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SEMOP(2) BSD System Calls Manual SEMOP(2)
NAME
semop -- atomic array of operations on a semaphore set
SYNOPSIS
#include <sys/sem.h>
int
semop(int semid, struct sembuf *sops, size_t nsops);
DESCRIPTION
The semop() system call atomically performs the array of operations indi-cated indicated
cated by sops on the semaphore set indicated by semid. The length of
sops is indicated by nsops. Each operation is encoded in a struct
sembuf, which is defined as follows:
struct sembuf {
u_short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
};
For each element in sops, sem_op and sem_flg determine an operation to be
performed on semaphore number sem_num in the set. The values SEM_UNDO
and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify
the behavior of the given operation.
The operation performed depends as follows on the value of sem_op:
oo When sem_op is positive and the process has alter permission, the
semaphore's value is incremented by sem_op's value. If SEM_UNDO is
specified, the semaphore's adjust on exit value is decremented by
sem_op's value. A positive value for sem_op generally corresponds to
a process releasing a resource associated with the semaphore.
oo The behavior when sem_op is negative and the process has alter per-mission, permission,
mission, depends on the current value of the semaphore:
oo If the current value of the semaphore is greater than or equal to
the absolute value of sem_op, then the value is decremented by
the absolute value of sem_op. If SEM_UNDO is specified, the sem-aphore's semaphore's
aphore's adjust on exit value is incremented by the absolute
value of sem_op.
oo If the current value of the semaphore is less than the absolute
value of sem_op, one of the following happens:
oo If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
oo Otherwise, the calling process is put to sleep until one of
the following conditions is satisfied:
oo Some other process removes the semaphore with the
IPC_RMID option of semctl(2). In this case, semop()
returns immediately with a return value of EIDRM.
oo The process receives a signal that is to be caught. In
this case, the process will resume execution as defined
by sigaction(2).
oo The semaphore's value is greater than or equal to the
absolute value of sem_op. When this condition becomes
true, the semaphore's value is decremented by the abso-lute absolute
lute value of sem_op, the semaphore's adjust on exit
value is incremented by the absolute value of sem_op.
A negative value for sem_op generally means that a process is waiting
for a resource to become available.
oo When sem_op is zero and the process has read permission, one of the
following will occur:
oo If the current value of the semaphore is equal to zero then
semop() can return immediately.
oo If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
oo Otherwise, the calling process is put to sleep until one of the
following conditions is satisfied:
oo Some other process removes the semaphore with the IPC_RMID
option of semctl(2). In this case, semop() returns immedi-ately immediately
ately with a return value of EIDRM.
oo The process receives a signal that is to be caught. In this
case, the process will resume execution as defined by
sigaction(2).
oo The semaphore's value becomes zero.
For each semaphore a process has in use, the kernel maintains an ``adjust
on exit'' value, as alluded to earlier. When a process exits, either
voluntarily or involuntarily, the adjust on exit value for each semaphore
is added to the semaphore's value. This can be used to insure that a
resource is released if a process terminates unexpectedly.
RETURN VALUES
The semop() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The semop() system call will fail if:
[E2BIG] Too many operations are specified. [SEMOPM]
[EACCES] Permission is denied, due to a mismatch between the
operation and the mode of the semaphore set.
[EAGAIN] The semaphore's value would result in the process
being put to sleep and IPC_NOWAIT is specified.
[EFBIG] sem_num is not in the range of valid semaphores for
the set.
[EIDRM] The semaphore set is removed from the system.
[EINTR] The semop() system call is interrupted by a signal.
[EINVAL] No semaphore set corresponds to semid, or the process
would exceed the system-defined limit for the number
of per-process SEM_UNDO structures.
[ENOSPC] The system SEM_UNDO pool [SEMMNU] is full.
[ERANGE] The requested operation would cause either the sema-phore's semaphore's
phore's current value [SEMVMX] or its adjust-on-exit
value [SEMAEM] to exceed the system-imposed limits.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
The include files <sys/types.h> and <sys/ipc.h> are necessary.
SEE ALSO
semctl(2), semget(2), sigaction(2), compat(5)
BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT
was specified.
BSD September 22, 1995 BSD
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