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2011-11-23T20:24:26.823
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Body
I do think your approach with a precedence graph is correct, but the problem statement is a bit unclear, for example, from the graph is looks like <code>B</code> and <code>C</code> can occur in any order, but no indication of this in the original a) and b) sequences. (EDIT: it become clear that <code>B</code> and <code>C</code> have to alternate) So, for case a) the following (infinite) sequence is acceptable: <pre><code>ABCD ACBD ABCD ACBD ABCD ACBD ... A<--+ / \ | B<->C | \ / | D---+ </code></pre> <code>A</code> precedes <code>B</code> by program logic and they are in the same thread. Likewise <code>C</code> precedes <code>D</code> for the same reasons. Hence, semaphores are needed to enforce precedence only along the edges <code>A->C</code>, <code>B->D</code> and <code>D->A</code>. The edge between <code>B</code> and <code>C</code> changes direction each cycle, hence we need an additional bit of state, to determine the direction: <code>B->C</code> or <code>B<-C</code> It can be done with an extra variable, or we can maintain the state implicitly by duplicating the loop bodies, like below: <pre><code>#include <semaphore.h> #include <pthread.h> #include <stdio.h> #define NLOOPS 100000 sem_t s0, s1, s2, s3; void * process_A (void *unused) { int n = NLOOPS; while (n--) { sem_wait (&s0); putchar ('A'); sem_post (&s1); putchar ('B'); sem_post (&s3); sem_post (&s2); sem_wait (&s0); putchar ('A'); sem_post (&s1); sem_wait (&s3); putchar ('B'); sem_post (&s2); } return 0; } void * process_B (void *unused) { int n = NLOOPS; while (n--) { sem_wait (&s1); sem_wait (&s3); putchar ('C'); sem_wait (&s2); putchar ('D'); sem_post (&s0); sem_wait (&s1); putchar ('C'); sem_post (&s3); sem_wait (&s2); putchar ('D'); sem_post (&s0); } return 0; } int main () { pthread_t a, b; sem_init (&s0, 0, 1); sem_init (&s1, 0, 0); sem_init (&s2, 0, 0); sem_init (&s3, 0, 0); pthread_create (&a, 0, process_A, 0); pthread_create (&b, 0, process_B, 0); pthread_join (a, 0); pthread_join (b, 0); putchar ('\n'); return 0; } </code></pre> I will leave you to implement b) by yourself :)
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1. POProcess syncronization with semaphore primitives
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