C what is size_t

The post will certainly aid the reader understand what size_t and also ptrdiff_t kinds are, what they are provided for, and also as soon as they must be provided. The write-up will be exciting for those developers that start production of 64-bit applications wbelow usage of size_t and ptrdiff_t forms offers high performance possibility to run big information sizes, and also portcapability between various platcreates.

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Before we begin I would favor to note that the definitions and referrals given in the post refer to the the majority of famous architectures for the minute (IA-32, Intel 64, IA-64), and might not completely use to some exotic architectures.

The forms size_t and ptrdiff_t were developed to perform correct attend to arithmetic. It had been assumed for a long time that the size of int corresponds via the size of a computer system word (microprocessor"s capacity) and also it have the right to be offered as indexes to save sizes of objects or pointers. Correspondingly, deal with arithmetic was constructed via the usage of int and also unsigned types too. int form is supplied in many training materials on programming in C and C++ in the loops" bodies and also as indexes. The complying with example is almost canon:

for (int i = 0; i As microprocessors arisen over time and their capacity raised, it became irrational to additionally increase int type"s sizes. Tbelow are many reasons for that: economic situation of memory offered, maximum portcapacity and so on. As an outcome, several data models appeared advertising the relationships of C/C++ base types. Table N1 shows the main information models, and lists the most well-known systems utilizing them.


Table N1. Data models

As you deserve to view from the table, it is not so simple to select a variable"s form to save a reminder or an object"s size. To uncover the smartest solution to this trouble dimension _t and also ptrdiff_t forms were created. They are guaranteed to be provided for deal with arithmetic. And currently the following code need to become canon:

for (ptrdiff_t i = 0; i It is this code that have the right to administer safety, portcapacity, and great performance. The rest of the post explains why.

size_t type

size_t type is a base unsigned integer form of C/C++ language. It is the kind of the result returned by sizeof operator. The type"s size is favored so that it can keep the maximum size of a theoretically possible variety of any type of kind. On a 32-little mechanism size_t will certainly take 32 bits, on a 64-bit one 64 bits. In various other words, a variable of size_t type have the right to safely keep a tip. The exemption is pointers to class functions, but this is a distinct situation. Although size_t can save a guideline, it is much better to use an additional unsigned integer kind uintptr_t for that function (its name shows its capability). The kinds size_t and also uintptr_t are synonyms. size_t type is commonly offered for loop counters, array indexing, and resolve arithmetic.

The maximum possible worth of size_t kind is constant SIZE_MAX.

ptrdiff_t type

ptrdiff_t kind is a base signed integer form of C/C++ language. The type"s size is liked so that it deserve to save the maximum size of a theoretically possible variety of any type of kind. On a 32-little bit device ptrdiff_t will certainly take 32 bits, on a 64-little one 64 bits. Like in size_t, ptrdiff_t have the right to safely save a guideline other than for a tip to a class attribute. Also, ptrdiff_t is the type of the outcome of an expression where one reminder is subtracted from the other (ptr1-ptr2). ptrdiff_t kind is commonly offered for loop counters, array indexing, size storage, and deal with arithmetic. ptrdiff_t form has its synonym intptr_t whose name shows even more plainly that it can store a pointer.

Portcapability of size_t and also ptrdiff_t

The kinds size_t and also ptrdiff_t permit you to write well-portable code. The code developed via the use of size_t and ptrdiff_t types is quickly portable. The size of size_t and also ptrdiff_t constantly coincide via the pointer"s size. Thus, it is these types which must be provided as indexes for big arrays, for storage of pointers and also, tip arithmetic.

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Linux-application developers often usage long type for these purposes. Within the framework of 32-little bit and also 64-little bit information models accepted in Linux, this really functions. long type"s size coincides with the pointer"s dimension. But this code is incompatible via Windows information model and also, consequently, you cannot consider it conveniently portable. An even more correct solution is to usage forms size_t and also ptrdiff_t.

As an different to size_t and ptrdiff_t, Windows-developers deserve to usage kinds DWORD_PTR, SIZE_T, SSIZE_T and so on However before, it is still preferable to confine to size_t and also ptrdiff_t types.

Safety of ptrdiff_t and also size_t types in address arithmetic

Address arithmetic problems have occured typically considering that the start of the adaptation of 64-little devices. Most problems in porting 32-little bit applications to 64-little bit units relate to the usage of such types as int and long, which are unsuitable for working via pointers and kind arrays. The problems of porting applications to 64-little systems are not restricted by this, but most errors relate to address arithmetic and also operation with indexes.

Here is an easy example:

size_t n = ...;for (unsigned i = 0; i If we attend to the variety consisting of even more than UINT_MAX items, this code is incorrect. It is not basic to detect an error and predict the behavior of this code. The debug-version will hang however hardly anyone will process gigabytes of information in the debug-version. And the release-version, depending on the optimization settings and also code"s peculiarities, can either hang, or suddenly fill all the array cells appropriately, thereby producing the illusion of correct procedure. As an outcome, tright here shows up floating errors in the program, occurring and vanishing through the subtlest readjust of the code. To learn even more about such phantom errors and their dangerous results see the article "A 64-little bit equine that deserve to count" <1>.

Another example of another "sleeping" error which occurs at a particular combicountry of the input data (worths of A and B variable):

int A = -2;unsigned B = 1;int array<5> = 1, 2, 3, 4, 5 ;int *ptr = variety + 3;ptr = ptr + (A + B); //Errorprintf("%i ", *ptr); This code will be properly perdeveloped in the 32-little version, and also print number "3". After compilation in 64-bit mode tbelow will certainly be a fail when executing the code. Let"s examine the sequence of code execution and the reason of the error:

A variable of int type is actors right into unsigned type;A and also B are summed. As a result, we gain 0xFFFFFFFF worth of unsigned type;"ptr + 0xFFFFFFFFu" expression is calculated. The result counts on the pointer"s dimension on the current platcreate. In the 32-little program, the expression will be equal to "ptr - 1" and we will certainly successfully print number 3. In the 64-little regime, 0xFFFFFFFFu value will be added to the reminder, and as a result, the pointer will certainly be far past the array"s limits.

Such errors deserve to be easily avoided by utilizing size_t or ptrdiff_t types. In the initially instance, if the kind of "i" variable is size_t, tright here will be no unlimited loop. In the second case, if we usage size_t or ptrdiff_t types for "A" and also "B" variable, we will properly print number "3".

Let"s formulate a guideline: wherever before you address pointers or arrays, you need to use size_t and ptrdiff_t types.

To learn even more around the errors you have the right to protect against by utilizing size_t and also ptrdiff_t kinds, view the adhering to articles:

Performance of code utilizing ptrdiff_t and also size_t

Besides code safety and security, the usage of ptrdiff_t and also size_t types in address arithmetic have the right to provide you an additional acquire in performance. For example, using int type as an index, the former"s capacity being various from that of the guideline, will cause the binary code containing added data convariation regulates. We sheight around 64-bit code where pointers" dimension is 64 bits, and int type"s size remains 32 bits.

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It is a challenging task to give a brief instance of size_t type"s advantage over unsigned type. To be objective we must use the compiler"s optimizing abilities. And the two variants of the optimized code generally come to be as well different to display this incredibly difference. We controlled to produce something prefer a basic instance only on our 6th try. And still the instance is not right bereason it demonstrates not those unessential data kind conversions we spoke of above, yet that the compiler have the right to develop an extra reliable code when using size_t kind. Let"s take into consideration a routine code arvarying an array"s items in the inverse order:

unsigned arraySize;...for (unsigned i = 0; i In the example, "arraySize" and also "i" variables have unsigned form. This kind have the right to be easily replaced via size_t type, and now compare a tiny fragment of assembler code presented on Figure 1.

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