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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
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().
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().
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.
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 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.
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;
/* 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);
}
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 Section 2.7, XSI Interprocess
Communication can be easily modified to use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, exec, exit(), fork(),
semctl(), semget(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(),
sem_post(), sem_trywait(), sem_unlink()
The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore, <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, 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
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