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SEMOP(3P) POSIX Programmer's Manual SEMOP(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
semop — XSI semaphore operations
#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 POSIX.1‐2008, Section 4.16, Semaphore). It is
unspecified whether this function interoperates with the realtime
interprocess communication facilities defined in Section 2.8,
Realtime.
The semop() function shall perform atomically a user-defined array of
semaphore operations in array order 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 │
├───────────────┼───────────────┼──────────────────────────┤
│unsigned 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
semadj value of the calling process 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 semadj value of the calling process 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 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(3p).
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 semadj value of the calling process 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 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(3p).
Upon successful completion, the value of sempid for each semaphore
specified in the array pointed to by sops shall be set to the process
ID of the calling process. Also, the sem_otime timestamp shall be set
to the current time, as described in Section 2.7.1, IPC General
Description.
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
Section 2.7, 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 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.
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;
/* Code to initialize semid. */
...
/* 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 calls to semctl() and
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.
Processes that obtain semid without creating it check that sem_otime
is non-zero, to ensure that the creating process has completed the
semop() initialization.
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 <stdio.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdlib.h>
...
key_t semkey;
int semid;
struct sembuf sbuf;
union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
} arg;
struct semid_ds ds;
...
/* 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. */
arg.val = 0;
sbuf.sem_num = 0;
sbuf.sem_op = 2; /* This is the number of runs without queuing. */
sbuf.sem_flg = 0;
if (semctl(semid, 0, SETVAL, arg) == -1
|| 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);
}
goto check_init;
}
else {
perror("IPC error 2: semget"); exit(1);
}
}
else
{
/* Check that semid has completed initialization. */
/* An application can use a retry loop at this point rather than
exiting. */
check_init:
arg.buf = &ds;
if (semctl(semid, 0, IPC_STAT, arg) < 0) {
perror("IPC error 3: semctl"); exit(1);
}
if (ds.sem_otime == 0) {
perror("IPC error 4: semctl"); 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 Section 2.7, XSI Interprocess Communication can
be easily modified to use the alternative interfaces.
None.
None.
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
exec(1p), exit(3p), fork(3p), semctl(3p), semget(3p), sem_close(3p),
sem_destroy(3p), sem_getvalue(3p), sem_init(3p), sem_open(3p),
sem_post(3p), sem_trywait(3p), sem_unlink(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore,
sys_ipc.h(0p), sys_sem.h(0p), sys_types.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SEMOP(3P)
Pages that refer to this page: sys_sem.h(0p), ipcs(1p), exec(3p), _Exit(3p), fork(3p), sem_close(3p), semctl(3p), sem_destroy(3p), semget(3p), sem_getvalue(3p), sem_open(3p), sem_post(3p), sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p)