c - Internally capture/redirect stdout?

Question

This seems like a bit of a computing systems 101 question, but I'm stumped.

I am integrating existing code from C/C++ project A into my own project B. Both A and B will be linked into a single executable, threaded process. Project A's code makes extensive use of printf for output. This is fine, but I want also to capture that output into my own buffers. Is there a way I can read from stdout once the printf calls have written to it? I cannot fork the process or pipe. And my efforts to poll() stdout, or to dup() it, have not succeeded (I may be doing something wrong here).

Answer 1

You can use freopen to change the descriptor.

#include<stdio.h>

main(int argc, char** argv) {
    FILE *fp = freopen("output.txt", "w", stdout);
    printf("Hello
");
    fclose(fp);
}

If you run that you'll see the printf output in output.txt and nothing will go to your screen.

You can now open the file to read the data or you could even mmap it into your memory space and process it that way.