How to use pthread to modify pointers in main process? - c

Question

In the sample below, I tried to modify a char pointer in the main process using pthread. However, I dont see the pointer value changed. What is the reason and how can I achieve my goal?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> //Header file for sleep(). man 3 sleep for details.
#include <pthread.h>
#include <errno.h>
#include <string.h>
char *target = "thread";
void *modify(void *vargp) {
vargp = target;
printf("Thread vargp = %s\n", (char*)vargp);
return NULL;
}
int main() {
char *pt = "main";
pthread_t thread_id;
printf("Before Thread, pt = %s\n", pt);
pthread_create(&thread_id, NULL, modify, pt);
pthread_join(thread_id, NULL);
printf("After Thread, pt = %s\n", pt);
exit(0);
}
$ ./pthread_simple
Before Thread, pt = main
Thread vargp = thread
After Thread, pt = main
What I want to achieve is:
$ ./pthread_simple
Before Thread, pt = main
Thread vargp = thread
After Thread, pt = thread

Answer 1

Function arguments are copies of what are passed and change to them is local to the functions. To have functions modify things, you have to pass pointers to what should be modified.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> //Header file for sleep(). man 3 sleep for details.
#include <pthread.h>
#include <errno.h>
#include <string.h>
char *target = "thread";
void *modify(void *vargp) {
*(char**)vargp = target; /* add cast and dereference to modify what is pointed */
printf("Thread vargp = %s\n", *(char**)vargp); /* add cast and dereference */
return NULL;
}
int main() {
char *pt = "main";
pthread_t thread_id;
printf("Before Thread, pt = %s\n", pt);
pthread_create(&thread_id, NULL, modify, &pt); /* pass pointer to pt */
pthread_join(thread_id, NULL);
printf("After Thread, pt = %s\n", pt);
exit(0);
}