Conditional Variables with Posix Threads - c

Question

I want to print for infinity the below message "What a wonderful world!" , I have from the begin the 3 threads to do this job the first one "What a , second one "Wonderful" and third one "world" , ( I am giving as parameter in ./a.out 3) , the problem in the below code is that the message displayed like this : What a wonderful what a wonderful the World is missing. I think Something is bad with the conditional variables, any suggestions?
int p;
void *disp(void *arg);
pthread_mutex_t muta,mutb,mutc;
pthread_cond_t condition_cond= PTHREAD_COND_INITIALIZER;
void *disp(void *arg)
{
int id=*(int*)arg;
free(arg);
arg=0;
pthread_mutex_lock(&muta);
while (id==0)
{
pthread_cond_wait (&condition_cond,&muta);
}
printf("What a");
pthread_mutex_unlock(&muta);
pthread_mutex_lock(&mutb);
while (id==1)
{
pthread_cond_wait (&condition_cond,&mutb);
}
printf(" Wonderful");
pthread_mutex_unlock(&mutb);
pthread_mutex_lock(&mutc);
while(id==2)
{
pthread_cond_wait (&condition_cond,&mutc);
}
printf(" World!\n");
pthread_mutex_unlock(&mutc);
return NULL;
}
EDIT: updated code based on the answer by dbush:
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <semaphore.h>
#include <unistd.h>
int p;
void *disp(void *arg);
pthread_mutex_t mut=PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond1= PTHREAD_COND_INITIALIZER;
pthread_cond_t cond2= PTHREAD_COND_INITIALIZER;
pthread_cond_t cond3= PTHREAD_COND_INITIALIZER;
void *disp(void *arg)
{
int id=*(int*)arg;
free(arg);
arg=0;
pthread_mutex_lock(&mut);
while(1)
{
if (id==0)
{
pthread_cond_wait (&cond1,&mut);
printf("What a ");
pthread_cond_signal (&cond2);
}
else if(id==1)
{
pthread_cond_wait (&cond2,&mut);
printf(" wonderful ");
pthread_cond_signal (&cond3);
}
else if(id==2)
{
pthread_cond_wait (&cond3,&mut);
printf(" world!\n ");
pthread_cond_signal (&cond1);
}
}
return NULL;
}
int main (int argc,char *argv[])
{
int i;
pthread_t *tid;
if(argc!=2)
{
printf("Provide number of threads.\n");
exit(1);
}
p=atoi(argv[1]);
tid=(pthread_t *) malloc (p*sizeof(pthread_t));
if (tid==NULL)
{
printf("Could not allocate memory.\n");
exit(1);
}
for (i=0;i<p;++i)
{
int *a;
a=malloc(sizeof(int)); //pointer to pass
*a=i;
pthread_create(&tid[i],NULL,disp,a);
}
pthread_cond_signal(&cond1);
for (i=0;i<p;++i)
pthread_join(tid[i],NULL);
return 0;
}

Answer 1

You've got your usage of mutexes and condition variables reversed. You need one mutex to control all three threads, and three condition variables (one for each thread).
After each thread finishes its work, it should signal the next thread using the condition variable for the target thread. Additionally, the main function needs to signal the first thread to get things started. You'll want it to sleep for a brief period so that all threads have time to start.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
void *disp(void *arg);
pthread_mutex_t mux = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond1= PTHREAD_COND_INITIALIZER;
pthread_cond_t cond2= PTHREAD_COND_INITIALIZER;
pthread_cond_t cond3= PTHREAD_COND_INITIALIZER;
void *disp(void *arg)
{
int id=*(int*)arg;
free(arg);
arg=0;
pthread_mutex_lock(&mux);
while (1) {
if (id==0) {
pthread_cond_wait (&cond1,&mux);
printf("What a");
pthread_cond_signal(&cond2);
} else if (id == 1) {
pthread_cond_wait (&cond2,&mux);
printf(" Wonderful");
pthread_cond_signal(&cond3);
} else if (id == 2) {
pthread_cond_wait (&cond3,&mux);
printf(" World!\n");
pthread_cond_signal(&cond1);
}
}
return NULL;
}
int main()
{
pthread_t t1, t2, t3;
int *id = malloc(sizeof(int));
*id=0;
pthread_create(&t1, NULL, disp, id);
id = malloc(sizeof(int));
*id=1;
pthread_create(&t2, NULL, disp, id);
id = malloc(sizeof(int));
*id=2;
pthread_create(&t3, NULL, disp, id);
sleep(1);
pthread_cond_signal(&cond1);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
pthread_join(t3, NULL);
return 0;
}