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Another way to solve the signal issue is to use a unique condition variable for each thread (maintaining all associated with a single mutex associated with it). In this case, the signal operation (cnd_signal()) only wakes up the only thread that is waiting on it. This solution turns out to be more efficient than using cnd_broadcast(), because only the desired thread is awakened.
NOTE: The condition predicate of the signaled thread must be true; otherwise, a deadlock can will occur.
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#include <stdio.h>
#include <threads.h>
enum { NTHREADS = 5 };
mtx_t mutex;
cnd_t cond[NTHREADS];
int run_step(void *t) {
static int current_step = 0;
int my_step = (int)t;
if (thrd_success != mtx_lock(&mutex)) {
/* Handle error condition. */
}
printf("Thread %d has the lock\n", my_step);
while (current_step != my_step) {
printf("Thread %d is sleeping...\n", my_step);
if (thrd_success != cnd_wait(&cond[my_step], &mutex)) {
/* Handle error condition. */
}
printf("Thread %d woke up\n", my_step);
}
/* Do processing... */
printf("Thread %d is processing...\n", my_step);
current_step++;
/* Signal next step thread. */
if ((my_step + 1) < NTHREADS) {
if (thrd_success != cnd_signal(&cond[my_step + 1])) {
/* Handle error condition */
}
}
printf("Thread %d is exiting...\n", my_step);
if (thrd_success != mtx_unlock(&mutex)) {
/* Handle error condition. */
}
return 0;
}
int main(int argc, char** argv) {
int i;
thrd_t threads[NTHREADS];
int step[NTHREADS];
if (thrd_success != mtx_init(&mutex, mtx_plain)) {
/* Handle error condition. */
}
for (i = 0; i< NTHREADS; ++i) {
if (thrd_success != cnd_init(&cond[i])) {
/* Handle error condition. */
}
}
/* Create threads. */
for (i = 0; i < NTHREADS; ++i) {
step[i] = i;
if (thrd_success != thrd_create(&threads[i], run_step,
(void *)step[i])) {
/* Handle error condition. */
}
}
/* Wait for all threads to complete. */
for (i = NTHREADS - 1; i >= 0; --i) {
if (thrd_success != thrd_join(threads[i], NULL)) {
/* Handle error condition. */
}
}
mtx_destroy(&mutex);
for (i = 0; i < NTHREADS; ++i) {
cnd_destroy(&cond[i]);
}
return 0;
}
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Risk Assessment
Signaling a single thread instead of all waiting threads can pose a threat to the liveness property of the system.
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