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system that the part under test will run We showed a tiny example for testing binary search earlier in this chapter It's easy to build scaffolds for testing mathematical functions, string functions, sort routines, and so on, since the scaffolding is likely to consist mostly of setting up input parameters, calling the functions to be tested, then checking the results It's a bigger job to create scaffolding for testing a partly-completed program To illustrate, we'll walk through building a test for memset, one of the mem functions in the C/C++ standard library These functions are often written in assembly language for a specific machine, since their performance is important The more carefully tuned they are, however, the more likely they are to be wrong and thus the more thoroughly they should be tested The first step is to provide the simplest possible C versions that are known to work; these provide a benchmark for performance and, more important, for correctness To move to a new environment, one carries the simple versions and uses them until the tuned ones are working The function memset (s, c , n) sets n bytes of memory to the byte c, starting at address s, and returns s This function is easy if speed is not an issue:
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/+ memset: s e t f i r s t n bytes o f s to c void *memset(void +s, i n t c , size- t n)
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s i z e - t i; char +p; p = (char +) s; f o r (i 0 ; i < n; i++) = p[i] = c; r e t u r n s;
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But when speed is an issue, tricks like writing full words of 32 or 64 bits at a time are used These can lead to bugs, so extensive testing is mandatory Testing is based on a combination of exhaustive and boundary-condition checks at likely points of failure For memset, the boundaries include obvious values of n such as zero, one and two, but also values that are powers of two or nearby values including both small ones and large ones like 216, which corresponds to a natural boundary in many machines, a 16-bit word Powers of two deserve attention because one way to make memset faster is to set multiple bytes at one time; this might be done by special instructions or by trying to store a word at a time instead of a byte Similarly, we want to check array origins with a variety of alignments in case there is some error based on starting address or length We will place the target array inside a larger array, thus creating a buffer zone or safety margin on each side and giving us an easy way to vary the alignment We also want to check a variety of values for c, including zero, Ox7F (the largest signed value, assuming 8-bit bytes), 0x80 and OxFF (probing at potential errors involving signed and unsigned characters), and some values much bigger than one
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byte (to be sure that only one byte is used) We should also initialize memory to some known pattern that is different from any of these character values so we can check whether memset wrote outside the valid area We can use the simple implementation as a standard of comparison in a test that allocates two arrays, then compares behaviors on combinations of n, c and offset within the array: big
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SO = ma1 loc(bi g)
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s l = ma1 loc(bi g) f o r each combination of t e s t parameters n, c , and offset: set a l l of SO and s l t o known pattern run slow memset(s0 + o f f s e t , c , n) run f a s t memset(s1 + o f f s e t , c , n) check return values compare a l l of SO and sl byte by byte An error that causes rnemset to write outside the limits of its array is most likely to affect bytes near the beginning or the end of the array, so leaving a buffer zone makes it easier to see damaged bytes and makes it less likely that an error will overwrite some other part of the program To check for writing out of bounds, we compare all the bytes of SO and s l , not just the n bytes that should be written Thus a reasonable set of tests might include all combinations of: offset = 10, 11, , 20 c = 0, 1 , Ox7F, 0x80, OxFF, Ox11223344
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n=0,1,2,3,4,5,7,8,9,15,16,17,
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The values of n would include at least 2' - l,2' and 2' + 1 for i from 0 to 16 These values should not be wired into the main pan of the test scaffold but should appear in arrays that might be created by hand or by program Generating them automatically is better; that makes it easy to specify more powers of two or to include more offsets and more characters These tests will give memset a thorough workout yet cost very little time even to create, let alone run, since there are fewer than 3500 cases for the values above The tests are completely portable, so they can be carried to a new environment as necessary As a warning, consider this story We once gave a copy of a memset tester to someone developing an operating system and libraries for a new processor Months later, we (the authors of the original test) started using the machine and had a large application fail its test suite We traced the problem to a subtle bug involving sign extension in the assembly language implementation of memset For reasons unknown the library implementer had changed the memset tester so it did not check values of c above Ox7F Of course, the bug was isolated by running the original working tester, once we realized that rnemset was a suspect
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