用gdb理解C宏(#和##)

/* glibc-2.25/sysdeps/powerpc/powerpc64/sysdep.h */ 
207 #define tostring(s) #s
208 #define stringify(s) tostring(s)
209 #define XGLUE(a,b) a##b
210 #define GLUE(a,b) XGLUE(a,b)

 1 (gdb) help macro
 2 Prefix for commands dealing with C preprocessor macros.
 3 
 4 List of macro subcommands:
 5 
 6 macro define -- Define a new C/C++ preprocessor macro
 7 macro expand -- Fully expand any C/C++ preprocessor macro invocations in EXPRESSION
 8 macro expand-once -- Expand C/C++ preprocessor macro invocations appearing directly in EXPRESSION
 9 macro list -- List all the macros defined using the `macro define‘ command
10 macro undef -- Remove the definition of the C/C++ preprocessor macro with the given name
11 ...<snip>...
12 (gdb) #
13 (gdb) macro define tostring(s)  #s
14 (gdb) macro define stringify(s) tostring(s)
15 (gdb) macro define XGLUE(a,b)   a##b
16 (gdb) macro define GLUE(a,b)    XGLUE(a,b)
17 (gdb) #
18 (gdb) macro list
19 macro define GLUE(a, b) XGLUE(a,b)
20 macro define XGLUE(a, b) a##b
21 macro define stringify(s) tostring(s)
22 macro define tostring(s) #s
23 (gdb) #
24 (gdb) macro expand tostring(12345)
25 expands to: "12345"
26 (gdb) macro expand tostring("hello")
27 expands to: "\"hello\""
28 (gdb) #
29 (gdb) macro expand GLUE(123, 456)
30 expands to: 123456
31 (gdb) macro expand GLUE("hello", "world")
32 expands to: "hello""world"