Embedded Systems Design - July 2008 - (Page 10)

programming pointers BY Dan Saks making it remarkably easy to recently presented arguments insert customized memory for and against using dynamic managers into existing code. memory allocation in C and 1 I do agree that You can even use allocation C++ programs. functions to place objects reptruly safety-critical systems resenting device registers at should avoid using dynamic almemory-mapped locations. location because the associated This month, I’ll contrast risks outweigh the advantages. the standard allocation and However, I strongly suspect that deallocation facilities in C with many other embedded systems those in C++. Understanding could be improved by judiciousthe differences between these ly using dynamic memory allofacilities is insightful regardless cation. In places where the stanof which language you use. dard C or C++ allocation and deallocation functions are less THE STANDARD C than ideal, a customized memoFUNCTIONS ry manager might work much Standard C provides two membetter. ory allocation functions, malIdeally, a customized memory manager should look and loc and calloc, and one dealact like a standard one to the greatlocation function, free. A fourth C and C++ have strikingly function, realloc, does both dealest extent possible. Although the different approaches to customized allocation policy will location and allocation. All four be different from the policy of a functions are declared in the stanmanaging dynamic standard one (that’s the whole dard header . memory. point of using a custom scheme), The storage for an object allothe parameters and return values cated using an allocation function of the customized allocation and deallocation functions (malloc, calloc, or realloc) remains allocated until the should be as similar as possible to the standard ones. This program passes the object’s address as an argument to a similarity lets you build and test your code using standard deallocation function (free or realloc). Attempting to memory management functions and then slip customized deallocate storage that isn’t currently allocated produces functions in or out as needed with minimal fuss. undeﬁned behavior. (I explained the distinctions among Understanding the ins and outs of dynamic memory undeﬁned, unspeciﬁed, and implementation-deﬁned bemanagement is especially worthwhile for C++ programhavior in earlier columns.)1, 2 mers. C++ offers many facilities—most notably classes The standard declares malloc as: with constructors and destructors—that dramatically diminish the incidence of memory leaks. C++ also lets you void *malloc(size_t size); deﬁne allocation and deallocation functions for each class, Calling malloc(s) allocates storage for an object whose size is s bytes. If the allocation succeeds, the call returns a Dan Saks is president of Saks & Associates, pointer to the allocated storage. If it fails, the call returns a a C/C++ training and consulting company. For more information about Dan Saks, visit his null pointer. website at www.dansaks.com. Dan also welParameter size has type size_t. As I explained in a comes your feedback: e-mail him at column last year, size_t is a typedef declared in dsaks@wittenberg.edu. as well as several other headers. The type is 10 JULY 2008 | embedded systems design | www.embedded.com Dynamic allocation in C and C++ I http://www.dansaks.com http://www.embedded.com

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

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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