How to capture output of printf?

Question

I am calling a function funcB from funcA. funcB uses several printf statements to output data. Is there a way for me to capture that data via funcA? I can not modify funcB.

funcB(){
     printf( "%s", "My Name is" );
     printf( "%s", "I like ice cream" );
}

funcA(){
    funcB();
}

Answer 1

(This answer is a corrected version based on this answer.)

This answer is POSIX centric. Use open to create a file descriptor for the file you want to redirect to. Then, use dup2 to STDOUT_FILENO to change stdout to write to the file instead. But, you'll want to dup the STDOUT_FILENO before you do that, so you can restore stdout with another dup2.

fflush(stdout);
int stdout_fd = dup(STDOUT_FILENO);
int redir_fd = open(redirected_filename, O_WRONLY);
dup2(redir_fd, STDOUT_FILENO);
close(redir_fd);
funcB();
fflush(stdout);
dup2(stdout_fd, STDOUT_FILENO);
close(stdout_fd);

If funcB is using std::cout, use std::cout.flush() instead of fflush(stdout).

If you want to manipulate C++ streams more directly, you can use Johnathan Wakely's answer.

Answer 2

If nothing else in your program uses printf, you can write your own version and link it explicitly. The linker will not look in the standard library if the function is already defined. You can probably use vsprintf for the implementation, or some safer version with overrun checking if it is supplied by your compiler.