NAME
semop —
semaphore operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/sem.h>
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.