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Man Pages


Manual Reference Pages  -  SEMCTL (2)

NAME

semctl - control operations on a semaphore set

CONTENTS

Library
Synopsis
Description
Return Values
Errors
See Also
Bugs

LIBRARY


.Lb libc

SYNOPSIS


.In sys/types.h
.In sys/ipc.h
.In sys/sem.h int semctl int semid int semnum int cmd ...

DESCRIPTION

The semctl system call performs the operation indicated by cmd on the semaphore set indicated by semid. A fourth argument, a union semun arg, is required for certain values of cmd. For the commands that use the arg argument, union semun is defined as follows:
union semun {
        int     val;            /* value for SETVAL */
        struct  semid_ds *buf;  /* buffer for IPC_STAT & IPC_SET */
        u_short *array;         /* array for GETALL & SETALL */
};

Commands are performed as follows:
IPC_STAT Fetch the semaphore set’s struct semid_ds, storing it in the memory pointed to by arg.buf.
IPC_SET Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode members of the semaphore set’s struct semid_ds to match those of the struct pointed to by arg.buf. The calling process’s effective uid must match either sem_perm.uid or sem_perm.cuid, or it must have superuser privileges.
IPC_RMID Immediately removes the semaphore set from the system. The calling process’s effective uid must equal the semaphore set’s sem_perm.uid or sem_perm.cuid, or the process must have superuser privileges.
GETVAL Return the value of semaphore number semnum.
SETVAL Set the value of semaphore number semnum to arg.val. Outstanding adjust on exit values for this semaphore in any process are cleared.
GETPID Return the pid of the last process to perform an operation on semaphore number semnum.
GETNCNT Return the number of processes waiting for semaphore number semnum ’s value to become greater than its current value.
GETZCNT Return the number of processes waiting for semaphore number semnum ’s value to become 0.
GETALL Fetch the value of all of the semaphores in the set into the array pointed to by arg.array.
SETALL Set the values of all of the semaphores in the set to the values in the array pointed to by arg.array. Outstanding adjust on exit values for all semaphores in this set, in any process are cleared.

The
.Vt struct semid_ds is defined as follows:

struct semid_ds {
        struct  ipc_perm sem_perm;      /* operation permission struct */
        struct  sem *sem_base;  /* pointer to first semaphore in set */
        u_short sem_nsems;      /* number of sems in set */
        time_t  sem_otime;      /* last operation time */
        time_t  sem_ctime;      /* last change time */
                                /* Times measured in secs since */
                                /* 00:00:00 GMT, Jan. 1, 1970 */
};

RETURN VALUES

On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT, semctl returns the corresponding value; otherwise, 0 is returned. On failure, -1 is returned, and errno is set to indicate the error.

ERRORS

The semctl system call will fail if:
[EINVAL]
  No semaphore set corresponds to semid.
[EINVAL]
  The semnum argument is not in the range of valid semaphores for given semaphore set.
[EPERM]
  The calling process’s effective uid does not match the uid of the semaphore set’s owner or creator.
[EACCES]
  Permission denied due to mismatch between operation and mode of semaphore set.
[ERANGE]
  SETVAL or SETALL attempted to set a semaphore outside the allowable range [0 .. SEMVMX].

SEE ALSO

semget(2), semop(2)

BUGS

SETALL may update some semaphore elements before returning an error.
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