SEMCTL(2)

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, union semun arg);

DESCRIPTION
The semctl() system call provides a number of control operations on the semaphore specified by semnum and semid. The operation to be performed is specified in cmd (see below). arg is a union of the following fields:

        int     val;            /* value for SETVAL */
        struct  semid_ds *buf;  /* buffer for IPC_{STAT,SET} */
        u_short *array;         /* array for GETALL & SETALL */

The semid_ds structure used in the IPC_SET and IPC_STAT commands is defined as follows in <sys/sem.h>:

    struct semid_ds {
        struct ipc_perm sem_perm;  /* operation permissions */
        struct  sem *sem_base;     /* semaphore set */
        u_short sem_nsems;         /* number of sems in set */
        time_t  sem_otime;         /* last operation time */
        time_t  sem_ctime;         /* last change time */
    };

The ipc_perm structure used inside the semid_ds structure is defined in <sys/ipc.h> and looks like this:

    struct ipc_perm {
        ushort cuid; /* creator user id */
        ushort cgid; /* creator group id */
        ushort uid;	 /* user id */
        ushort gid;	 /* group id */
        ushort mode; /* r/w permission (see chmod(2)) */
        ushort seq;	 /* sequence # (to generate unique msg/sem/shm id) */
        key_t key;	 /* user specified msg/sem/shm key */
    };

semctl() provides the following operations:

GETVAL
    Return the value of the semaphore.

SETVAL
    Set the value of the semaphore to arg.val.

GETPID
    Return the pid of the last process that did an operation on this semaphore.

GETNCNT
    Return the number of processes waiting to acquire the semaphore.

GETZCNT
    Return the number of processes waiting for the value of the semaphore to reach 0.

GETALL
    Return the values of all the semaphores associated with semid.

SETALL
    Set the values of all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array.

IPC_STAT
    Gather statistics about a semaphore and place the information in the semid_ds structure pointed to by arg.buf (see above).

IPC_SET
    Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the structure associated with the semaphore. The values are taken from the corresponding fields in the structure pointed to by arg.buf. This operation can only be executed by the super-user, or a process that has an effective user id equal to either sem_perm.cuid or sem_perm.uid in the data structure associated with the message queue.

IPC_RMID
    Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the super-user or a process with an effective uid equal to the sem_perm.cuid or sem_perm.uid values in the data structure associated with the semaphore can do this.

The permission to read or change a message queue (see semop(2)) is determined by the sem_perm.mode field in the same way as is done with files (see chmod(2) ), but the effective uid can match either the sem_perm.cuid field or the sem_perm.uid field, and the effective gid can match either sem_perm.cgid or sem_perm.gid.

RETURN VALUES
For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns one of the values described above if successful. All other operations will make semctl() return 0 if no errors occur. Otherwise -1 is returned and errno set to reflect the error.

ERRORS
semctl() will fail if:

[EPERM]
    cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the sem_perm.uid or sem_perm.cuid fields of the data structure associated with the message queue.

[EACCES]
    The caller has no operation permission for this semaphore.

[EINVAL]
    semid is not a valid message semaphore identifier.

    cmd is not a valid command.

[EFAULT]
    arg.buf specifies an invalid address.

SEE ALSO
semget(2), semop(2)