c - Redirect stdout to a file

Question

I am trying to do the equivalent of the bash command ls>foo.txt in C.

The code bellow redirects the output to a variable.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>

int main(){
  int pfds[2];
  char buf[30];

  pipe(pfds);

  if (!fork()) {
    close(pfds[0]);
     //close(1);//Close stdout
    //dup(pfds[1]);
    //execlp("ls", "ls", NULL);
    write(pfds[1], "test", 5); //Writing in the pipe
    exit(0);
  } else {
    close(pfds[1]);  
    read(pfds[0], buf, 5); //Read from pipe
    wait(NULL);
  }
  return 0;
}

The comments lines refer to those operations that I believe that are required for the redirection. What should I change to redirect the output of ls to foo.txt?

Answer 1

While dealing with redirecting output to a file you may use freopen().

Assuming you are trying to redirect your stdout to a file 'output.txt' then you can write-

freopen("output.txt", "a+", stdout); 

Here "a+" for append mode. If the file exists then the file open in append mode. Otherwise a new file is created.

After reopening the stdout with freopen() all output statement (printf, putchar) are redirected to the 'output.txt'. So after that any printf() statement will redirect it's output to the 'output.txt' file.

If you want to resume printf()'s default behavior again (that is printing in terminal/command prompt) then you have to reassign stdout again using the following code-

freopen("/dev/tty", "w", stdout); /*for gcc, ubuntu*/  

Or -

freopen("CON", "w", stdout); /*Mingw C++; Windows*/ 

However similar technique works for 'stdin'.