Originální popis anglicky:
semop - XSI semaphore operations
Návod, kniha: POSIX Programmer's Manual
#include <sys/sem.h>
int semop(int
semid, struct sembuf
*sops , size_t
nsops );
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:
| 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.
Upon successful completion,
semop() shall return 0; otherwise, it shall
return -1 and set
errno to indicate the error.
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.
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);
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);
}
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 ,
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>
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
.
