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 */
 #define tostring(s) #s
 #define stringify(s) tostring(s)
 #define XGLUE(a,b) a##b
 #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()
 expands to: ""
 (gdb) macro expand tostring("hello")
 expands to: "\"hello\""
 (gdb) #
 (gdb) macro expand GLUE(, )
 expands to:
 (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 -
 mov $, %eax
 mov $, %eax
 $