__attribute__ (( __cleanup__))

1. Brief description:

cleanup as an optional attribute value in the __attribute__ attribute

Its effect is that when the variable it declares leaves its life cycle, then

The destroy function you specify will be called automatically

 

2. Examples:

#include <stdio.h>  
#include <stdlib.h>  
#include <string.h>  
  
void destroy_string(char **str) {  
    printf("final str : %s\n", *str);  
    free(*str);  
}  
  
int main(int argc, char **argv) {  
  
    char *str __attribute__ ((__cleanup__(destroy_string))) = NULL;  
    str = (char*)malloc((sizeof(char)) * 100);  
    strcpy(str, "hello world!");  
    printf("current str : %s\n", str);  
    return 0;  
}  

Results of the:

current str : hello world!

final str : hello world!

pass again

valgrind --tool=memcheck --leak-check=full Check for memory leaks

 

==6298== All heap blocks were freed -- no leaks are possible

You can see that the memory is correctly released

 

3. Principle:

In fact, the operation of releasing the code is to transfer the manual writing method to the compiler implementation.

It can be viewed by gcc -S main.c

call    printf  
movl    $0, %ebx  
leaq     - 32 (% rbp),% rax  
movq     % rax,% rdi  
call    destroy_string

There is a section of this function in the main function, you will find that after calling the printf function, it will continue to call the destroy_string destroy function

 

But this destroy function call is not explicitly written by our code, so this is added implicitly by the compiler.