Desktop/xinu_mips-1.0.2.tar/xinu_mips-1.0.2/system/getmem.c

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/* Embedded XINU, Copyright (C) 2007.  All rights reserved. */

#include <kernel.h>
#include <memory.h>

void *getmem(ulong nbytes)
{
        irqmask ps;    
        struct memblock *prev, *next, *leftover;

        if ( (0 == nbytes) || (NULL == freelist.next) )
        {
                return (void *)SYSERR;
        }

        /* pad nbytes with room for accounting information and */
        /* round to the nearest bytes                          */
        nbytes = (ulong)roundmb(nbytes + sizeof(struct memblock));

        /* modifying memory layout, do not interrupt */
        ps = disable();

        prev = &freelist;
        next = freelist.next;
        while ( NULL != next )
        {
                /* if the current node has enough room, we'll allocate */
                if (next->length >= nbytes)
                {
                        /* this block is big enough, check if it can hold nbytes   */
                        /* and still have room for the leftover block's acct. info */
                        if ( next->length < (nbytes + sizeof(struct memblock)) ) 
                        {
                                /* there was not enough room for nbytes and the new    */
                                /* block's accounting information, so allocate block   */
                                prev->next = next->next;
                                next->next = next;

                                /* maintain freelist length */
                                freelist.length -= next->length;

                                restore(ps);
                                return (void *)(next + 1); /* + 1 to skip acct. data   */
                        } 
                        else 
                        {
                                /* block is big enough to split in two relevant blocks */
                                leftover   = (struct memblock *)((ulong)next + nbytes);
                                prev->next = leftover;

                                /* set up data for the new (smaller) node */
                                leftover->next   = next->next;
                                leftover->length = next->length - nbytes;
                                
                                /* prepare block for checks in freemem                */
                                /* a memory location pointing to itself is unlikely   */
                                next->next   = next;
                                next->length = nbytes;

                                /* maintain freelist length */
                                freelist.length -= next->length;

                                restore(ps);
                                return (void *)(next + 1); /* + 1 to skip acct. data  */
                        }
                }

                /* iterate to next pointer, this node was too small */
                prev = next;
                next = next->next;
        }

        restore(ps);
        return (void *)SYSERR;
}

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