Originální popis anglicky:
semget - get set of XSI semaphores
Návod, kniha: POSIX Programmer's Manual
#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:
- EACCES
- 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 .
- EEXIST
- A semaphore identifier exists for the argument key
but (( semflg &IPC_CREAT) &&( semflg
&IPC_EXCL)) is non-zero.
- EINVAL
- 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.
- ENOENT
- A semaphore identifier does not exist for the argument
key and ( semflg &IPC_CREAT) is equal to 0.
- ENOSPC
- 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.
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.
None.
None.
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>
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 http://www.opengroup.org/unix/online.html
.
