Problem with running a function in 2 threads in C - c

Question

I have written a program with a function that prints a character received as an argument continuously, and made 2 threads running that function. The program runs as intended and keeps printing 2 characters interlaced indefinitely. Here is the code:
#include <pthread.h>
#include <stdio.h>
void* print_char(void* th_param) {
char* ch = (char*)th_param;
while(1)
printf("%c", *ch);
return NULL;
}
int main() {
char* c1 = "a";
char* c2 = "b";
pthread_t th_id;
pthread_create(&th_id, NULL, &print_char, (void*)c1);
pthread_t th_id2;
pthread_create(&th_id2, NULL, &print_char, (void*)c2);
return 0;
}
Now I have to modify this program so the print function takes a number argument as well, and prints the character that number of times. What I try is this:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
struct Params {
char* ch;
int num;
};
void* print_char(void* th_param) {
int i;
struct Params* par = (struct Params*)th_param;
for(i=0; i<par->num; i++) {
printf("%s", par->ch);
}
return NULL;
}
int main() {
struct Params* p1 = (struct Params*)malloc(sizeof(struct Params));
p1->ch = "a"; p1->num = 5;
struct Params* p2 = (struct Params*)malloc(sizeof(struct Params));
p2->ch = "b"; p2->num = 10;
pthread_t th_id;
pthread_create(&th_id, NULL, &print_char, (void*)p1);
pthread_t th_id2;
pthread_create(&th_id2, NULL, &print_char, (void*)p2);
while(1) {}
return 0;
}
But to no avail. Doesn't print a single character, the cursor just stands there blinking doing nothing. I have tried tampering with the code for hours and nothing has worked.
Note that there's not a problem with the struct, because the program works if I remove the for loop and put while(1) instead, and prints the character given to par->ch infinite times again.

Answer 1

The problem is buffered I/O.
Your main()-function loops infinitely, so the process doesn't terminate properly (only by being killed via an external signal), so the stdout-buffer is never flushed by regular process termination.
Since stdout is line-buffered by default, and your printf()s don't contain any newlines, and the amount of characters you try to print is smaller than the I/O-buffer, your program never writes out the buffer, so nothing is printed.
One approach would be to add fflush(stdout); after the printf()s in your thread function, or include a '\n' in the output.
Another (more sane) approach would be to have main() actually wait for the threads to finish, and gracefully terminate when they are done:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
struct Params {
char* ch;
int num;
};
void* print_char(void* th_param) {
int i;
struct Params* par = (struct Params*)th_param;
for(i=0; i<par->num; i++) {
printf("%s", par->ch);
//fflush(stdout);
}
return NULL;
}
int main() {
struct Params* p1 = (struct Params*)malloc(sizeof(struct Params));
p1->ch = "a"; p1->num = 5;
struct Params* p2 = (struct Params*)malloc(sizeof(struct Params));
p2->ch = "b"; p2->num = 10;
pthread_t th_id;
pthread_create(&th_id, NULL, &print_char, (void*)p1);
pthread_t th_id2;
pthread_create(&th_id2, NULL, &print_char, (void*)p2);
//while(1) {}
pthread_join(th_id, NULL);
pthread_join(th_id2, NULL);
free(p1); //make LeakSanitizer happy, don't leak memory
free(p2);
puts(""); //one newline for command-line beautification
fflush(stdout); //redundant after the puts(""), unless stdout is redirected to a non-interactive stream
return 0;
}

Answer 2

Thanks everyone, using fflush did the job.
To answer the person who asked what platform I ran this on, I was running this on Linux terminal. This was the homework for my OS lab class and pthread_create is all they have taught us so far, pthread_join and more thread management stuff come in later sessions.

Answer 3

Because of your while loop, stdout never get's flushed
Try this :
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
struct Params {
char* ch;
int num;
};
void* print_char(void* th_param) {
int i;
struct Params* par = (struct Params*)th_param;
for(i=0; i<par->num; i++) {
printf("%s", par->ch);
}
return NULL;
}
int main() {
struct Params* p1 = malloc(sizeof(struct Params));
p1->ch = "a"; p1->num = 5;
struct Params* p2 = malloc(sizeof(struct Params));
p2->ch = "b"; p2->num = 10;
pthread_t th_id;
pthread_create(&th_id, NULL, &print_char, p1);
pthread_t th_id2;
pthread_create(&th_id2, NULL, &print_char, p2);
fflush(stdout);
while(1) {;}
return 0;
}