GCC如何在宏中引用過程？

Question

我正在嘗試爲其宏擴展測試gcc預處理器。

我寫下面的代碼：（只是用於測試）

#include <stdio.h> 

#define QUOTE " 
#define TMPL hello 

int main(){ 
    printf(QUOTE TMPL QUOTE); 
    return 0; 
} 

編譯結果是：

$ gcc main.c -o main 
main.c:3:15: warning: missing terminating " character 
main.c: In function ‘main’: 
main.c:7: error: missing terminating " character 
main.c:7: error: missing terminating " character 
main.c:7: error: ‘hello’ undeclared (first use in this function) 
main.c:7: error: (Each undeclared identifier is reported only once 
main.c:7: error: for each function it appears in.) 
main.c:7: warning: format not a string literal and no format arguments 
main.c:7: warning: format not a string literal and no format arguments 
$ 

然後我嘗試看看預處理的結果

$ gcc -E main.c -o tmp.c 
main.c:3:15: warning: missing terminating " character 
$ 

雖然發出警告，但它以某種方式在中產生正確的預處理代碼10

int main(){ 
printf(" hello "); 
return 0; 
} 

我編譯tmp.c，hello被正確打印。

我想知道爲什麼gcc -E可以產生正確的代碼，而使用gcc直接編譯失敗。 gcc預處理器的兩種方法有什麼區別？

$ gcc --version 
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00) 

Answer 1

正如我所評論的，預處理器宏應擴展爲一系列詞法分析符號。使用GCC源代碼，libcpp（負責預處理和標記）產生了一個令牌流（不是普通的字符串）。最近GCC 4.8，當爲gcc -Wall endyul.c -o endyl上你的榜樣運行，給人非常有幫助診斷：

endyul.c:3:15: warning: missing terminating " character [enabled by default] 
#define QUOTE " 
      ^
endyul.c: In function 'main': 
endyul.c:7:5: error: missing terminating " character 
    printf(QUOTE TMPL QUOTE); 
    ^
endyul.c:7:5: error: missing terminating " character 
endyul.c:4:14: error: 'hello' undeclared (first use in this function) 
#define TMPL hello 
      ^
endyul.c:7:18: note: in expansion of macro 'TMPL' 
    printf(QUOTE TMPL QUOTE); 
       ^
endyul.c:4:14: note: each undeclared identifier is reported only once for each function it appears in 
#define TMPL hello 
      ^
endyul.c:7:18: note: in expansion of macro 'TMPL' 
    printf(QUOTE TMPL QUOTE); 
       ^

你的GCC 4.2已經很老了。你應該考慮升級它。

而且clang（3.3）也提供了一個很好的診斷：

clang -Wall endyul.c -o endyul 
endyul.c:3:15: warning: missing terminating '"' character [-Winvalid-pp-token] 
#define QUOTE " 
      ^
endyul.c:7:12: error: expected expression 
    printf(QUOTE TMPL QUOTE); 
      ^
endyul.c:3:15: note: expanded from macro 'QUOTE' 
1 warning and 1 error generated. 

再次閱讀CPP manual of GCC，尤其是在Stringification和Concatenation的章節。