[C,GCC,CPP] 巨集與不定變數 (## __VA_ARGS__)

為了debug方便，我們常會利用marcro將客製化printf，其中牽扯了不定變數巨集的問題，如下說明:

#include <stdio.h>

#if VERSION==0

#define PRINTF(fmt,...)

#elif VERSION==1

#define PRINTF(fmt,...) printf(fmt,__VA_ARGS__) //C99 standard version. fmt is replaced by strings.

#else

#define PRINTF(fmt,...) printf(fmt,##__VA_ARGS__) //Use ## to solve the comma problem.

#endif

int main()

{

        PRINTF("Only one argument test\n");

        PRINTF("Two arguments test, and the time is %s\n",__TIME__);

}

VERSION 1會產生compiler error(")"前少了一個描述):

user@user:~/works/test/gnu_lab$ gcc -c -DVERSION=1 test.c -o test.i

test.c: In function ‘main’:

test.c:6:47: error: expected expression before ‘)’ token

 #define PRINTF(fmt,...) printf(fmt,__VA_ARGS__) //C99 standard version

test.c:10:2: note: in expansion of macro ‘PRINTF’

  PRINTF("Only one argument test\n");

利用-E(CPP)來將Macro展開，查看出了什麼問題(當只有一個參數時展開多了一個",")

user@user:~/works/test/gnu_lab$ gcc -E -DVERSION=1 test.c -o test.i

user@user:~/works/test/gnu_lab$ cat test.i

# 8 "test.c"

int main()

{

 printf("Only one argument test\n",);

 printf("Two arguments test, and the time is %s\n","10:57:36");

}

為了避免只有一個參數的錯誤，GNU CPP 使用 "##" 做為標記，當沒有不定變數時(只有一個參數)，CPP會將"##"前的","移除

user@user:~/works/test/gnu_lab$ gcc -E -DVERSION=2 test.c -o test.i

int main()

{

 printf("Only one argument test\n");

 printf("Two arguments test, and the time is %s\n","10:57:36");

}

另外，為了增加可讀性，GCC允許在並定變數前增加其使用方式如下:

#define PRINTF(fmt,args...) printf(fmt,args)

Note:

1. …: variable argument
2. __VA_ARGS__: this identifier is replaced by variable argument

Reference:

1. http://stackoverflow.com/questions/5588855/standard-alternative-to-gccs-va-args-trick
2. https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html