exercise8-8.md

Exercise 8-8

Write a routine bfree(p,n) that will free an arbitrary block p of n characters into the free list maintained by malloc and free. By using bfree, a user can add a static or external array to the free list at any time.

Process

Well, this took way less time than expected, was able to do it in a day.

I think the large amount of time I spent playing around trying to understand how the program works, meant I had the knowledge necessary to implement a solution straightaway.

My Solution:

1. check recieved memory is valid (not NULL)
2. check memory can hold at least one Header and one free block
3. if malloc things aren't initialised, initalise them (freep, base)
4. free the memory to add it to the circularly linked list

I like the idea of this function, taking different pieces of memory and adding them together for a free list to allocate to.

Being the last exercise in the book, I thought it would be more complex, but I'm happy to end on something straightforward!

Other Solutions - CLC Wiki

Some interesting solutions, some were kinda complex and buggy, and then Anonymous coming in with a great solution again.

Anonymous points out that if memory isn't bigger than two Header blocks then isn't not really useful, so shouldn't be allowed.

Other Solutions - C Answer Book

A nice short solution as always.

I really like their decision to make bfree return the size allocated. This allows a way to determine if the free process worked and to so see how much wasted space there was due to Header alignment.

I was playing around with returning a void * with my solution so I could return NULL if it failed, but didn't feel ffree should return a pointer. Returning the size a a much better solution.

Code

#include <stdio.h>
#include <string.h>
#include "malloc.h" /* from previous exercise */

void bfree(void *p, unsigned n)
{
    Header *bp;
    unsigned nunits;
    
    if (n == NULL) {
        printf("bfree error: *p can't be NULL\n");
        return;
    }
    
    if (n < sizeof(Header)) {
        printf("bfree error: n must be > sizeof(Header)\n");
        return;
    }
    
    bp = (Header *) p;
    bp->s.size = n / sizeof(Header);

    if (freep == NULL) {
        freep = &base;
        base.s.ptr = &base;
        base.s.size = 0;        
    }        

    ffree(bp + 1);
}

#define LEN (sizeof(Header) * 2)
static char arr[LEN];

int main()
{
    char *p;
    
    bfree(&arr, LEN);
    p = (char *) mmalloc(LEN / 2);
    strncpy(p, "hello world", 12);
    
    printf("\nString stored:\n%s\n\n", p);
    printf("%u Byte Array:\n", LEN);    
    printf("%32s%32s\n", "End of Header 1 ->|", "End of Header 2 ->|");
    for (int i = 0; i < LEN; i++)
        printf("%c|", arr[i]);
    printf("\n");
}

Output

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