Results of printf() and system() are in the wrong order when output is redirected to a file

I have a C program that compiles to an executable called myprogram. This is its main function:

int main(int argc, char ** argv) {
  printf("this is a test message.\n");
  system("ls");

  return 0;
}

When I run myprogram > output.txt in a Linux shell and then examine output.txt, I see the output of ls listed above "this is a test message."

I feel like it should be the other way around. Why is this happening, and what can I do so that "this is a test message" appears at the top of output.txt?

If it matters, I'm new to both C and working in a command line.

By default output to stdout is line-buffered when connected to a terminal. That is, the buffer is flushed when it's full or when you add a newline.

However, if stdout is not connected to a terminal, like what happens when you redirect the output from your program to a file, then stdout becomes fully buffered. That means the buffer will be flushed and actually written either when it's full or when explicitly flushed (which happens when the program exits).

This means that the output of a separate process started from your code (like what happens when you call system) will most likely be written first, since the buffer of that process will be flushed when that process ends, which is before your own process.

What happens when using redirection (or pipes for that matter):

1. Your printf call writes to the stdout buffer.
2. The system function starts a new process, which writes to its own buffer.
3. When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.
4. Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout, or call setbuf before any output to disable buffering completely.

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

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

int main(int argc, char ** argv) {
  printf("this is a test message.\n");
  fflush(stdout);
  system("ls");

  return 0;
}

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

I suspect it's because of the order in which the stdout buffer gets flushed, which is not necessarily deterministic. It's possible that the parent spawns the ls process and doesn't flush its own stdout until after that returns. It may not actually flush stdout until the process exits.

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.