C - Write malloc

分手后的思念是犯贱 2023-01-13 08:47 104阅读 0赞

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/*
     * malloc accepts a size request without checking its plausibility;
     * free believes that the block it is asked to free contains a valid size field.
     * Improve these routines so they make more pains with error checking.
     *
     * MAlloc.c - by FreeMan
     */
    
    #include <unistd.h> 
    #include <limits.h>
    #include <string.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <stddef.h>
    
    #define NALLOC 1024
    
    typedef long Align;
    
    union header {
    	struct {
    		union header *ptr;
    		unsigned size;
    	} s;
    	Align x;
    };
    
    typedef union header Header;
    
    static Header base;
    static Header *freep = NULL;
    
    static void *_malloc(unsigned nbytes);
    static void *_calloc(unsigned nbytes, unsigned size);
    static Header *morecore(unsigned nu);
    static void _free(void *ap);
    
    int main(void)
    {
    	char *c, *d;
    
    	if ((c = (char *)_malloc(sizeof(char) * 1000)) == NULL
    		|| (d = (char *)_calloc(sizeof(char), 1000)) == NULL)
    	{
    		fprintf(stderr, "malloc: returned NULL\n");
    		exit(1);
    	}
    	strcpy(c, "hello,world");
    	strcpy(d, "hello,china");
    	printf("==> %s\n", c);
    	printf("==> %s\n", d);
    	_free(c);
    	_free(d);
    
    	return 0;
    }
    
    static void *_calloc(unsigned nbytes, unsigned size)
    {
    	void *p;
    	unsigned t = nbytes * size;
    
    	if ((p = _malloc(t)) == NULL)
    	{
    		return NULL;
    	}
    	memset(p, 0, t);
    	return p;
    }
    
    static void *_malloc(unsigned nbytes)
    {
    	Header *p, *prevp;
    	Header *morecore(unsigned);
    	unsigned nunits;
    
    	if (nbytes == 0 || nbytes >= UINT_MAX - NALLOC)
    	{
    		fprintf(stderr, "malloc: invalid size %u\n", nbytes);
    		return NULL;
    	}
    	nunits = (nbytes + sizeof(Header) - 1) / sizeof(Header) + 1;
    	if ((prevp = freep) == NULL)
    	{
    		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)
    		{
    			if (p->s.size == nunits)
    			{
    				prevp->s.ptr = p->s.ptr;
    			}
    			else
    			{
    				p->s.size -= nunits;
    				p += p->s.size;
    				p->s.size = nunits;
    			}
    			freep = prevp;
    			return (void *)(p + 1);
    		}
    		if (p == freep)
    		{
    			if ((p = morecore(nunits)) == NULL)
    			{
    				return NULL;
    			}
    		}
    	}
    }
    
    static Header *morecore(unsigned nu)
    {
    	char *cp;
    	Header *up;
    
    	if (nu < NALLOC)
    	{
    		nu = NALLOC;
    	}
    	cp = heapalloc(nu * sizeof(Header));
    	if (cp == (char *)-1)
    	{
    		return NULL;
    	}
    	up = (Header *)cp;
    	up->s.size = nu;
    	_free((void *)(up + 1));
    	return freep;
    }
    
    static void _free(void *ap)
    {
    	Header *bp, *p;
    
    	bp = (Header *)ap - 1;
    	if (bp->s.size == 0 || bp->s.size == UINT_MAX - NALLOC)
    	{
    		fprintf(stderr, "free: invalid block size %u\n", bp->s.size);
    		return;
    	}
    	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;
    		}
    	}
    	if (bp + bp->s.size == p->s.ptr)
    	{
    		/* we should check for UINT_MAX overflow before merge */
    		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)
    	{
    		/* we should check for UINT_MAX overflow before merge */
    		p->s.size += bp->s.size;
    		p->s.ptr = bp->s.ptr;
    	}
    	else
    	{
    		p->s.ptr = bp;
    	}
    	freep = p;
    }

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