Operations on semaphores

This section describes how to use the semop system call. The accompanying program illustrates its use.

Using semop

The synopsis found on the semop(2) manual page is as follows:

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/sem.h>
   

   int semop (int semid, struct sembuf *sops, size_t nsops)

The semop system call requires three arguments to be passed to it and returns an integer value which will be zero for successful completion or -1 otherwise.

The semid argument must be a valid, non-negative, integer value. In other words, it must have already been created by using the semget system call.

The sops argument points to an array of structures in the user memory area that contains the following for each semaphore to be changed:

• the semaphore number (sem_num)

• the operation to be performed (sem_op)

• the control flags (sem_flg)

The *sops declaration means that either an array name (which is the address of the first element of the array) or a pointer to the array can be used. sembuf is the ``tag'' name of the data structure used as the template for the structure members in the array; it is located in the sys/sem.h header file.

The nsops argument specifies the length of the array (the number of structures in the array). The maximum size of this array is determined by the SEMOPM system-tunable parameter. Therefore, a maximum of SEMOPM operations can be performed for each semop system call.

The semaphore number (sem_num) determines the particular semaphore within the set on which the operation is to be performed.

The operation to be performed is determined by the following:

• if sem_op is positive, the semaphore value is incremented by the value of sem_op

• if sem_op is negative, the semaphore value is decremented by the absolute value of sem_op

• if sem_op is zero, the semaphore value is tested for equality to zero

The following operation commands (flags) can be used:

• IPC_NOWAIT--this operation command can be set for any operations in the array. The system call will return unsuccessfully without changing any semaphore values at all if any operation for which IPC_NOWAIT is set cannot be performed successfully. The system call will be unsuccessful when trying to decrement a semaphore more than its current value, or when testing for a semaphore to be equal to zero when it is not.

• SEM_UNDO--this operation command is used to tell the system to undo the process's semaphore changes automatically when the process exits; it allows processes to avoid deadlock problems. To implement this feature, the system maintains a table with an entry for every process in the system. Each entry points to a set of undo structures, one for each semaphore used by the process. The system records the net change.