gcc -D：gcc的预定义功能

gcc编译的时候提供了预定义功能，参数是-D,  通过man gcc可以了解更多：
-D name
           Predefine name as a macro, with definition 1.

       -D name=definition
           Predefine name as a macro, with definition 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 new-line characters.

可以通过以下例子解释一下：
int main()
{
    #ifdef HELLO
    printf("HELLO defined\n");
    #else
    printf("HELLO not define\n");
    #endif

    return 0;
}
在该程序中，判断是否已经定义了宏DEBUG,从而输出相应的语句。
如果编译该程序时采用了gcc -DHELLO, 则输出：HELLO defined