semget

Name

semget - get a semaphore set identifier

Library

libc.lib

Synopsis

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

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

Return values

The semget call returns the id of a semaphore set if successful; otherwise, -1 is returned and errno is set to indicate the error.

Detailed description

Based on the values of key and flag, semget returns the identifier of a newly created or previously existing set of semaphores. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key:

IPC_PRIVATE may be specified, in which case a new IPC object will be created.
An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created.
The ftok function may be used to generate a key from a pathname.

The mode of a newly created IPC object is determined by OR ’ing the following constants into the flag argument:

`SEM_R`	Read access for user.
`SEM_A`	Alter access for user.
`( SEM_R>>3`)
	Read access for group.
`( SEM_A>>3`)
	Alter access for group.
`( SEM_R>>6`)
	Read access for other.
`( SEM_A>>6`)
	Alter access for other.

If a new set of semaphores is being created, nsems is used to indicate the number of semaphores the set should contain. Otherwise, nsems may be specified as 0.

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;
    int perm;
    /*
     * Create 2 semaphores in a set, with access only to
     * the owner
     */
    perm = SEM_R | SEM_A;
    if((sem_set_id = semget(SEM_SET_KEY, NO_OF_SEMAPHORES, IPC_CREAT | perm))
        == -1) {
       printf("Semaphore creation failed with errno %d\n", errno);
       return -1;
    }
    return 0;
}

Errors

The semget call will fail with:

[`EACCES`]
	Access permission failure.
[`EEXIST`]
	IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding to key already exists.
[`EINVAL`]
	The number of semaphores requested exceeds the system imposed maximum per set.
[`ENOSPC`]
	A semaphore set has to be created but the system wide maximum number of semaphores would be exceeded.
[`ENOSPC`]
	The kernel could not allocate a struct semid_ds.
[`ENOENT`]
	No semaphore set was found corresponding to key, and IPC_CREAT was not specified.

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