semop(2)semop(2)Namesemop - semaphore operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop (semid, sops, nsops)
int semid;
struct sembuf *sops[];
int nsops;
Description
The system call is used to atomically perform an array of semaphore
operations on the set of semaphores associated with the semaphore iden‐
tifier specified by The sops is a pointer to the array of semaphore-
operation structures. The nsops is the number of such structures in
the array. The contents of each structure includes the following mem‐
bers:
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 corre‐
sponding semaphore specified by and sem_num.
The sem_op specifies one of three semaphore operations as follows:
1. If sem_op is a negative integer, one of the following occurs:
· If semval is greater than or equal to the absolute value of sem_op,
the absolute value of sem_op is subtracted from semval. For fur‐
ther information, see Also, 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. For further information, see
· If semval is less than the absolute value of sem_op and (sem_flg &
IPC_NOWAIT) is true, returns immediately.
· If semval is less than the absolute value of sem_op and (sem_flg &
IPC_NOWAIT ) is false, increments the semncnt associated with the
specified semaphore and suspend execution of the calling process
until one of the following occurs:
· If the 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 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 for which the calling process is awaiting action is removed
from the system. For further information, see 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
2. 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.
3. If sem_op is zero, one of the following occurs:
· If semval is zero, returns immediately.
· If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is true,
returns immediately.
· If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is false,
increments the semzcnt associated with the specified semaphore and
suspend execution of the calling process, until one of the follow‐
ing occurs:
The semval became zero, at which time the value of semzcnt associ‐
ated with the specified semaphore is decremented.
The 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 semaphore is
decremented, and the calling process resumes execution in the manner
prescribed in
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 callingprocess.Return Values
Upon successful completion, a value of 0 is returned. Otherwise, a
value of -1 is returned, and errno is set to indicate the error.
Diagnostics
The fails if any of the following is true for any of the semaphore
operations specified by sops:
[EINVAL] The sempid is not a valid semaphore identifier.
[EFBIG] The sem_num is less than zero or greater than or equal
to the number of semaphores in the set associated with
[E2BIG] The nsops is greater than the system-imposed maximum.
[EACCESS] Operation permission is denied to the calling process.
For further information, see
[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 request‐
ing an SEM_UNDO would be exceeded.
[EINVAL] The number of individual semaphores for which the call‐
ing process request a SEM_UNDO would exceed the limit.
[ERANGE] An operation would cause a semval to overflow the sys‐
tem-imposed limit.
[ERANGE] An operation would cause a semadj value to overflow the
system-imposed limit.
[EFAULT] The sops points to an illegal address.
[EINTR] The returns due to the receipt of a signal.
[EIDRM] The has been removed from the system.
See Alsoexecve(2), exit(2), fork(2), semctl(2), semget(2), signal(3)semop(2)
