gdb: Macros
12 C Preprocessor Macros
************************
Some languages, such as C and C++, provide a way to define and invoke
"preprocessor macros" which expand into strings of tokens. GDB can
evaluate expressions containing macro invocations, show the result of
macro expansion, and show a macro's definition, including where it was
defined.
You may need to compile your program specially to provide GDB with
information about preprocessor macros. Most compilers do not include
macros in their debugging information, even when you compile with the
'-g' flag. 
Compilation.
A program may define a macro at one point, remove that definition
later, and then provide a different definition after that. Thus, at
different points in the program, a macro may have different definitions,
or have no definition at all. If there is a current stack frame, GDB
uses the macros in scope at that frame's source code line. Otherwise,
GDB uses the macros in scope at the current listing location; see
List.
Whenever GDB evaluates an expression, it always expands any macro
invocations present in the expression. GDB also provides the following
commands for working with macros explicitly.
'macro expand EXPRESSION'
'macro exp EXPRESSION'
Show the results of expanding all preprocessor macro invocations in
EXPRESSION. Since GDB simply expands macros, but does not parse
the result, EXPRESSION need not be a valid expression; it can be
any string of tokens.
'macro expand-once EXPRESSION'
'macro exp1 EXPRESSION'
(This command is not yet implemented.) Show the results of
expanding those preprocessor macro invocations that appear
explicitly in EXPRESSION. Macro invocations appearing in that
expansion are left unchanged. This command allows you to see the
effect of a particular macro more clearly, without being confused
by further expansions. Since GDB simply expands macros, but does
not parse the result, EXPRESSION need not be a valid expression; it
can be any string of tokens.
'info macro [-a|-all] [--] MACRO'
Show the current definition or all definitions of the named MACRO,
and describe the source location or compiler command-line where
that definition was established. The optional double dash is to
signify the end of argument processing and the beginning of MACRO
for non C-like macros where the macro may begin with a hyphen.
'info macros LOCATION'
Show all macro definitions that are in effect at the location
specified by LOCATION, and describe the source location or compiler
command-line where those definitions were established.
'macro define MACRO REPLACEMENT-LIST'
'macro define MACRO(ARGLIST) REPLACEMENT-LIST'
Introduce a definition for a preprocessor macro named MACRO,
invocations of which are replaced by the tokens given in
REPLACEMENT-LIST. The first form of this command defines an
"object-like" macro, which takes no arguments; the second form
defines a "function-like" macro, which takes the arguments given in
ARGLIST.
A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the 'macro
undef' command, described below. The definition overrides all
definitions for MACRO present in the program being debugged, as
well as any previous user-supplied definition.
'macro undef MACRO'
Remove any user-supplied definition for the macro named MACRO.
This command only affects definitions provided with the 'macro
define' command, described above; it cannot remove definitions
present in the program being debugged.
'macro list'
List all the macros defined using the 'macro define' command.
Here is a transcript showing the above commands in action. First, we
show our source files:
$ cat sample.c
#include <stdio.h>
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We're so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the GNU C compiler, GCC. We pass
the '-gdwarf-2'(1) _and_ '-g3' flags to ensure the compiler includes
information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start GDB on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
We can expand macros and examine their definitions, even when the
program is not running. GDB uses the current listing position to decide
which macro definitions are in scope:
(gdb) list main
3
4 #define M 42
5 #define ADD(x) (M + x)
6
7 main ()
8 {
9 #define N 28
10 printf ("Hello, world!\n");
11 #undef N
12 printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that 'macro expand-once' expands only the
macro invocation explicit in the original text -- the invocation of
'ADD' -- but does not expand the invocation of the macro 'M', which was
introduced by 'ADD'.
Once the program is running, GDB uses the macro definitions in force
at the source line of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10 printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro 'N' at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove 'N''s definition, and then
give it a new definition, GDB finds the definition (or lack thereof) in
force at each point:
(gdb) next
Hello, world!
12 printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14 printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
In addition to source files, macros can be defined on the compilation
command line using the '-DNAME=VALUE' syntax. For macros defined in
such a way, GDB displays the location of their definition as line zero
of the source file submitted to the compiler.
(gdb) info macro __STDC__
Defined at /home/jimb/gdb/macros/play/sample.c:0
-D__STDC__=1
(gdb)
---------- Footnotes ----------
(1) This is the minimum. Recent versions of GCC support '-gdwarf-3'
and '-gdwarf-4'; we recommend always choosing the most recent version of
DWARF.
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