Thread: about C malloc and free

below is the code

Code:

#include <stdio.h>


typedef struct _test{
    int a;
    int b;
    char data[0];
}Test;




/****************************************************************************************
  * this eample proved that the data part is not deleted while free!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
  **/
int main(int argc, char *argv[]){
    Test *tt = malloc(sizeof(Test) + 8);
    tt->a = 20;
    tt->b = 20;
    tt->data[4] = ',';
    printf("before free, tt->a is %d, tt->b is %d, tt->data[4] is %c\n", tt->a, tt->b, tt->data[4]);
    free(tt);
    printf("after free, tt->a is %d, tt->b is %d, tt->data[4] is %c\n", tt->a, tt->b, tt->data[4]);
    //free(tt->data);
    //printf("after free the data part, tt->a is %d, tt->b is %d, tt->data[4] is %c\n", tt->a, tt->b, tt->data[4]);
}

and the display

Code:

before free, tt->a is 20, tt->b is 20, tt->data[4] is ,
after free, tt->a is 0, tt->b is 0, tt->data[4] is ,

so the data[] is not freed? how can I delete it ?

The memory block is freed.

Freeing a block does not necessarily wipe the contents, it only frees the block. If you want to wipe the contents, memset it before freeing it.

And what you're doing by accessing a pointer after it's been freed is undefined behavior anyway. You're lucky it didn't crash the program.

but the tt->a tt->b is set to be zero, the data part stays the same?. Now I understand that some time later this memory is not usable. but the cleaning .
seems it only clean the struct _test part?