Page 700 in Software

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Debugging To paraphrase Thomas Edison, programming is 10 percent inspiration and 90 percent debugging All really good programmers are good debuggers To help avoid bugs, it is useful to review some the common ways in which bugs can occur Order-of-Evaluation Errors The increment and decrement operators are used in most C programs, and the order in which the operations take place is affected by whether these operators precede or follow the variable Consider the following:
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y = 10; x = y++; y = 10; x = ++y;
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These two sequences are not the same The one on the left assigns the value of 10 to x and then increments y The one on the right increments y to 11 and then assigns the value 11 to x Therefore, in the first case x contains 10; in the second, x contains 11 In the general case, a prefix increment (or decrement) operation occurs before the value of the operand is obtained for use in the larger expression A postfix increment (or decrement) occurs after the value of the operand is obtained for use in the larger expression If you forget these rules, problems will result The way an order-of-evaluation error usually occurs is through changes to an existing statement sequence For example, when optimizing a piece of code, you might change this sequence
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/* original code */ x = a + b; a = a + 1;
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/* "improved" code -- wrong! */ x = ++a + b;
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The trouble is that the two code fragments do not produce the same results The reason is that the second way increments a before it is added to b This was not the case in the original code! Errors like this can be very hard to find There may be clues such as loops that don't run right or routines that are off by one If you have any doubt about a statement, recode it in a way that you are sure about
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Page 701
Pointer Problems A very common error in C programs is the misuse of pointers Pointer problems fall into two general categories: misunderstanding indirection and the pointer operators, and accidentally using invalid or uninitialized pointers The solution to the first problem is easy: Simply be clear on what the * and & operators mean! The second type of pointer problems is a bit trickier Here is a program that illustrates both types of pointer errors:
/* This program has an error */ #include <stdlibh> #include <stdioh> int main(void) { char *p; *p = (char *) malloc(100); /* this line is wrong */ gets(p); printf (p); return 0; }
This program will most likely crash The reason is that the address returned by malloc( ) was not assigned to p but rather to the memory location pointed to by p, which in this case is completely unknown This type of error represents a fundamental misunderstanding of the * pointer operator It is usually created by novice C programmers and sometimes by experienced pros who just make a silly mistake! To correct this program, substitute
p = (char *) malloc(100); /* this is correct */
for the wrong line The program also contains a second and more insidious error There is no runtime check on the address returned by malloc( ) Remember, if memory is exhausted, malloc( ) returns NULL, and the pointer should not be used Using a NULL pointer is invalid and nearly always leads to a program crash Here is a corrected version of the program, which includes a check for pointer validity:
/* This program is now correct */
Page 702 #include <stdioh> #include <stdlibh> int main(void) { char *p; p = (char *) malloc(100); /* this is correct */ if(!p) { printf(''Out of memory\n"); exit(1); } gets(p); printf(p); return 0; }
Another common error is forgetting to initialize a pointer before using it Consider the following code fragment:
int *x; *x = 100;
This will cause trouble because x has not been initialized to point to anything Thus, you don't know where x is pointing Assigning a value to that unknown location may destroy something of value, such as other code or data The troublesome thing about wild pointers is that they are so hard to track down If you are making assignments through a pointer that does not contain a valid address, your program may appear to function correctly some of the time and crash at other times The smaller your program, the more likely it will run correctly, even with a stray pointer This is because very little memory is in use, and the odds are that the offending pointer is pointing to memory that is not being used As your program grows, failures will become more common, but you will be thinking about current additions or changes to your program, not about pointer errors Hence, you will tend to look in the wrong spot for the bug The way to recognize a pointer problem is that errors are often erratic Your program will work correctly one time, wrong another Sometimes other variables will contain garbage for no apparent reason If these problems begin to occur, check your pointers As a matter of procedure, you should always check all pointers when bugs begin to occur
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