how can I put a breakpoint on “something is printed to the terminal” in gdb?

Question

I would like to know from where inside a huge application a certain message is printed. The application is so big and old that it uses all conceivable ways of printing text to the terminal; for example printf(), fprintf(stdout, ...) etc.

I write to put a breakpoint on the write() system call but then I'm flooded with too many breakpoint stops because of various file I/O operations that use write() as well.

So basically I want gdb to stop whenever the program prints something to the terminal but at the same time I don't want gdb to stop when the program writes something to a file.

Answer 1

Use a conditional breakpoint that checks the first parameter. On 64-bit x86 systems the condition would be:

(gdb) b write if 1==$rdi

On 32-bit systems, it is more complex because the parameter is on the stack, meaning that you need to cast $esp to an int * and index the fd parameter. The stack at that point has the return address, the length, buffer and finally fd.

This varies greatly between hardware platforms.

Answer 2

With gdb 7.0, you can set conditional breakpoint on syscall write():

(gdb) catch syscall write
Catchpoint 1 (syscall 'write' [4])
(gdb) condition 1 $ebx==1

$ebx contains first syscall parameter - FD number here