Program calls printf() then loops forever. Why don't I see the printf output?-CodePudding

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <math.h>
  
struct my_Struct{
    int a;
    int b;
};
void *myThreadFun(void *received_struct)
{
    struct my_Struct *struct_ptr = (struct my_Struct*) received_struct;
    printf("%.1lf",pow(struct_ptr->a,struct_ptr->b));
    return NULL;
}
   
int main(int argc, char* argv[])
{
    struct my_Struct s;
    s.a = atoi(argv[1]);
    s.b = atoi(argv[2]);
    
    pthread_t thread_id;
    pthread_create(&thread_id, NULL, myThreadFun, &s);
    pthread_join(thread_id, NULL);
    while(1);
    exit(0);
}

Why does this code do nothing? It is supposed to print statement in the spawned thread and then go into infinite loop.

CodePudding user response：

Using gdb we can see the thread forked successfully

[New Thread 0x7ffff7a2d640 (LWP 8261)]
[Switching to Thread 0x7ffff7a2d640 (LWP 8261)]

Thread 2 "a.out" hit Breakpoint 1, myThreadFun (received_struct=0x7fffffffd960) at tmp.cpp:15

If you delete the while(1) you can find the output. The delay in output may related to the stdio's buffer, while(1) prevented the buffer from flushing.

You can try another approach: add fflush(stdout); after printf, this works the same.

CodePudding user response：

It's because stdout buffers whole lines, so it means it won't print your values until you do one of three things:

end your string with \n: printf("%.1lf\n", ...)
flush the stdio buffer: fflush(stdout)
terminate the program (it will flush all buffers automatically)