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. See 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-2¹⁴ 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)