by including <sys/sem.h> */
#else
/* according to X/OPEN we have to define it ourselves */
union semun {
        int val;                    /* value for SETVAL */
        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
        unsigned short int *array;  /* array for GETALL, SETALL */
        struct seminfo *__buf;      /* buffer for IPC_INFO */
};
#endif

int semctl (int semid, int semnum, int cmd, union semun arg)

DESCRIPTION
The function performs the control operation specified by cmd on the semaphore set (or on the semnum-th semaphore of the set) identified by semid. The first semaphore of the set is indicated by a value 0 for semnum.

Legal values for cmd are

IPC_STAT
    Copy info from the semaphore set data structure into the structure pointed to by arg.buf. The argument semnum is ignored. The calling process must have read access privileges on the semaphore set.

IPC_SET
    Write the values of some members of the semid_ds structure pointed to by arg.buf to the semaphore set data structure, updating also its sem_ctime member. Considered members from the user supplied struct semid_ds pointed to by arg.buf are

    	sem_perm.uid
    	sem_perm.gid
    	sem_perm.mode	/* only lowest 9-bits */

    The calling process effective user-ID must be one among super-user, creator or owner of the semaphore set. The argument semnum is ignored.

IPC_RMID
    Remove immediately the semaphore set and its data structures awakening all waiting processes (with an error return and errno set to EIDRM). The calling process effective user-ID must be one among super-user, creator or owner of the semaphore set. The argument semnum is ignored.

GETALL
    Return semval for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have read access privileges on the semaphore set.

GETNCNT
    The system call returns the value of semncnt for the semnum-th semaphore of the set (i.e. the number of processes waiting for an increase of semval for the semnum-th semaphore of the set). The calling process must have read access privileges on the semaphore set.

GETPID
    The system call returns the value of sempid for the semnum-th semaphore of the set (i.e. the pid of the process that executed the last semop call for the semnum-th semaphore of the set). The calling process must have read access privileges on the semaphore set.

GETVAL
    The system call returns the value of semval for the semnum-th semaphore of the set. The calling process must have read access privileges on the semaphore set.

GETZCNT
    The system call returns the value of semzcnt for the semnum-th semaphore of the set (i.e. the number of processes waiting for semval of the semnum-th semaphore of the set to become 0). The calling process must have read access privileges on the semaphore set.

SETALL
    Set semval for all semaphores of the set using arg.array, updating also the sem_ctime member of the semid_ds structure associated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the wait queue are awakened if some semval becomes 0 or increases. The argument semnum is ignored. The calling process must have alter access privileges on the semaphore set.

SETVAL
    Set the value of semval to arg.val for the semnum-th semaphore of the set, updating also the sem_ctime member of the semid_ds structure associated to the set. Undo entry is cleared for altered semaphore in all processes. Processes sleeping on the wait queue are awakened if semval becomes 0 or increases. The calling process must have alter access privileges on the semaphore set.

RETURN VALUE
On fail the system call returns -1 with errno indicating the error. Otherwise the system call returns a nonnegative value depending on cmd as follows:

GETNCNT
    the value of semncnt.

GETPID
    the value of sempid.

GETVAL
    the value of semval.

GETZCNT
    the value of semzcnt.

ERRORS
For a failing return, errno will be set to one among the following values:

EACCES
    The calling process has no access permissions needed to execute cmd.

EFAULT
    The address pointed to by arg.buf or arg.array isn't accessible.

EIDRM
    The semaphore set was removed.

EINVAL
    Invalid value for cmd or semid.

EPERM
    The argument cmd has value IPC_SET or IPC_RMID but the calling process effective user-ID has insufficient privileges to execute the command.

ERANGE
    The argument cmd has value SETALL or SETVAL and the value to which semval has to be set (for some semaphore of the set) is less than 0 or greater than the implementation value SEMVMX.

NOTES
The IPC_INFO, SEM_STAT and SEM_INFO control calls are used by the ipcs(8) program to provide information on allocated resources. In the future these can be modified as needed or moved to a proc file system interface.

Various fields in a struct semid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)

The following system limit on semaphore sets affects a semctl call:

SEMVMX
    Maximum value for semval: implementation dependent (32767).

CONFORMING TO
SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.

SEE ALSO
ipc(5), shmget(2), shmat(2), shmdt(2)