Embedded Systems Design Europe - June/July 2008 - (Page 19)

memory Listing 1 Static memory allocation in C. // // static memory allocation void alloc_static() { static int array[10]; // static allocation int n; // // populate “array” for (n=0;n<10;n++) { array[n]=n+1; printf(“%d:%d\n”,n,array[n]); } Listing 2 Stack allocation in C. // // stack allocation void alloc_stack() { int array[10]; // stack allocation int n; // // populate “array” for (n=0;n<10;n++) { array[n]=n+1; printf(“%d:%d\n”,n,array[n]); } } Listing 3 Heap allocation in C. // // heap allocation void alloc_heap() { int* array; int n; if (needMemory) // allocate memory only if needed { // // allocate array on heap array = (int *)malloc(10*sizeof(int)); free(array); } } // free memory works even on processors without stack support. However, this method inefﬁciently allocates all storage memory for the lifetime of the program, so more memory is allocated than is actually in use. In addition, the amount of memory is ﬁxed and must be speciﬁed at compile time. For example in Listing 1, the memory for variable “array” is allocated statically. In stack allocation, the processor allocates a ﬁxed amount of memory when a function is invoked. The memory for the currently executing function is maintained on a stack, and when the function exits, the memory on the stack is released. This method uses memory more efﬁciently than static allocation, because memory is allocated and released as needed by each function. However, the stack is limited to a ﬁnite area of memory. It can be difﬁcult to predict how large the overall stack needs to be, and stack overﬂow can be difﬁcult to detect. Like static allocation, the size of variables on the stack must be speciﬁed at compile time. Listing 2 illustrates stack allocation for variable “array.” Finally, in dynamic allocation, you manually allocate memory at run time. The memory is drawn from the heap, using C’s malloc and free commands. Because you have complete control over memory usage, dynamic allocation allows more efﬁcient use of memory than either static or stack allocation. Listing 3 shows an example of dynamic allocation. The ﬂexibility of dynamic memory allocation, however, comes at a price: • • • • In C, the developer has the choice of allocating memory either on the stack, in static memory, or dynamically (on the heap). Each allocation method has its advantages and disadvantages. Static allocation, in which the compiler sets aside a ﬁxed amount for each variable, is the simplest method and Manual memory management is error prone, and heap-related software problems, such as memory leaks, are particularly difﬁcult to debug. Searching the heap for an appropriate size block takes an indeterminate amount of time, which is unacceptable for “hard” real-time systems. Repeatedly allocating and freeing memory blocks of different sizes eventually leads to fragmentation. Custom memory allocators, such 19 www.embedded.com/europe | embedded systems design europe | JUNE – JULY 2008 http://www.embedded.com/europe

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Embedded Systems Design Europe - June 2008 Contents Work in Progress to Define Compact PCI Plus Power.org Demonstrates New Tools Project Supports Multi-core System Programming Altium Links Electronic to Mechanical Design PLDs Look to Cut Power Budget and Costs Project to Provide Coverage Analysis Tool Microsoft Details Windows Embedded Update Cover Feature: Leveraging Virtual Hardware Platforms for Software Allocating Memory in MATLAB-to-C Code MDD & IDEs: Making the Twain Meet in Embedded System Designs Debugging Mixed Signal Designs for Infrequent & Random Events Why Open Source is the Natural Choice for High-security Systems Bringing the Benefits of Low Power CPUs to Modular Design New Products Advertising Contacts

Embedded Systems Design Europe - June/July 2008

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