GDB: redirect target stdout temporarly

debugging, gdb, linux, stdout

Solution

i cannot find a way to restore the stdout of the inferior back to normal

Here is how you can do it:

Reading symbols from /tmp/./a.out...done.
(gdb) list main
1   #include <stdio.h>
2
3   int main() {
4     int i;
5
6     for (i = 0; i < 1000; ++i) {
7       printf("A line we don't care about: %d\n", i);
8     }
9     printf("An important line\n");
10    return 0;
11  }
(gdb) b 9
Breakpoint 1 at 0x400579: file t.c, line 9.
(gdb) run > /dev/null
Starting program: /tmp/./a.out > /dev/null

Breakpoint 1, main () at t.c:9
9     printf("An important line\n");
(gdb) call fflush(0)
$1 = 0

Since we are about to switch output, we want to make sure to flush any buffered data.

Next we call `open("/dev/tty", O_WRONLY)`. You can find the value of `O_WRONLY` by `grep`ping for it in `/usr/include`.

(gdb) shell grep WRONLY /usr/include/bits/*.h
/usr/include/bits/fcntl.h:#define O_WRONLY       01
(gdb) p open("/dev/tty", 1)
$2 = 3

So we now have a new file descriptor `3` that will output to the current terminal. Finally, we switch `STDOUT_FILENO` to it, like so:

(gdb) call dup2(3, 1)
$3 = 1
(gdb) c
Continuing.
An important line
[Inferior 1 (process 22625) exited normally]

Voilà: "An important line" was printed to the terminal.

Problem

Original source