Embedded Systems Design - August 2008 - (Page 38)

0808esd.p35to42 7/17/08 11:57 AM Page 38 20 years ago Listing 1 CALLOC.C. /* * Produced by Programming ARTS * 8/18/88 * Programmers: Les Aldridge, Travis I. Seay */ #deﬁne NULL (void *)0 typedef struct hdr { struct hdr unsigned int } HEADER; *ptr; size; /* Deﬁned in the linker ﬁle. _heapstart is the ﬁrst byte allocated to the heap; _heapend is the last. */ extern HEADER _heapstart, _heapend; extern void static HEADER static short warm_boot(char *str); *frhd; memleft; /* memory left */ void free(char *ap) { /* Return memory to free list. Where possible, make contiguous blocks of free memory. (Assumes that 0 is not a valid address for allocation. Also, i_alloc() must be called prior to using either free() or malloc(); otherwise, the free list will be null.) */ HEADER *nxt, *prev, *f; f = (HEADER *)ap - 1; memleft += f->size; /* Point to header of block being returned. */ /* frhd is never null unless i_alloc() wasn't called to initialize package. */ if (frhd > f) { /* Free-space head is higher up in memory than returnee. */ nxt = frhd; frhd = f; prev = f + f->size; if (prev==nxt) f->size += nxt->size; f->ptr = nxt->ptr; } else f->ptr = nxt; return; } Listing continues on page 40. /* old head */ /* new head */ /* right after new head */ /* Old and new are contiguous. */ /* Form one block. */ The application must call i_alloc() prior to calling malloc() or free(). Normally called from main(), i_alloc() initializes the free-space pointer to indicate the ﬁrst available byte of heap space. (By convention, heap generally refers to the pool of memory available for dynamic allocation at run-time.) The free-space pointer initially indicates a header showing that the entire heap area is free and no other free blocks exist (see 38 Figure 1). The size of the free space is calculated in HEADER-size units. When pointers to a structure are added or subtracted in C, the result is divided by the size of the structure. For example, if _heapstart and _heapend differ by 4,096 bytes and the size of the structure HEADER is four bytes, the result of the pointer arithmetic is 1,024. The available space can be thought of as an array of header blocks and the calculation as ﬁnding the number of entries in the array. ALLOCATING & FREEING MEMORY Malloc() calculates the number of HEADER-size units required to satisfy the request. It rounds the result up and adds one unit for the header, which is part of the block allocated. For speed, the calculation could contain a shift instead of a divide. AUGUST 2008 | embedded systems design | www.embedded.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - August 2008

Embedded Systems Design - August 2008 Contents Number Include Parity Bit Programmer's Toolbox Cover Feature: Virtualization for Embedded X86 Multiprocessor Applications Wanted: Industry Standards for Benchmarking Embedded VMM Hypervisors Achieving Cache Coherence in a MIPS32 Multicore Design Memory Allocation in C Advertising Index Break Points Marketplace

Embedded Systems Design - August 2008

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