man semop () - XSI semaphore operations

NAME

semop - XSI semaphore operations

SYNOPSIS

#include <sys/sem.h>

int semop(int semid, struct sembuf *sops, size_t nsops);

DESCRIPTION

The semop() 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 semop() function shall perform atomically a user-defined array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by the argument semid.

The argument sops is a pointer to a user-defined array of semaphore operation structures. The implementation shall not modify elements of this array unless the application uses implementation-defined extensions.

The argument nsops is the number of such structures in the array.

Each structure, sembuf, includes the following members: center; l2 l2 l. Member Type Member Name Description short sem_num Semaphore number. short sem_op Semaphore operation. short sem_flg Operation flags.

Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num.

The variable sem_op specifies one of three semaphore operations:

1.
If sem_op is a negative integer and the calling process has alter permission, one of the following shall occur:
*
If semval(see <sys/sem.h>) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. Also, if (sem_flg &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the calling process' semadj value for the specified semaphore.

*
If semval is less than the absolute value of sem_op and (sem_flg &IPC_NOWAIT) is non-zero, semop() shall return immediately.

*
If semval is less than the absolute value of sem_op and (sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semncnt associated with the specified semaphore and suspend execution of the calling thread until one of the following conditions occurs:
*
The value of semval becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of semncnt associated with the specified semaphore shall be decremented, the absolute value of sem_op shall be subtracted from semval and, if (sem_flg &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the calling process' semadj value for the specified semaphore.

*
The semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set equal to [EIDRM] and -1 shall be returned.

*
The calling thread receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction() .

2.
If sem_op is a positive integer and the calling process has alter permission, the value of sem_op shall be added to semval and, if (sem_flg &SEM_UNDO) is non-zero, the value of sem_op shall be subtracted from the calling process' semadj value for the specified semaphore.

3.
If sem_op is 0 and the calling process has read permission, one of the following shall occur:
*
If semval is 0, semop() shall return immediately.

*
If semval is non-zero and (sem_flg &IPC_NOWAIT) is non-zero, semop() shall return immediately.

*
If semval is non-zero and (sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semzcnt associated with the specified semaphore and suspend execution of the calling thread until one of the following occurs:
*
The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore shall be decremented.

*
The semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set equal to [EIDRM] and -1 shall be returned.

*
The calling thread receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction() .

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops shall be set equal to the process ID of the calling process.

RETURN VALUE

Upon successful completion, semop() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

The semop() function shall fail if:

E2BIG
The value of nsops is greater than the system-imposed maximum.
EACCES
Operation permission is denied to the calling process; see XSI Interprocess Communication .
EAGAIN
The operation would result in suspension of the calling process but (sem_flg &IPC_NOWAIT) is non-zero.
EFBIG
The value of sem_num is less than 0 or greater than or equal to the number of semaphores in the set associated with semid.
EIDRM
The semaphore identifier semid is removed from the system.
EINTR
The semop() function was interrupted by a signal.
EINVAL
The value of semid is not a valid semaphore identifier, or the number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the system-imposed limit.
ENOSPC
The limit on the number of individual processes requesting a SEM_UNDO would be exceeded.
ERANGE
An operation would cause a semval to overflow the system-imposed limit, or an operation would cause a semadj value to overflow the system-imposed limit.

The following sections are informative.

EXAMPLES

Setting Values in Semaphores

The following example sets the values of the two semaphores associated with the semid identifier to the values contained in the sb array. 

#include <sys/sem.h>
...
int semid;
struct sembuf sb[2];
int nsops = 2;
int result;


/* Adjust value of semaphore in the semaphore array semid. */
sb[0].sem_num = 0;
sb[0].sem_op = -1;
sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
sb[1].sem_num = 1;
sb[1].sem_op =  1;
sb[1].sem_flg = 0;


result = semop(semid, sb, nsops);



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.


The final call to semop() acquires the semaphore and waits until
it is free; the SEM_UNDO option releases the semaphore
when the process exits, waiting until there are less than two processes
running concurrently.


#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);
    }
}
...
sbuf.sem_num = 0;
sbuf.sem_op = -1;
sbuf.sem_flg = SEM_UNDO;
if (semop(semid, &sbuf, 1) == -1) {
    perror("IPC Error: semop"); exit(1);
}



APPLICATION USAGE

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

XSI Interprocess Communication , Realtime , exec()
,
exit() , fork() , semctl() , semget() , 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/ipc.h>,
<sys/sem.h>, <sys/types.h>

COPYRIGHT
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 .