A simple example for using pthread_cond_wait() and pthread_cond_signal() with mutexes and conditional variables.

lockwait.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>

/* compile with gcc -pthread lockwait.c */

pthread_cond_t cv;
pthread_mutex_t lock;

void *thread(void *v) {
	printf("Locking and waiting. Type unlock to unlock me.\n");
	pthread_mutex_lock(&lock);
	pthread_cond_wait(&cv, &lock);
	printf("I've been unlocked.\n");
	pthread_mutex_unlock(&lock);
	return NULL;
}

main() {

	char cmd[1024];
	pthread_t *t;


	printf("Type lock to run a thread that locks and waits.\n");
	printf("Type unlock to unlock the same thread.\n");
	while(fscanf(stdin, "%s", cmd) != EOF) {
		if(strcmp(cmd, "lock") == 0) { 
			t = (pthread_t *) malloc(sizeof(pthread_t));
			pthread_create(t, NULL, thread, NULL);		
		} else if(strcmp(cmd, "unlock") == 0) {
			pthread_cond_signal(&cv);
		}

	}	

}

No need to initialise cv and lock?

I'd be inclined to initialise cv & lock
pthread_cond_t cv = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
BTW this is a dump of pthread_cond_t internals, without and then with the initialiser:
L=20182272,F=0,T=140733535085552,Wa=140733535084496,Wo=4252151,M=0x133f500,N=341697520,B=32767
L=0,F=0,T=0,Wa=0,Wo=0,M=0,N=0,B=0

Definitely, @brewmanz. I'd be surprised, especially with what you're showing, if this code worked.

I prefer this (scroll down)

http://man7.org/linux/man-pages/man3/pthread_cond_destroy.3p.html

I agree with @farrellit, you should wrap pthread_cond_wait(...) in a while (!condition) { .. } block and before signaling set the condition to true. Since "Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur." (see https://linux.die.net/man/3/pthread_cond_wait). So you need to check the condition again, also if the signal happens before the wait the thread will not finish.

Thank you so much.