First, the current value of ptr may be a trap representation as per 6. If it is, the behavior is undefined. Next is the question of whether 0x1 points to a valid object. Adding a pointer and an integer is only valid if both the pointer operand and the result point to elements of an array object a single object counts as an array of size 1 or one element past the array object. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression.
If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. On a hosted implementation the value 0x1 almost certainly does not point to a valid object, in which case the addition is undefined.
An embedded implementation could however support setting pointers to specific values, and if so it could be the case that 0x1 does in fact point to a valid object.
If so, the behavior is well defined , otherwise it is undefined. As this is strictly less than you need to include the upper bound. But this will compare, for example a[2] to a[3] and then a[3] to a[2].
The Overflow Blog. Does ES6 make JavaScript frameworks obsolete? Podcast Do polyglots have an edge when it comes to mastering programming Featured on Meta. Now live: A fully responsive profile. Linked Related Hot Network Questions. Question feed. Stack Overflow works best with JavaScript enabled. Before we begin I would like to note that the definitions and recommendations given in the article refer to the most popular architectures for the moment IA , Intel 64 , IA , and may not fully apply to some exotic architectures.
It had been assumed for a long time that the size of int coincides with the size of a computer word microprocessor's capacity and it can be used as indexes to store sizes of objects or pointers. Correspondingly, address arithmetic was built with the use of int and unsigned types as well. The following example is nearly canon:.
As microprocessors developed over time and their capacity increased, it became irrational to further increase int type's sizes. There are a lot of reasons for that: economy of memory used, maximum portability etc. Table N1 shows the main data models , and lists the most popular systems using them.
As you can see from the table, it is not so easy to choose a variable's type to store a pointer or an object's size. They are guaranteed to be used for address arithmetic. And now the following code must become canon:. It is this code that can provide safety, portability, and good performance. The rest of the article explains why. It is the type of the result returned by sizeof operator.
The type's size is chosen so that it can store the maximum size of a theoretically possible array of any type. The exception is pointers to class functions, but this is a special case. Because of this, it is these types which should be used as indexes for large arrays, for storage of pointers and, pointer arithmetic.
Linux-application developers often use long type for these purposes. Within the framework of bit and bit data models accepted in Linux, this really works.
But this code is incompatible with Windows data model and, consequently, you cannot consider it easily portable. Address arithmetic issues have occured frequently since the beginning of the adaptation of bit systems.
Most problems in porting bit applications to bit systems relate to the use of such types as int and long, which are unsuitable for working with pointers and type arrays. The problems of porting applications to bit systems are not limited by this, but most errors relate to address arithmetic and operation with indexes.
It is not easy to detect an error and predict the behavior of this code. The debug-version will hang but hardly anyone will process gigabytes of data in the debug-version. And the release-version, depending on the optimization settings and code's peculiarities, can either hang, or suddenly fill all the array cells correctly, thereby producing the illusion of correct operation.
As a result, there appears floating errors in the program, occurring and vanishing with the subtlest change of the code. To learn more about such phantom errors and their dangerous consequences see the article " A bit horse that can count " [1]. Another example of one more "sleeping" error which occurs at a particular combination of the input data values of A and B variable :. This code will be correctly performed in the bit version, and print number "3".
Although the d specifier has been around as long as C, the t modifier is fairly new. Few C libraries support it yet. For more information about Dan Saks, visit his website at www. Dan also welcomes your feedback: e-mail him at. For more information about Dan. Saks, Dan. You must Sign in or Register to post a comment. This site uses Akismet to reduce spam.
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