用gdb理解C宏(#和##)

在Unix/Linux内核代码以及GNU libc源代码中，有两个C的宏被广泛使用。 例如：

/* 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)

在gdb中用命令macro定义并展开看看，

(gdb) help macro
Prefix for commands dealing with C preprocessor macros.

List of macro subcommands:

macro define -- Define a new C/C++ preprocessor macro
macro expand -- Fully expand any C/C++ preprocessor macro invocations in EXPRESSION
macro expand-once -- Expand C/C++ preprocessor macro invocations appearing directly in EXPRESSION
macro list -- List all the macros defined using the `macro define' command
macro undef -- Remove the definition of the C/C++ preprocessor macro with the given name
...<snip>...
(gdb) #
(gdb) macro define tostring(s)  #s
(gdb) macro define stringify(s) tostring(s)
(gdb) macro define XGLUE(a,b)   a##b
(gdb) macro define GLUE(a,b)    XGLUE(a,b)
(gdb) #
(gdb) macro list
macro define GLUE(a, b) XGLUE(a,b)
macro define XGLUE(a, b) a##b
macro define stringify(s) tostring(s)
macro define tostring(s) #s
(gdb) #
(gdb) macro expand tostring(12345)
expands to: "12345"
(gdb) macro expand tostring("hello")
expands to: ""hello""
(gdb) #
(gdb) macro expand GLUE(123, 45)
expands to: 12345
(gdb) macro expand GLUE("hello", "world")
expands to: "hello""world"

由此可见，

• #是将单个宏参数转换成一个字符串
• ##则是将两个宏参数连接在一起

可以简记为“单#字符串，双#连接参”。

另外，也可以用"gcc -E"查看宏展开。例如，

$ cat foo.S
#define SYS_ify(syscall_name)   __NR_##syscall_name
#define __NR_mmap               115
#define __NR_munmap             117

mov $SYS_ify(mmap), %eax
mov $SYS_ify(munmap), %eax

$ gcc -E foo.S | tail -2
mov $115, %eax
mov $117, %eax
$