original_kr_malloc.c

// Transcribed from pp. 185 – 189 of K&R, 2nd edition. Copyright 1988, 1978, by
// Bell Telephone Laboratories, Incorporated.

#include <stddef.h>

#include "original_kr_malloc.h"

typedef long Align;   /* for alignment to long boundary */

union header {        /* block header: */
    struct {
        union header *ptr;  /* next block if on free list */
        unsigned size;      /* size of this block */
    } s;
    Align x;          /* force alignment of blocks */
};

typedef union header Header;

static Header base;       /* empty list to get started */
static Header *freep = NULL;     /* start of free list */

#define NALLOC   1024    /* minimum #units to request */

/* morecore:   ask system for more memory */
static Header *morecore(unsigned nu)
{
    char *cp, *sbrk(int);
    Header *up;

    if (nu < NALLOC)
        nu = NALLOC;
    cp = sbrk(nu * sizeof(Header));
    if (cp == (char *) -1)  /* no space at all */
        return NULL;
    up = (Header *) cp;
    up->s.size = nu;
    kr_free((void *)(up+1));
    return freep;
}

/* malloc:  general-purpose storage allocator */
void *kr_malloc(unsigned nbytes)
{
    Header *p, *prevp;
    //Header *morecore(unsigned);
    unsigned nunits;

    nunits = (nbytes+sizeof(Header)-1)/sizeof(Header) + 1;
    if ((prevp = freep) == NULL) {  /* no free list yet */
        base.s.ptr = freep = prevp = &base;
        base.s.size = 0;
    }
    for (p = prevp->s.ptr; ;  prevp = p, p = p->s.ptr) {
        if (p->s.size >= nunits) {     /* big enough */
            if (p->s.size == nunits)      /* exactly */
                prevp->s.ptr = p->s.ptr;
            else {              /* allocate tail end */
                p->s.size -= nunits;
                p += p->s.size;
                p->s.size = nunits;
            }
            freep = prevp;
            return (void *)(p+1);
        }
        if (p == freep)  /* wrapped around free list */
            if ((p = morecore(nunits)) == NULL)
                return NULL;   /* none left */
    }
}

/* free:   put block ap in free list */
void kr_free(void *ap)
{
    Header *bp, *p;

    bp = (Header *)ap - 1;     /* point to block header */
    for (p = freep; !(bp > p && bp < p->s.ptr); p = p->s.ptr)
        if (p >= p->s.ptr && (bp > p || bp < p->s.ptr))
            break;  /* freed block at start of end of arena */

    if (bp + bp->s.size == p->s.ptr) { /* join to upper nbr */
        bp->s.size += p->s.ptr->s.size;
        bp->s.ptr = p->s.ptr->s.ptr;
    } else
        bp->s.ptr = p->s.ptr;
    if (p + p->s.size == bp) {         /* join to lower nbr */
        p->s.size += bp->s.size;
        p->s.ptr = bp->s.ptr;
    } else
        p->s.ptr = bp;
    freep = p;
}