Thread: problem with realloc

hi can anyone tell me whats wrong with my realloc
I get segmentationfault when my testprogram does: p = realloc(p, 4711);

Code:

void *realloc(void *oldp, size_t size)
{
    if (oldp == NULL){                    
        return(malloc(size));
    }
    void *newp;                       
    size_t oldsize;                       
    oldsize = ((Header *)oldp - 1)->s.size * sizeof(Header);    
    newp = malloc(size);                    
    if (NULL != newp){                   
        memcpy(newp, oldp, min(size, oldsize));       
        free((void *)oldp);                
    }
    return(newp);                                                   
}

thanks for your help!

Why are you trying to redefine realloc? It's already a built-in C function that does relatively the same as what you're trying to do.

im writing a little library with malloc, realloc and free and then im gonna do some performance analysis and compare with the built in functions. any ideas of why i get segmentationsfaults?

What is Header? I'd be willing to bet that the segfault occurs on that line, as the code compiles and works fine without it, and in place, a random number.

Originally Posted by mattholm

im writing a little library with malloc, realloc and free and then im gonna do some performance analysis and compare with the built in functions. any ideas of why i get segmentationsfaults?

How are you going to do malloc (assuming you don't just calloc()) and free?

the malloc first-fit and free part is from a book (so I will only write realloc and worst and bestfit for malloc) and the Header comes from this:

Code:

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;

I was hoping this line

Code:

oldsize = ((Header *)oldp - 1)->s.size * sizeof(Header);

would make the oldp point to the header

Thank you!!! it should be + sizeof(Header);

thanks alot!