Preprocessor

• # and# in macros
  • #: stringified what comes after
  • ##: concatenation
• What is concatenation ?
  • Concatenation
  • It is often useful to merge two tokens into one while expanding macros. This is called token pasting or token concatenation. The ‘##’ preprocessing operator performs token pasting. When a macro is expanded, the two tokens on either side of each ‘##’ operator are combined into a single token, which then replaces the ‘##’ and the two original tokens in the macro expansion.

## 將左右兩遍的內容結合起來，例子如下：

struct command
{
  char *name;
  void (*function) (void);
};
 
struct command commands[] =
{
  { "quit", quit_command },
  { "help", help_command },
  …
};

#define COMMAND(NAME)  { #NAME, NAME ## _command }
 
struct command commands[] =
{
  COMMAND (quit),
  COMMAND (help),
  …
};

container_of()

• ‘weird’ kernel macros — container_of
• The (char *) casting in container_of() macro in linux kernel

BUILD_BUD_ON_ZERO(e)

• C 語言的 bit-field
  • zero-width bit field 有個規定是必須 unnamed ，再來 zero-width bit field 宣告不會使用到任何空間，但是會強制 structure 中下一個 bit field 對齊到下一個 unit 的 boundary
  • What is zero-width bit field

Linux 核心原始程式碼巨集: max, min

• Linux 核心原始程式碼巨集: max min
• What is the function of (void) (& min1 == & min2) in the min macro in kernel.h?

Reference

• 你所不知道的 C 語言：前置處理器應用篇