NAME
semget —
obtain a semaphore id
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semget(key_t
key, int nsems,
int flag);
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_PRIVATEmay 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_CREATbit is set in flag, a new one will be created. ftok() may be used to generate a key from a pathname. See ftok(3).
The mode of a newly created IPC object is determined by OR'ing the following constants into the flag parameter:
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.
RETURN VALUES
Semget() returns the id of a semaphore set
if successful; otherwise, -1 is returned and errno is
set to indicate the error.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
Semget() will fail if:
- [
EACCES] - Access permission failure.
- [
EEXIST] IPC_CREATandIPC_EXCLwere specified, and a semaphore set corresponding to key already exists.- [
EINVAL] - The number of semaphores requested exceeds the system imposed maximum per set.
- [
ENOSPC] - Insufficiently many semaphores are available.
- [
ENOSPC] - The kernel could not allocate a struct semid_ds.
- [
ENOENT] - No semaphore set was found corresponding to key, and
IPC_CREATwas not specified.