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What is the pthread_cond_wait() & pthread_cond_signal() calling sequence?

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

pthread_mutex_t lockid;
pthread_cond_t cvar = PTHREAD_COND_INITIALIZER;
char name[14];

void *entry()
{
    pthread_mutex_lock(&lockid);

    printf("Enter Name\n");
    scanf("%s",name);

    pthread_cond_signal(&cvar);
    pthread_mutex_unlock(&lockid);
    pthread_exit(NULL);

}

void *display()
{
    pthread_mutex_lock(&lockid);
    pthread_cond_wait(&cvar,&lockid);

    printf("Name: %s\n",name);

    pthread_mutex_unlock(&lockid);
    pthread_exit(NULL);
}

int main()
{
    pthread_t id1,id2;
    if(pthread_mutex_init(&lockid,NULL)== 0){
        pthread_create(&id1,NULL,&entry,NULL);
        pthread_create(&id2,NULL,&display,NULL);

        pthread_join(id1,NULL);
        pthread_join(id2,NULL);
    }
    return 0;
}

I am expecting that the thread id2 to print the name that I have entered on thread id1. I know it's the pthread_cond_wait()doesn't allow the thread id2 to display the name entered on thread id1 but signal() should be called before wait() right ? Is the way of calling wait() and signal() methods correct? I am new to linux programming please help me on this.

11 months ago · Santiago Trujillo

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Accepted Answer · 2022-04-28T16:40:58.250Z

If you have a sequential task don't use threads:

#include <stdio.h>

char name[14];

int main() {
    printf("Enter Name\n");
    scanf("%s",name);
    printf("Name: %s\n", name);
    return 0
}

You can use a state variable to ensure that entry() run before display():

#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>

pthread_mutex_t lockid;
pthread_cond_t cvar = PTHREAD_COND_INITIALIZER;
char name[14];
enum { START, ENTRY } state = START;
#define CHECK(e) { int error = (e); if(error) printf("%s:%d %s", __FILE__, __LINE__, strerror(error)); }
#define CHECK2(p, e) { if(p) CHECK(e); }


void *entry() {
    CHECK(pthread_mutex_lock(&lockid));
    printf("Enter Name\n");
    CHECK2(scanf("%s", name) == EOF, errno);
    state = ENTRY;
    pthread_cond_signal(&cvar);
    CHECK(pthread_mutex_unlock(&lockid));
    return 0;
}

void *display() {
    CHECK(pthread_mutex_lock(&lockid));
    while(state != ENTRY) pthread_cond_wait(&cvar, &lockid);
    printf("Name: %s\n", name);
    CHECK(pthread_mutex_unlock(&lockid));
    return 0;
}

int main() {
    pthread_mutex_init(&lockid, NULL);
    pthread_t id1, id2;
    CHECK(pthread_create(&id1, NULL, &entry, NULL));
    CHECK(pthread_create(&id2, NULL, &display, NULL));
    CHECK(pthread_join(id1,NULL));
    CHECK(pthread_join(id2,NULL));
    return 0;
}