semctl

Name

semctl - semaphore control operations

Library

libc.lib

Synopsis

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

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

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.

Detailed 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.
`IPC_RMID`	Immediately removes the semaphore set from the system. The calling process’s uid must equal the semaphore set’s sem_perm.uid or sem_perm.cuid,
`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 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 */
        long    sem_pad1;       /* SVABI/386 says I need this here */
        time_t  sem_ctime;      /* last change time */
                                /* Times measured in secs since */
                                /* 00:00:00 GMT, Jan. 1, 1970 */
        long    sem_pad2;       /* SVABI/386 says I need this here */
        long    sem_pad3[4];    /* SVABI/386 says I need this here */
};

Examples

#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
#include <errno.h>

         
#define SEM_SET_KEY 1000
#define NO_OF_SEMAPHORES 2

         
int main(void)
{
    int sem_set_id;
    union semun sem_val;
    /*
     * Create 2 semaphores in a set, with access only to
     * the owner
     */
    if((sem_set_id = semget(SEM_SET_KEY, NO_OF_SEMAPHORES, IPC_CREAT | 0600))
        == -1) {
       printf("Semaphore creation failed with errno %d\n", errno);
       return -1;
    }
    /*
     * Initialize the first semaphore in our set to 1
     */
    sem_val.array = NULL;
    sem_val.buf = NULL;
    sem_val.val = 1;
    if(semctl(sem_set_id, 0, SETVAL, sem_val) == -1) {
       printf("Could not initialize first semaphore (errno %d)\n", errno);
       return -1;
    }
    /*
     * Initialize the second semaphore in our set to 0
     */
    sem_val.val = 0;
    if(semctl(sem_set_id, 1, SETVAL, sem_val) == -1) {
       printf("Could not initialize second semaphore (errno %d)\n", errno);
       return -1;
    }
    /*
     * Delete the semaphore set
     */
    if(semctl(sem_set_id, NO_OF_SEMAPHORES, IPC_RMID) == -1) {
       printf("Could not delete semaphore set (errno %d)\n", errno);
       return -1;
    }
    return 0;
}

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 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.`]

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