GCC类型的扩展

Question

我不明白为什么会这样:

/* gcc range extension */
__extension__ static int fn(int n)
{
    switch (n) {
        case 0: return 0;
        case 1 ... 1000: return 1;
        default: return -1;
    }
}

但这不是:

/* gcc typeof extension */
__extension__ static void fn(int n)
{
    typeof(n) a = n;

    printf("%d\n", a);
}

gcc返回:

demo.c:14: warning: implicit declaration of function ‘typeof’
demo.c:14: warning: nested extern declaration of ‘typeof’
demo.c:14: error: expected ‘;’ before ‘a’
demo.c:16: error: ‘a’ undeclared (first use in this function)
demo.c:16: error: (Each undeclared identifier is reported only once
demo.c:16: error: for each function it appears in.)

我知道我可以编译-std=gnu99以避免错误,但第一个使用-std=c99并使用扩展

Answer 1

非ANSI兼容关键字不会被重新启用__extension__(唯一的影响__extension__是警告抑制-pedantic).__typeof__如果要在ANSI模式下编译,请使用.