NAME
semop
- semaphore operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
int
semop(
int semid
, struct sembuf *sops
, size_t nsops
)
DESCRIPTION
semop(
)
provides a number of atomic operations on a set of semaphores.
The semaphore set is specified by
semid
,
sops
is an array of semaphore operations, and
nsops
is the number of operations in this array.
The
sembuf
structures in the array contain the following members:
unsigned short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Each operation (specified in
sem_op)
is applied to semaphore number
sem_num
in the set of semaphores specified by
semid
.
The value of
sem_op
determines the action taken in the following way:
-
sem_op
is less than 0.
The current process is blocked until the value of the
semaphore is greater than or equal to the absolute value of
sem_op.
The absolute value of
sem_op
is then subtracted from the value of the semaphore, and the calling
process continues.
Negative values of
sem_op
are thus used to enter critical regions.
-
sem_op
is greater than 0.
Its value is added to the value of the specified semaphore.
This is used to leave critical regions.
-
sem_op
is equal to 0.
The calling process is blocked until the value of the
specified semaphore reaches 0.
The behaviour of each operation is influenced by the flags set in
sem_flg
in the following way:
IPC_NOWAIT
-
In the case where the calling process would normally block, waiting
for a semaphore to reach a certain value,
IPC_NOWAIT
makes the
call return immediately, returning a value of -1 and setting
errno
to
EAGAIN
.
- SEM_UNDO
-
Keep track of the changes that this call makes to the value of a semaphore,
so that they can be undone when the calling process terminates.
This is useful to prevent other processes waiting on a semaphore to block
forever, should the process that has the semaphore locked terminate in a
critical section.
RETURN VALUES
Upon successful completion, a value of 0 is returned.
Otherwise, -1 is returned and the global variable
errno
is set to indicate the error.
ERRORS
semop(
)
will fail if:
- [
EINVAL
] -
There is no semaphore associated with
semid
.
- [
EIDRM
] -
The semaphore set was removed while the process was waiting for one of
its semaphores to reach a certain value.
- [
EACCES
] -
The calling process has no permission to access the specified semaphore set.
- [
E2BIG
] -
The value of
nsops
is too big.
The maximum is defined as
MAX_SOPS
in
<
sys/sem.h
>.
- [
EFBIG
] -
sem_num
in one of the sem_buf structures is less than 0, or greater than the actual
number of semaphores in the set specified by
semid
.
- [
ENOSPC
] -
SEM_UNDO
was requested, and there is not enough space left in the kernel to
store the undo information.
- [
EAGAIN
] -
The requested operation can not immediately be performed, and
IPC_NOWAIT
was set in
sem_flg.
- [
EFAULT
] -
sops
points to an illegal address.
SEE ALSO
semctl(2),
semget(2)
STANDARDS
The
semop
system call conforms to
X/Open System Interfaces and Headers Issue 5 (``XSH5'') .
HISTORY
Semaphores appeared in the first release of
AT&T
System V UNIX
.