Question

I have a program which prints (by printf) to the stdout some data and also calls to function *foo*
which also prints to the stdout some data [the way (implementation) of how printing is done from foo is unknown and I can`t see the code of foo].

I have to redirect everything from stdout to buffer or file. I tried to do it in several ways

1. freopen(file.txt, stdout) – only my code prints are written to the file.txt. What was printed from foo is lost.
2. setbuf(buffer, stdout) – only my code prints are written to the buffer. What was printed from foo is appears in the stdout.(It appears on the screen)

What can explain this behavior? How can the problem be solved?

Note:This code has to work in cross-OS( lunux/wind && mac OS).I use gcc in order compile the code and I have cygwin

Answer 1

It’s likely that foo isn’t using stdio for printing and directly calling the OS for this.

I don’t know about win32, but on POSIX you could use dup2 to take care of it.

/* Before the function foo is called, make `STDOUT_FILENO` refer to `fd` */
int fd;
fd = open(...);
dup2(fd, STDOUT_FILENO);

EDIT

Much to my surprise, win32 has _dup2 but it does something else.