C language. Read from stdout

Question

I have some troubles with a library function. I have to write some C code that uses a library function which prints on the screen its internal steps. I am not interested to its return value, but only to printed steps. So, I think I have to read from standard output and to copy read strings in a buffer. I already tried fscanf and dup2 but I can't read from standard output. Please, could anyone help me?

Answer 1

An expanded version of the previous answer, without using files, and capturing stdout in a pipe, instead:

#include <stdio.h>
#include <unistd.h>

main()
{
   int  stdout_bk; //is fd for stdout backup

   printf("this is before redirection\n");
   stdout_bk = dup(fileno(stdout));

   int pipefd[2];
   pipe2(pipefd, 0); // O_NONBLOCK);

   // What used to be stdout will now go to the pipe.
   dup2(pipefd[1], fileno(stdout));

   printf("this is printed much later!\n");
   fflush(stdout);//flushall();
   write(pipefd[1], "good-bye", 9); // null-terminated string!
   close(pipefd[1]);

   dup2(stdout_bk, fileno(stdout));//restore
   printf("this is now\n");

   char buf[101];
   read(pipefd[0], buf, 100); 
   printf("got this from the pipe >>>%s<<<\n", buf);
}

Generates the following output:

this is before redirection
this is now
got this from the pipe >>>this is printed much later!
good-bye<<<