The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


semget - get set of XSI semaphores


[XSI] [Option Start] #include <sys/sem.h>

int semget(key_t
key, int nsems, int semflg); [Option End]


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:

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


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.






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>


First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 6

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/122 is applied, updating the DESCRIPTION from ``each semaphore in the set shall not be initialized'' to ``each semaphore in the set need not be initialized''.

End of informative text.

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