Clang's blocks produce atrocious code. Look at the disassembly for:

    int main(){ __block int (^foo)(int x) = ^ int (int x) { if(x<100) return foo(x+10); return x; }; return foo(69); }

versus the GCC:

    int main(){ int foo(int x) { if(x<100) return foo(x+10); return x; }; return foo(69); }

if you want to see what I'm talking about.

CLANG: http://pastebin.com/37A9by4V

GCC: http://pastebin.com/RMEDnwxi

However, defer does look interesting, and implementing it for GCC is very easy:

    #define defer_(x) do{}while(0); \
            auto void _dtor1_##x(); \
            auto void _dtor2_##x(); \
            int __attribute__((cleanup(_dtor2_##x))) _dtorV_##x=69; \
            void _dtor2_##x(){if(_dtorV_##x==42)return _dtor1_##x();};_dtorV_##x=42; \
            void _dtor1_##x()
    #define defer__(x) defer_(x)
    #define defer defer__(__COUNTER__)

You don't have to use the stupid block-syntax either, just:

    in = fopen("whatever", "r");
    defer { fclose(in); }