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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017, Section 4.17, 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.1‐2017, Section 4.17,
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 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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