gdb - print macros names instead of a values - c

Question

Is it possible to UNexpand macros in gdb ?
I've compiled binary with -g3 , and I can see macro definitions:
(gdb) info macro NGX_CORE_MODULE
Defined at /path/src/core/ngx_conf_file.h:70
included at /path/src/core/ngx_core.h:86
included at /path/src/core/ngx_cycle.c:9
#define NGX_CORE_MODULE 0x45524F43
(gdb)
but when i see at the variable, i see numeric value instead of the macro name
(gdb) p/x cycle->modules[0]->type
$17 = 0x45524f43
(gdb)
is it possible to get something like this:
(gdb) p cycle->modules[0]->type
$17 = NGX_CORE_MODULE
(gdb)
I'v tried user-defined function like this:
define expand_module_type_macro
if $arg0 == 0x45524F43
print "NGX_CORE_MODULE"
else
if $arg0 == 0x464E4F43
print "NGX_CONF_MODULE"
else
print $arg0
end
end
end
and it works
(gdb) expand_module_type_macro cycle->modules[0]->type
$18 = "NGX_CORE_MODULE"
(gdb)
but it has no practical use, because i can't use it with complex output:
(gdb) printf "module type=%s\n", expand_module_type_macro cycle->modules[0]->type
No symbol "expand_module_type_macro" in current context.
(gdb) set $m = expand_module_type_macro cycle->modules[0]->type
No symbol "expand_module_type_macro" in current context.
(gdb)

Answer 1

You might get something useful by creating a convenience function in python. I tested using this C program:
int var1 = 1234;
int var2 = 4567;
int
main ()
{
return 0;
}
And using this python script:
class ExpandMacro (gdb.Function):
"""Expand a value to a string, replacing with
macro name where possible."""
def __init__ (self):
super (ExpandMacro, self).__init__ ("expand")
def invoke (self, value):
if (value == 1234):
return "MY_MACRO"
else:
return str (value)
ExpandMacro ()
Then my GDB session:
(gdb) start
# .... snip ....
7 return 0;
(gdb) source lookup.py
(gdb) p $expand (var1)
$1 = "MY_MACRO"
(gdb) p $expand (var2)
$2 = "4567"
(gdb) printf "var1=%s\n", $expand (var1)
var1=MY_MACRO
(gdb) printf "var2=%s\n", $expand (var2)
var2=4567
In order to use the returned value with a %s format inside printf, I always return a string.