() 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 regions.
- sem_op is greater than 0. Its value is
added to the value of the specified semaphore. This is used to leave
- sem_op is equal to 0. The calling process
is blocked until the value of the specified semaphore reaches 0.
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
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
- 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 semid.
was requested, and there is not enough space left in the kernel to store
the undo information.
- The requested operation cannot immediately be performed, and
IPC_NOWAIT was set in
- sops points to an illegal address.