__attribute__系列之cleanup

cleanup属性：当变量离开它的作用域时，设置的cleanup_function函数将被调用。

cleanup (cleanup_function)

The cleanup attribute runs a function when the variable goes out of scope. This attribute can only be applied to auto function scope variables; it may not be applied to parameters or variables with static storage duration. The function must take one parameter, a pointer to a type compatible with the variable. The return value of the function (if any) is ignored.

If -fexceptions is enabled, then cleanup_function will be run during the stack unwinding that happens during the processing of the exception. Note that the cleanupattribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally.

#include <stdio.h>
#include <stdlib.h>

#define local_type  __attribute__ ((cleanup(my_free)))

static void my_free(void* pmem)//the type of paramter must be a void pointer
{
      printf("pmem=%p...\n", pmem);
      printf("&pmem=%p...\n", &pmem);
      void** ppmem = (void**) pmem;
      printf("*ppmem=%p...\n", *ppmem);
      free(*ppmem);
}

#if 0
//warning: passing argument 1 of ‘my_free’ from incompatible pointer type
static void my_free(void** ppmem)
{
      printf("ppmem=%p...\n", ppmem);
      printf("*ppmem=%p...\n", *ppmem);
}
#endif

int foo(void)
{
    local_type int* p = (int*) malloc(sizeof(int));
    printf("do something, p=%p\n", p);

    return 0;
}

int main(int argc, char** argv)
{

    foo();
    // when return, the memory block pointed by p is freed automatically
    printf("11111111\n");

    return 0;
}

利用cleanup属性来实现智能指针：

#include <stdlib.h>
#include <stdio.h>

struct shared_ptr_s
{
    //    struct impl_t* inst;
    int *use_cnt;
};

typedef struct shared_ptr_s shared_ptr_t;           // unadorned type
#define shared_ptr struct shared_ptr_s __attribute__((cleanup(free_shared)))

#define SHARED_PTR_GET_ADD_REF(sp_in, sp_name) ++(*sp_in.use_cnt); \
                        printf("add use_cnt = %d, sp_in=%p\n", *sp_in.use_cnt, sp_in); \
                        shared_ptr sp_name = sp_in; \
                        printf("&sp_name=%p, sp_name:%p\n", &sp_name, sp_name);

void free_shared(struct shared_ptr_s* ptr)
{
    printf("ptr:%p, usr_cnt:%p\n", ptr, ptr->use_cnt);
    if(!ptr) return;
    printf("del use_cnt = %d\n", *ptr->use_cnt - 1); 
    if(0 == --(*ptr->use_cnt)) {
        //dtor(ptr->inst);
        printf("freeing %p\n", (void *)ptr->use_cnt);
        free(ptr->use_cnt);
    }
    //ptr->inst = 0;
    ptr->use_cnt = 0;
}

void func(shared_ptr_t sp)
{
    SHARED_PTR_GET_ADD_REF(sp, sp_loc);
    printf("111111111\n");
    return;
}

int main(void)
{
    shared_ptr_t sp = {         // original type does not use __attribute__(cleanup)
        //.inst = ctor(), 
        .use_cnt = malloc(sizeof(int)) 
    };
    printf("use_cnt content addr:%p, &use_cnt=%p, &sp=%p,sp=%p\n", sp.use_cnt, &sp.use_cnt, &sp,sp);
    SHARED_PTR_GET_ADD_REF(sp, sp_loc);

    printf("222222222222\n");
    func(sp_loc);
    printf("333333333\n");

    return 0;
}

运行结果：

use_cnt content addr:0x9ee7008, &use_cnt=0xbfa11dac, &sp=0xbfa11dac,sp=0x9ee7008
add use_cnt = 1, sp_in=0x9ee7008
&sp_name=0xbfa11da8, sp_name:0x9ee7008
222222222222
add use_cnt = 2, sp_in=0x9ee7008
&sp_name=0xbfa11d74, sp_name:0x9ee7008
111111111
ptr:0xbfa11d74, usr_cnt:0x9ee7008
del use_cnt = 1
333333333
ptr:0xbfa11da8, usr_cnt:0x9ee7008
del use_cnt = 0
freeing 0x9ee7008