You can use mutex instead of semaphores if you need to execute only one
case at a time.
FROM,
V.VIGNESH.
M.Sc Theoretical Computer Science.
PSG College of Technology
On 25 November 2012 22:37, jagannath <jpdasi...@gmail.com> wrote:
> Folks, I have one pthread question.I know that its not the right place but
> i thought this is the right place to post this. Code snippet:
> #include <stdlib.h>
> #include <stdio.h>
>
> #include <unistd.h> /* defines _POSIX_THREADS if pthreads are available */
> #ifdef _POSIX_THREADS
> # include <pthread.h>
> #endif
>
> #include <semaphore.h>
>
> void *text(void *arg);
>
> int code[] = { 4, 6, 3, 1, 5, 0, 2 };
>
> int main()
> {
> int i;
> pthread_t tid[7];
>
> for (i = 0; i < 7; i++)
> {
> pthread_create(&tid[i], NULL, text, (void*)&code[i]);
> pthread_join(tid[i],NULL);
> }
>
> return 0;
> }
>
> void *text(void *arg)
> {
> int n = *(int*)arg;
>
> switch (n)
> {
> case 0:
> printf("A semaphore S is an integer-valued variable which can take only
> non-negative\n");
> printf("values. Exactly two operations are defined on a semaphore:\n\n");
> break;
>
> case 1:
> printf("Signal(S): If there are processes that have been suspended on this
> semaphore,\n");
> printf(" wake one of them, else S := S+1.\n\n");
> break;
>
> case 2:
> printf("Wait(S): If S>0 then S:=S-1, else suspend the execution of this
> process.\n");
> printf(" The process is said to be suspended on the semaphore S.\n\n");
> break;
>
> case 3:
> printf("The semaphore has the following properties:\n\n");
> break;
>
> case 4:
> printf("1. Signal(S) and Wait(S) are atomic instructions. In particular,
> no\n");
> printf(" instructions can be interleaved between the test that S>0 and
> the\n");
> printf(" decrement of S or the suspension of the calling process.\n\n");
> break;
>
> case 5:
> printf("2. A semaphore must be given an non-negative initial value.\n\n");
> break;
>
> case 6:
> printf("3. The Signal(S) operation must waken one of the suspended
> processes. The\n");
> printf(" definition does not specify which process will be
> awakened.\n\n");
> break;
> }
>
> pthread_exit(0);
> }
>
> The threads are not synchronized and therefore the text output is garbled.
> How to add semaphores of POSIX to this program to synchronize the threads.?
>
> primitives to rejig your memory:
>
> sem_init(), sem_wait(),sem_post(),sem_destroy().
>
> --
>
>
>
--
