Question

I am debugging a large application and I would like to break before a certain string is written to std::cerr. I there a way to do this?

There are many places in the code where this string is written.
The string is not always of the form

std::cerr << "some error code";

It might be of other forms such as

std::cerr << write_error_code();

Also note that there are many things being written to std::cerr so I can’t just break on std::cerr.

I also have the same question but for std::cout but I will post another question if the answer does not apply to that as well.

Answer 1

This is a С++ example I have made on Linux. The example has been built with gcc 4.3.

#include <iostream>
#include <stdio.h>
#include <setjmp.h>

void write_error_code()
{
  std::cerr << "some error code";
}

int main()
{
  std::cout << "Start, main()\n";
  std::cerr << "some error code" << std::endl;
  std::cerr << "some error code?" << std::endl;
  std::cerr << "some error code?" << std::endl;
  write_error_code();
  std::cout << "End, main()\n";

  return 0;
}

I took a look at disassemble and saw that std::cerr << "" is a function with the symbol name _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc.
The function has 2 args: __out and __s.
So I created a .gdbinit file:

host: srv2-x64rh5-02, OS: Linux 2.6.18-238.el5>more .gdbinit
file a.out
b _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
commands
silent
if strcmp(__s, "some error code") == 0
  print "Some error code has been given as an arg"
  bt
  c
else
  c
end
end
r
quit

And here I test my program:

host: srv2-x64rh5-02, OS: Linux 2.6.18-238.el5>gdb -q ./a.out
Reading symbols from /import/home/sergey.kurenkov/src/linux.x64.5.0/tests/test.br_on_std_cerr/a.out...done.
Breakpoint 1 at 0x400710
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x2aaaaaaab000
Start, main()
$1 = "Some error code has been given as an arg"
#0  std::operator<< <std::char_traits<char> > (__out=..., __s=0x400a4c "some error code")
    at /home/zalex/test/gcc_build/x86_64-unknown-linux-gnu/libstdc++-v3/include/ostream:509
#1  0x0000000000400873 in main () at main.cpp:13
some error code
some error code?
some error code?
$2 = "Some error code has been given as an arg"
#0  std::operator<< <std::char_traits<char> > (__out=..., __s=0x400a4c "some error code")
    at /home/zalex/test/gcc_build/x86_64-unknown-linux-gnu/libstdc++-v3/include/ostream:509
#1  0x000000000040084f in write_error_code () at main.cpp:7
#2  0x00000000004008f5 in main () at main.cpp:18
some error codeEnd, main()

Program exited normally.