Release malloced structures in function

Question

I am creating source files containing buffer functions that I want to use for my other library that I am creating.

It works correctly, but it's hard for me to get rid of the buffer structure that I create in one of the functions. The following snippets should help illustrate my problem:

Header C:

//dbuffer.h
...

typedef struct{
    char *pStorage;
    int *pPosition;
    int next_position;
    int number_of_strings;
    int total_size;
    }DBUFF; 
...

Source C:

//dbuffer.c
...
DBUFF* dbuffer_init(char *init_pArray)
    {
    //Find out how many elements the array contains
    int size = sizeof_pArray(init_pArray);                         

    //Initialize buffer structure
    DBUFF *buffer = malloc(sizeof(DBUFF));                                       

    //Initialize the storage
    buffer->pStorage = malloc( (sizeof(char)) * (size) );

    strncpy( &(buffer->pStorage)[0] ,  &init_pArray[0] , size);
    buffer->number_of_strings = 1;

    buffer->total_size = size;
    buffer->next_position = size; //size is the next position because array allocates elements from 0 to (size-1)

    //Initialize the position tracker which keeps record of starting position for each string
    buffer->pPosition = malloc(sizeof(int) * buffer->number_of_strings );
    *(buffer->pPosition + (buffer->number_of_strings -1) ) = 0;

    return buffer;
    }

void dbuffer_destroy(DBUFF *buffer)
    {
    free(buffer->pStorage);
    free(buffer);
    }
...

Home:

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


int main(int argc, char** argv)
    {
    DBUFF *buff; 

    buff = dbuffer_init("Bring the action");
    dbuffer_add(buff, "Bring the apostles");
    printf("BUFFER CONTENTS: ");
    dbuffer_print(buff); 

    dbuffer_destroy(buff);

    // Looks like it has been succesfully freed because output is garbage
    printf("%s\n", buff->pStorage);   

    //Why am I still able to access struct contents after the pointer has been freed ?
    printf("buff total size: %d\n", buff->total_size);

    return (EXIT_SUCCESS);
    }

Exit:

BUFFER CONTENTS: Bring the action/0Bring the apostles/0
  / 
buff total size: 36

RUN SUCCESSFUL (total time: 94ms)

Question:

Why can I still access the contents of the structure using the next line after the pointer to the structure has been freed?

printf("buff total size: %d\n", buff->total_size);

Answer 1

free() , , undefined . .

C11, §7.22.3.4, free() function

free() , ptr, .. . [..]

, () .

, free() . ( ) (, ) malloc() . free() , C standard .

Answer 2

, free d, . . , undefined.

( ). free -

printf("buff total size: %d\n", buff->total_size);

free d NULL.

Answer 3

free() , . , , , . , malloc(), , .

, () , , NULL. - NULL UB, , , , , .

Answer 4

[ ]

For your destructor to set the pointer passed to NULL, change your code as follows:

void dbuffer_destroy(DBUFF ** buffer)
{
  if ((NULL == buffer) || (NULL == *buffer))
  {
     return;
  }

  free((*buffer)->pPosition);
  free((*buffer)->pStorage);
  free(*buffer);
  *buffer = NULL;
}

and name it as follows:

  ...
  dbuffer_destroy(&buff);
  ...