xvid算法看到的，不太明白为什么要这样做？
void * xvid_malloc(int size,unsigned char alignment)
{
unsigned char *mem_ptr;
if (!alignment)
{
/* We have not to satisfy any alignment */
if ((mem_ptr = (unsigned char *) malloc(size + 1)) != NULL)
{
/* Store (mem_ptr - "real allocated memory") in *(mem_ptr-1) */
*mem_ptr = 1;
/* Return the mem_ptr pointer */
return (void *)(mem_ptr+1);
}
}
else
{
unsigned char *tmp;
/*
* Allocate the required size memory + alignment so we
* can realign the data if necessary
*/
if ((tmp = (unsigned char *) malloc(size + alignment)) != NULL)
{
/* Align the tmp pointer */
mem_ptr =(unsigned char *) ((unsigned int) (tmp + alignment - 1) &
(~(unsigned int) (alignment - 1)));
/*
* Special case where malloc have already satisfied the alignment
* We must add alignment to mem_ptr because we must store
* (mem_ptr - tmp) in *(mem_ptr-1)
* If we do not add alignment to mem_ptr then *(mem_ptr-1) points
* to a forbidden memory space
*/
if (mem_ptr == tmp)
mem_ptr += alignment;
/*
* (mem_ptr - tmp) is stored in *(mem_ptr-1) so we are able to retrieve
* the real malloc block allocated and free it in xvid_free
*/
*(mem_ptr - 1) = (unsigned char) (mem_ptr - tmp);
/* Return the aligned pointer */
return (void *) mem_ptr;
}
}
return NULL;
}