() provides a number of atomic operations on a
set of semaphores. The semaphore set is specified by
is an array of semaphore operations, nsops
the number of operations in this array. The
structures in the array contain the
u_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
determines the action taken in the
- 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
- 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
The behavior of each operation is influenced by the flags set in
in the following way:
- 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
- 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.
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
is set to indicate the error.
() will fail if:
- There is no semaphore associated with
- The semaphore set was removed while the process was waiting
for one of its semaphores to reach a certain value.
- The calling process has no permission to access the
specified semaphore set.
- The value of nsops is too
big. The maximum is specified in MAX_SOPS in
- 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
was requested, and there is not enough space left in the kernel to store
the undo information.
- The requested operation cannot immediately be performed,
IPC_NOWAIT was set in
- sops points to an illegal