Why is the order of output of a C program different when its stdout is redirected to a file? [duplicate]

Question

Here is my program.

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

int main()
{
    printf("Hello\n");
    system("uname");
    return 0;
}

Here is the output.

$ gcc foo.c
$ ./a.out 
Hello
Linux

However, if I redirect the output of the program to a file, I see that the order of the output is reversed, i.e. Linux is printed before Hello.

$ ./a.out > out.txt
$ cat out.txt
Linux
Hello

Why is the order of the output different when redirection is involved?

Answer 1

This is because stdout is buffered in different ways. When you call your program without the redirection, the buffering defaults to linewise buffering. In the second call, the buffer is much larger, and get written when your program terminates. Since your call to uname terminated before, this output now shows up earlier in the file. When you depend on the ordering, you can either explicit call fflush(stdout) after your printf call, or you can call uname via popen and print its output explicit.