about C malloc and free

This is a discussion on about C malloc and free within the C Programming forums, part of the General Programming Boards category; below is the code Code: #include <stdio.h> typedef struct _test{ int a; int b; char data[0]; }Test; /**************************************************************************************** * this ...

  1. #1
    Registered User
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    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 ?

  2. #2
    Registered User TheBigH's Avatar
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    It is freed. Free simply tells your OS that it is now OK to store other data at those memory locations, it doesn't necessarily actually modify the data that's already there. If you refer to those memory locations later on, the data may still be there.
    Code:
    while(!asleep) {
       sheep++;
    }

  3. #3
    Registered User
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    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.

  4. #4
    a_capitalist_story
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    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.

  5. #5
    Registered User
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    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?

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