semget (3p) - Linux Manuals

semget: get set of XSI semaphores


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


semget - get set of XSI semaphores


#include <sys/sem.h>

int semget(key_t key, int nsems, int semflg);


The semget() function operates on XSI semaphores (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Realtime .

The semget() function shall return the semaphore identifier associated with key.

A semaphore identifier with its associated semid_ds data structure and its associated set of nsems semaphores (see <sys/sem.h>) is created for key if one of the following is true:

The argument key is equal to IPC_PRIVATE.

The argument key does not already have a semaphore identifier associated with it and (semflg &IPC_CREAT) is non-zero.

Upon creation, the semid_ds data structure associated with the new semaphore identifier is initialized as follows:

In the operation permissions structure sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid shall be set equal to the effective user ID and effective group ID, respectively, of the calling process.

The low-order 9 bits of sem_perm.mode shall be set equal to the low-order 9 bits of semflg.

The variable sem_nsems shall be set equal to the value of nsems.

The variable sem_otime shall be set equal to 0 and sem_ctime shall be set equal to the current time.

The data structure associated with each semaphore in the set shall not be initialized. The semctl() function with the command SETVAL or SETALL can be used to initialize each semaphore.


Upon successful completion, semget() shall return a non-negative integer, namely a semaphore identifier; otherwise, it shall return -1 and set errno to indicate the error.


The semget() function shall fail if:

A semaphore identifier exists for key, but operation permission as specified by the low-order 9 bits of semflg would not be granted; see XSI Interprocess Communication .
A semaphore identifier exists for the argument key but ((semflg &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
The value of nsems is either less than or equal to 0 or greater than the system-imposed limit, or a semaphore identifier exists for the argument key, but the number of semaphores in the set associated with it is less than nsems and nsems is not equal to 0.
A semaphore identifier does not exist for the argument key and (semflg &IPC_CREAT) is equal to 0.
A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system-wide would be exceeded.

The following sections are informative.


Creating a Semaphore Identifier

The following example gets a unique semaphore key using the ftok() function, then gets a semaphore ID associated with that key using the semget() function (the first call also tests to make sure the semaphore exists). If the semaphore does not exist, the program creates it, as shown by the second call to semget(). In creating the semaphore for the queuing process, the program attempts to create one semaphore with read/write permission for all. It also uses the IPC_EXCL flag, which forces semget() to fail if the semaphore already exists.

After creating the semaphore, the program uses a call to semop() to initialize it to the values in the sbuf array. The number of processes that can execute concurrently without queuing is initially set to 2. The final call to semget() creates a semaphore identifier that can be used later in the program.

#include <sys/types.h>
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <pwd.h>
#include <fcntl.h>
#include <limits.h>
key_t semkey;
int semid, pfd, fv;
struct sembuf sbuf;
char *lgn;
char filename[PATH_MAX+1];
struct stat outstat;
struct passwd *pw;
/* Get unique key for semaphore. */
if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
    perror("IPC error: ftok"); exit(1);

/* Get semaphore ID associated with this key. */
if ((semid = semget(semkey, 0, 0)) == -1) {

    /* Semaphore does not exist - Create. */
    if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
        S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
        /* Initialize the semaphore. */
        sbuf.sem_num = 0;
        sbuf.sem_op = 2;  /* This is the number of runs
                             without queuing. */
        sbuf.sem_flg = 0;
        if (semop(semid, &sbuf, 1) == -1) {
            perror("IPC error: semop"); exit(1);
    else if (errno == EEXIST) {
        if ((semid = semget(semkey, 0, 0)) == -1) {
            perror("IPC error 1: semget"); exit(1);
    else {
        perror("IPC error 2: semget"); exit(1);


The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in XSI Interprocess Communication can be easily modified to use the alternative interfaces.






Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .


XSI Interprocess Communication, Realtime, semctl(), semop(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(), sem_post(), sem_unlink(), sem_wait(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/sem.h>