Implementing RAII in pure C?

gooli_15109 0
Viewed 18650

Is it possible to implement RAII in pure C?

I assume it isn't possible in any sane way, but perhaps is it possible using some kind of dirty trick. Overloading the standard free function comes to mind or perhaps overwriting the return address on the stack so that when the function returns, it calls some other function that somehow releases resources? Or maybe with some setjmp/longjmp trick?

This is of a purely academic interest and I have no intention of actually writing such unportable and crazy code but I'm wondering if that is at all possible.

raii c
10 Answers

This is inherent implementation dependent, since the Standard doesn't include such a possibility. For GCC, the cleanup attribute runs a function when a variable goes out of scope:

#include <stdio.h>

void scoped(int * pvariable) {
    printf("variable (%d) goes out of scope\n", *pvariable);
}

int main(void) {
    printf("before scope\n");
    {
        int watched __attribute__((cleanup (scoped)));
        watched = 42;
    }
    printf("after scope\n");
}

Prints:

before scope
variable (42) goes out of scope
after scope

See here

litb_345090

Probably the easiest way is to use goto to jump to a label at the end of a function but that's probably too manual for the sort of thing you're looking at.

PrisonerOfPain_314860

I'd opt for overwriting the return address on the stack. It'd work out as the most transparent. Replacing free will only work with heap-allocated "objects".

Roger_Lipscombe_84460

Have you looked at alloca()? It will free when an var leaves scope. But to use it effecticly the caller must always do the alloca before sending it to things... If you were implementing strdup, well, you can't use alloca.

user_938630 
my implementation of raii for c in pure c and minimal asm
@ https://github.com/smartmaster/sml_clang_raii

**RAII for C language in pure C and ASM**

**featurs : **

-easy and graceful to use
- no need seperate free cleanup functions
- able to cleanup any resources or call any function on scope exits


**User guide : **

-add source files in src folder to your project
-include sml_raii_clang.h in.c file
-annote resource and its cleanup functions

/* sample code */

void sml_raii_clang_test()
{
    //start a scope, the scope name can be any string
    SML_RAII_BLOCK_START(0);


    SML_RAII_VOLATILE(WCHAR*) resA000 = calloc(128, sizeof(WCHAR)); //allocate memory resource
    SML_RAII_START(0, resA000); //indicate starting a cleanup code fragment, here 'resA000' can be any string you want
    if (resA000) //cleanup code fragment
    {
        free(resA000);
        resA000 = NULL;
    }
    SML_RAII_END(0, resA000); //indicate end of a cleanup code fragment


    //another resource
    //////////////////////////////////////////////////////////////////////////
    SML_RAII_VOLATILE(WCHAR*) res8000 = calloc(128, sizeof(WCHAR));
    SML_RAII_START(0, D000);
    if (res8000)
    {
        free(res8000);
        res8000 = NULL;
    }
    SML_RAII_END(0, D000);


    //scope ended, will call all annoated cleanups
    SML_RAII_BLOCK_END(0);
    SML_RAII_LABEL(0, resA000); //if code is optimized, we have to put labels after SML_RAII_BLOCK_END
    SML_RAII_LABEL(0, D000);
}
user_82371560
Related