How to redirect the output of system() to a file?

Question

In this C program

#include <stdio.h>
#include <fcntl.h>
int main()
{
    int file = open("Result", O_CREAT|O_WRONLY, S_IRWXU);

    dup2(stdout, file);
    system("ls -l");

    return 0;
}

I'm trying to redirect the output of system() to a file, for that i have used dup2 but it is not working.

What's wrong with this code?
and, please tell me if there is any better way to do this? (without using > at the terminal )

Answer 1

stdout is a FILE * pointer of the standard output stream. dup2 expects file descriptor, also you've messed up the parameters order. Use

dup2(file, 1);

instead.

On the better-way-to-do-this part. This way is bad because you probably want to restore your standard output after this system call completes. You can do this in a variety of ways. You can dup it somewhere and then dup2 it back (and close the dupped one). I personally don't like writing own cat implementations as suggested in other answers. If the only thing you want is redirecting a single shell command with system to a file in the filesystem, then probably the most direct and simple way is to construct the shell command to do this like

system("ls -l > Result");

But you have to be careful if filename (Result) comes from user input as user can supply something like 'Result; rm -rf /*' as the filename.

Also, on the topic of security, you should consider specifying the full path to ls as suggested in the comments:

system("/bin/ls -l > Result");