Question

I have an OpenGL library bug I’m trying to trace, and the bug prints out something that looks like C code into stdout. This is the program, and the bug occurs as soon as glutMainLoop() is called, but I suspect that it’s not that function that is faulty. How would I go tracing what function wrote to stdout?

As per request, the output:

arc_ccw_turn, p = 0
case b
arc_ccw_turn, p = 0
case d
arc_ccw_turn, p = 0
case a
arc_ccw_turn, p = 0
case c

I’ve reported the bug already, but I’d try and provide a GDB backtrace for the issue too.

Answer 1

If you are using Linux then set a breakpoint on write(), all output to stdout and stderr eventually goes through this function. The following is for x86-64, for other architectures you would need to change register names:

$ gdb /usr/bin/cat
Reading symbols from /usr/bin/cat...(no debugging symbols found)...done.
(gdb) set args /proc/cpuinfo
(gdb) b write
Breakpoint 1 at 0x401740
(gdb) condition 1 ($rdi == 1 || $rdi == 2)
(gdb) display $rdi
(gdb) display $rsi
(gdb) display $rdx
(gdb) r
Starting program: /usr/bin/cat /proc/cpuinfo
3: $rdx = 3368
2: $rsi = 6348800
1: $rdi = 1
(gdb) p (char*)$rsi
$4 = 0x60e000 "processor\t: 0\nvendor_id\t: GenuineIntel\ncpu family\t: 6\nmodel\t\t: 30\nmodel name\t: Intel(R) Core(TM) i7 CPU         870  @ 2.93GHz\nstepping\t: 5\nmicrocode\t: 0x5\ncpu MHz\t\t: 1199.000\ncache size\t: 8192 KB\nphy"...