神奇的 ## 符號

原文鏈接地址:http://starryalley.homelinux.net/blog/index.php?/archives/1134-Token-Concatenation-in-C-preprocessor.html

Token Concatenation in C preprocessor

使用程式碼來介紹比較快：

     #define foo() bar


     foo()baz


          ==> bar baz

not




          ==> barbaz  




所以加上’##’的用法，就可以達成barbaz，以下是範例：

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

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

經過C preprocessor處理後，會展開如下：

struct
 command commands[] = {
  { “quit”, quit_command },
  { “help”, help_command },
  //...
};

struct command commands[] = {
  { “quit”, quit_command },
  { “help”, help_command },
  //...
};

其中#NAME是Stringification的功能，所以配合’##’ token concatenation，我們可以結合兩個param。

之前在trace ffmpeg source code，其中竟然找不到put_pixels4_c()這類 function的definition，ctags找不到，grep好久也沒有。原來就是利用了token concatenation來產生的function。

以下是 節錄ffmpeg 0.5 source 裡面的一段code，用了C preprocessor token concatenation（其中省略了不相關的幾行程式碼）：

#define PIXOP2(OPNAME, OP) /
static
 
void
 OPNAME ## _pixels2_c(uint8_t *block, 
const
 uint8_t *pixels, 
int
 line_size, 
int
 h){/
  //...
  OP(*((uint16_t*)(block)), AV_RN16(pixels));/
  //...
}/
static
 
void
 OPNAME ## _pixels4_c(uint8_t *block, 
const
 uint8_t *pixels, 
int
 line_size, 
int
 h){/
  //...
  OP(*((uint32_t*)(block)), AV_RN32(pixels));/
  //...
}/
//...

#define PIXOP2(OPNAME, OP) /
static void OPNAME ## _pixels2_c(uint8_t *block, const uint8_t *pixels, int line_size, int h){/
  //... 
  OP(*((uint16_t*)(block)), AV_RN16(pixels));/
  //...
}/
static void OPNAME ## _pixels4_c(uint8_t *block, const uint8_t *pixels, int line_size, int h){/
  //...
  OP(*((uint32_t*)(block)), AV_RN32(pixels));/
  //...
}/
//...

在下面則使用這段code的方式:

#define op_avg(a, b) a = rnd_avg32(a, b)
#define op_put(a, b) a = b
PIXOP2(avg, op_avg)
PIXOP2(put,op_put)
#undef op_avg
#undef op_put
//from ffmpeg0.5, libavcodec/dsputil.c

#define op_avg(a, b) a = rnd_avg32(a, b)
#define op_put(a, b) a = b
PIXOP2(avg, op_avg)
PIXOP2(put,op_put)
#undef op_avg
#undef op_put
//from ffmpeg0.5, libavcodec/dsputil.c

使用gcc -E來展開preprocessor動作後，Line 1261和1262，呼叫PIXOP2的macro，就會自動產生新的function definition：

static
 
void
 avg_pixels2_c(uint8_t *block, 
const
 uint8_t pixels, 
int
 line_size, 
int
 h){
    ((uint16_t*)(block )) = rnd_avg32(*((uint16_t*)(block )), AV_RN16(pixels ));
}
static
 
void
 avg_pixels4_c(uint8_t *block, 
const
 uint8_t pixels, 
int
 line_size, 
int
 h){
    ((uint32_t*)(block )) = rnd_avg32(*((uint32_t*)(block )), AV_RN32(pixels ));
}
static
 
void
 put_pixels2_c(uint8_t *block, 
const
 uint8_t pixels, 
int
 line_size, 
int
 h){
    ((uint16_t*)(block )) = AV_RN16(pixels );
}
static
 
void
 put_pixels4_c(uint8_t *block, 
const
 uint8_t pixels, 
int
 line_size, 
int
 h){
    ((uint32_t*)(block )) = AV_RN32(pixels );
}

static void avg_pixels2_c(uint8_t *block, const uint8_t pixels, int line_size, int h){
    ((uint16_t*)(block )) = rnd_avg32(*((uint16_t*)(block )), AV_RN16(pixels ));
}
static void avg_pixels4_c(uint8_t *block, const uint8_t pixels, int line_size, int h){
    ((uint32_t*)(block )) = rnd_avg32(*((uint32_t*)(block )), AV_RN32(pixels ));
}
static void put_pixels2_c(uint8_t *block, const uint8_t pixels, int line_size, int h){
    ((uint16_t*)(block )) = AV_RN16(pixels );
}
static void put_pixels4_c(uint8_t *block, const uint8_t pixels, int line_size, int h){
    ((uint32_t*)(block )) = AV_RN32(pixels );
}

又學到一招，這code還真簡潔明瞭，不需要重複寫很多同質性的function definition。

有關C preprocessor的token concatenation介紹請參考這裡 。