Defer cleanup for C (gcc and llvm)

defer.h

#define DEFER_MERGE(a,b) a##b
#define DEFER_VARNAME(a) DEFER_MERGE(defer_scopevar_, a)
#define DEFER_FUNCNAME(a) DEFER_MERGE(defer_scopefunc_, a)
#define DEFER(BLOCK) void DEFER_FUNCNAME(__LINE__)(int *a) BLOCK; __attribute__((cleanup(DEFER_FUNCNAME(__LINE__)))) int DEFER_VARNAME(__LINE__)

// Usage:
/*
  void dosomething()
  {
    char* data = malloc(100);
    DEFER({ free(data); });
    
    dosomethingwith(data);
  }
*/

Inspired by http://fdiv.net/2015/10/08/emulating-defer-c-clang-or-gccblocks
But this version doesn't use the special extension "blocks" that doesn't exist in a normal gcc.

clang 10.0.0

    char (* buff_ptr_)[buffer_size()] = {0} ;
    DEFER({free(buff_ptr_) ;});

I get: "Function definition is not allowed here"? Perhaps nested functions need some switch for clang?

I'm not sure, I have a different piece of code utilizing this in my libwire wire_defer.h but that code is marked GCC only. I don't remember why is that anymore.

https://godbolt.org/z/YaKEGY

compiles using clang ... but "disappears" upon execution?

Try this https://godbolt.org/z/6Y8Ys4dPo

Any C version and any compiler.

Nice! You can add __attribute__((unused)) before the int *a parameter to get rid of the unused-parameter warning:

...
#define DEFER(BLOCK) void DEFER_FUNCNAME(__LINE__)(__attribute__((unused)) int *a) BLOCK; __attribute__((cleanup(DEFER_FUNCNAME(__LINE__)))) int DEFER_VARNAME(__LINE__)

Now simpler to re-use: https://godbolt.org/z/zsYre5Kfj

@sinecode
Did you manage to get rid of the warning: ISO C forbids nested functions [-Wpedantic] warning?

Try this https://godbolt.org/z/6Y8Ys4dPo

Any C version and any compiler.

Now simpler to re-use: https://godbolt.org/z/zsYre5Kfj

Problem with the for loop approach:

defer( close_file( dummsy_ )) {
    // Do something with the file
    if (/* something failed */)
        return;
    // ...and now close_file doesn't get called 🧨
}