SEMCTL

NAME

semctl - semaphore control operations

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION

The function semctl performs the control operation specified by cmd on the semaphore set identified by semid, or on the semnum-th semaphore of that set. (Semaphores are numbered starting at 0.)

This function has three or four arguments. When there are four, the call is semctl(semid,semnum,cmd,arg); where the fourth argument arg has a type union semun defined as follows:

#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 *array;    /* array for GETALL, SETALL */
                                /* Linux specific part: */
      struct seminfo *__buf;    /* buffer for IPC_INFO */
};
#endif

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

IPC_RMID

Immediately remove the semaphore set and its data structures awakening all waiting processes (with an error return and errno set to EIDRM). The effective user-ID of the calling process must be that of the super-user, or match the 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 entries are cleared for altered semaphores 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 failure semctl 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.

All other cmd values return 0 on success.

ERRORS

On failure, errno will be set to one of the following:

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 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).

For greater portability it is best to always call semctl with four arguments.

Under Linux, the function semctl is not a system call, but is implemented via the system call ipc(2).

CONFORMING TO

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