Implementation of Realloc in C

问题

int getmin(int a, int b)
{
    return a<b?a:b;
}


void *reallocation(void *ptr, size_t size) //size_t in bytes
{

    void *newptr;


    int msize;
    msize = getsize(ptr);

    msize = getmin(msize, size);

        printf("msize = %d", msize);

    newptr = malloc(size);
    newptr = memcpy(newptr, ptr, msize);
    free(ptr);


    return newptr;

}

I have implemented my own realloc, and in order to get the size of the allocated memory using malloc(however i know there isn't any method for this in c).

My reallocation function is working fine on my system How do we get the size of the memory allocated by malloc().

Also can we do inplace reallocation if the size of the previously allocated memory is greater than the new required?

回答1:

There is no portable way to get the size of memory allocated by malloc().

However, one can always do something like that to simulate what you want.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void myfree(void * p) {
    size_t * in = p;
    if (in) {
        --in; free(in);
    }
}

void * mymalloc(size_t n) {
    size_t * result = malloc(n + sizeof(size_t));
    if (result) { *result = n; ++result; memset(result,0,n); }
    return result;
}

size_t getsize(void * p) {
    size_t * in = p;
    if (in) { --in; return *in; }
    return -1;
}

#define malloc(_x) mymalloc((_x))
#define free(_x) myfree((_x))

void *reallocation(void *ptr,size_t size) {
    void *newptr;
    int msize;
    msize = getsize(ptr);
    printf("msize=%d\n", msize);
    if (size <= msize)
        return ptr;
    newptr = malloc(size);
    memcpy(newptr, ptr, msize);
    free(ptr);
    return newptr;
}
int main() {
    char * aa = malloc(50);
    char * bb ;
    printf("aa size is %d\n",getsize(aa));
    strcpy(aa,"my cookie");
    bb = reallocation(aa,100);
    printf("bb size is %d\n",getsize(bb));
    printf("<%s>\n",bb);
    free(bb);
}

回答2:

malloc does not initialize memory to zero. (calloc is the equivalent that does.) If you are seeing things set to zero, it's accidental.

I believe the library version of realloc uses length information in the heap that is not directly available. (And it may overestimate the original allocation, which means it might copy a little extra memory when using realloc to expand the allocation. This generally has no effect.)

realloc likely doesn't do a copy when shrinking an allocation.

Also, I should note that in same cases, you don't have to do a copy even when realloc increases the size, for example, if the next block in the heap is free.

回答3:

the memory allocated by malloc gets initialized to zero, so i am checking for that condition.

That's incorrect. From the draft:

Description

2 The malloc function allocates space for an object whose size is specified by size and whose value is indeterminate.

Your getsize needs to be fixed.

My reallocation function is working fine.

You are not even fixing the alignment -- it may fail for certain types. Read this SO question.

Also can we do inplace reallocation if the size of the previously allocated memory is greater than the new required?

What would in-place reallocation mean? Shouldn't this be a simple no-op?

来源：https://stackoverflow.com/questions/10864593/implementation-of-realloc-in-c

标签

malloc

free

realloc