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#include <memory.h>#include <stdio.h>struct Malloc {void *a;struct Malloc *next;int size;int isfree;};struct Malloc *malloc_begin = NULL;/* find first free region large enough */struct Malloc* findFirstFree(size_t size) {    struct Malloc * head = malloc_begin;    struct Malloc * prev = NULL;    if (!head) {        malloc_begin=head=sbrk(sizeof(struct Malloc));        if (!head) return NULL;        head->a=sbrk(size);        if (!head->a) return NULL;        head->next=NULL;        head->size=size;        head->isfree=0;        return head;    }    while (head) {        if (head->isfree && head->size >= size) {            return head;        }        prev=head;        head=head->next;    }    /* not found, so allocate */    prev->next=head=sbrk(sizeof(struct Malloc));    if (!head) return NULL;    head->a=sbrk(size);    if (!head->a) return NULL;    head->size=size;    head->isfree=0;    return head;    }void* calloc(size_t nmemb, size_t size){    struct Malloc * mem = findFirstFree(nmemb*size);    if (!mem) return NULL;    /* TODO: clear memory */    return mem->a;    }void* malloc(size_t size){    struct Malloc * mem = findFirstFree(size);    if (!mem) return NULL;        return mem->a;}void free(void *ptr){    struct Malloc * head = malloc_begin;    while (head) {        if (head->a==ptr) {            head->isfree=1;            return;        }        head=head->next;    }    fprintf(stderr,"attempt to free memory unknown to allocator\n"); return;}void * realloc(void * ptr, size_t size) {    struct Malloc * head = malloc_begin;    while (head) {        if (head->a==ptr) {            struct Malloc * newmem;            if (head->size >= size) return head->a; /* no problem */            /* otherwise allocate new and copy over */            newmem = findFirstFree(size);            if (!newmem) return NULL;            memcpy(newmem->a,head->a,size);            free(head->a);            return newmem->a;        }        head=head->next;    }    fprintf(stderr,"attempt to realloc memory unknown to allocator\n"); return NULL;}int main(void) {    void * first;    void * second;        first = malloc(50);    second = malloc(100);    printf ("first=%p\n",first);    printf ("second=%p\n",second);    free(second);    free(first);    first=malloc(100);    second=malloc(50);    printf ("first=%p\n",first);    printf ("second=%p\n",second);    return 0;}