semop(2) CLIX semop(2)
NAME
semop - Semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(
int semid ,
struct sembuf **sops ,
unsigned nsops );
PARAMETERS
semid Represents the identifier of an already existing semaphore
(created by the semget() function).
sops Points to an array of semaphore operation data structures.
nsops Specifies the number of operations in the sops array.
DESCRIPTION
The semop() function automatically performs an array of semaphore
operations on the set of semaphores associated with the semaphore
identifier specified by semid. The sops parameter is a pointer to the
array of semaphore-operation structures. The nsops parameter is the
number of such structures in the array. The contents of each structure
includes the following members:
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 sem_op member specifies one of three semaphore operations as follows:
⊕ If sem_op is a negative integer, one of the following will occur:
-- If semval (see intro(2)) 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 true, the absolute value
of sem_op is added to the calling process's semadj value (see
exit(2)) for the specified semaphore.
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-- If semval is less than the absolute value of sem_op and (sem_flg &
IPC_NOWAIT) is true, semop() returns immediately.
-- If semval is less than the absolute value of sem_op and (sem_flg &
IPC_NOWAIT) is false, semop() increments the semncnt associated with
the specified semaphore and suspends execution of the calling
process until one of the following conditions occurs:
-- The semval parameter becomes greater than or equal to the
absolute value of sem_op. When this occurs, the value of semncnt
associated with the specified semaphore is decremented, the
absolute value of sem_op is subtracted from semval and, if
(sem_flg & SEM_UNDO) is true, the absolute value of sem_op is
added to the calling process's semadj value for the specified
semaphore.
-- The semid parameter, for which the calling process is awaiting
action, is removed from the system. (See the semctl() function.)
When this occurs, errno is set equal to EIDRM, and a value of -1
is returned.
-- The calling process receives a signal that is to be caught. When
this occurs, the value of semncnt associated with the specified
semaphore is decremented, and the calling process resumes
execution in the manner prescribed in signal().
⊕ If sem_op is a positive integer, the value of sem_op is added to semval
and, if (sem_flg & SEM_UNDO) is true, the value of sem_op is subtracted
from the calling process's semadj value for the specified semaphore.
⊕ If sem_op is 0, one of the following will occur:
-- If semval is 0, semop() returns immediately.
-- If semval is not 0 and (sem_flg & IPC_NOWAIT) is true, semop()
returns immediately.
-- If semval is not 0 and (sem_flg & IPC_NOWAIT) is false, semop()
increments the semzcnt associated with the specified semaphore and
suspend execution of the calling process until one of the following
occurs:
-- The semval parameter becomes 0, at which time the value of
semzcnt associated with the specified semaphore is decremented.
-- The semid parameter, for which the calling process is awaiting
action, is removed from the system. When this occurs, errno is
set equal to EIDRM, and a value of -1 is returned.
-- The calling process receives a signal that is to be caught. When
this occurs, the value of semzcnt associated with the specified
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semaphore is decremented, and the calling process resumes
execution in the manner prescribed in signal().
EXAMPLES
1. To perform the P (down) semaphore operation on the first semaphore
described by semid:
struct sembuf sembuf;
int semid;
memset((char *)&sembuf, 0, sizeof(sembuf));
sembuf.sem_num = 0;
sembuf.sem_op = -1;
if (semop(semid, &sembuf, 1) < 0)
perror("Decrementing semaphore");
2. To perform the V (up) semaphore operation on the first semaphore
described by semid:
struct sembuf sembuf;
int semid;
memset((char *)&sembuf, 0, sizeof(sembuf));
sembuf.sem_num = 0;
sembuf.sem_op = 1;
if (semop(semid, &sembuf, 1) < 0)
perror("Incrementing semaphore");
RETURN VALUES
If semop() returns due to the receipt of a signal, a value of -1 is
returned to the calling process and errno is set to EINTR. If semop()
returns due to the removal of a semid from the system, a value of -1 is
returned and errno is set to EIDRM.
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error. Upon successful
completion, the value of sempid for each semaphore specified in the array
pointed to by sops is set equal to the process ID of the calling process.
ERRORS
The semop() function fails if one or more of the following are true for
any of the semaphore operations specified by sops:
[EINVAL] The value of semid is not a valid semaphore identifier.
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[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.
[E2BIG] The value of nsops is greater than the system-imposed maximum.
[EACCES] Operation permission is denied to the calling process. (See
intro(2).)
[EAGAIN] The operation would result in suspension of the calling process
but (sem_flg & IPC_NOWAIT) is true.
[ENOSPC] The limit on the number of individual processes requesting an
SEM_UNDO would be exceeded.
[EINVAL] The number of individual semaphores for which the calling
process requests a SEM_UNDO would exceed the limit.
[EIDRM] The specified msqid was removed while the process was suspended
waiting for a message to be sent.
[EINTR] A signal was caught.
[ERANGE] An operation would cause a semval to overflow the system-
imposed limit.
[ERANGE] An operation would cause a semadj value to overflow the
system-imposed limit.
[EFAULT] The value of sops points to an illegal address.
RELATED INFORMATION
Functions: exec(2), exit(2), fork(2), intro(2), semctl(2), semget(2)
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