gcc -D選項在man中的說明如下：

 -D name
           Predefine name as a macro, with definition 1.


       -D name=definition
           The contents of definition are tokenized and processed as if they
           appeared during translation phase three in a #define directive.  In
           particular, the definition will be truncated by embedded newline
           characters.


           If you are invoking the preprocessor from a shell or shell-like
           program you may need to use the shell’s quoting syntax to protect
           characters such as spaces that have a meaning in the shell syntax.


           If you wish to define a function-like macro on the command line,
           write its argument list with surrounding parentheses before the
           equals sign (if any).  Parentheses are meaningful to most shells,
           so you will need to quote the option.  With sh and csh,
           -D’name(args...)=definition’ works.


           -D and -U options are processed in the order they are given on the
           command line.  All -imacros file and -include file options are
           processed after all -D and -U options.

簡單點說，加上-D選項就是在編譯時增加對-D後面的巨集的定義。來看個簡單的例子吧，源程式(a.c)如下：

#include <stdio.h>

int main(int argc, char *argv[])
{
#ifdef MYDEBUG
	printf("test\n");
#endif
	printf("hello world\n");

	return 0;
}

[[email protected] #9]#gcc -DMYDEBUG -g -o a1 a.c       @1
[[email protected] #11]#./a1
test
hello world
[

可見，第2和第3條編譯指令的輸出結果是一樣的，為什麼呢？

先看@1,因為源程式中有條件判斷，是否定義了巨集“MYDEBUG”，而該編譯指令中剛好帶了"MYDEBUG"，所以就輸出了“test"；

接著看@2, 編譯指令中-D後是”DEBUG“，這個針對”#ifdef MYDEBUG"而言就是false,所以就不會輸出“test"；

最後看@3, 編譯指令中無-D選項，所以也不會輸出”test"。

這樣做比較，相信大家都明白這個-D的含義了吧。不信，就親自動手試試吧。