pvs-studio About size_t and ptrdiff_t

NOTE:

1、

size_t 对应的是 uintptr_t

ptrdiff_t 对应的是 intptr_t

它们都和pointer有着密切关联:

size_t length等于pointer length

ptrdiff_t length等于pointer length

在 "Portability of size_t and ptrdiff_t"段中，有着这样对待总结:

The size of size_t and ptrdiff_t always coincide with the pointer's size. Because of this, it is these types which should be used as indexes for large arrays, for storage of pointers and, pointer arithmetic.

The article will help the reader understand what size_t and ptrdiff_t types are, what they are used for, and when they must be used. The article will be interesting for those developers who begin creation of 64-bit applications where use of size_t and ptrdiff_t types provides high performance possibility to operate large data sizes, and portability between different platforms.

NOTE:

一、使用 size_t、ptrdiff_t 能够获得的: portability 、

1、"high performance possibility to operate large data sizes"

2、"portability between different platforms"

3、intentional programming

Introduction

The types size_t and ptrdiff_t were created to perform correct address arithmetic.

NOTE:

1、作为programmer，我们更多的是做address arithmetic，因此，我们更多的是关注pointer length，而不是Word length

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. int type is used in most training materials on programming in C and C++ in the loops' bodies and as indexes. The following example is nearly canon(标准):

for (int i = 0; i < n; i++)
  a[i] = 0;

As microprocessors developed over time and their capacity increased, it became irrational(不合理的) to further increase int type's sizes. There are a lot of reasons for that: economy of memory used, maximum portability etc. As a result, several data models appeared declaring the relations of C/C++ base types. Table N1 shows the main data models, and lists the most popular systems using them.

Table N1. Data models

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. To find the smartest solution to this problem size_t and ptrdiff_t types were created. They are guaranteed to be used for address arithmetic. And now the following code must become canon:

for (ptrdiff_t i = 0; i < n; i++)
  a[i] = 0;

It is this code that can provide safety, portability, and good performance. The rest of the article explains why.

`size_t` type

NOTE:

1、从这段内容可知: size_t length需要大于等于pointer length: "The type's size is chosen so that it can store the maximum size of a theoretically possible array of any type"

size_t type is a base unsigned integer type of C/C++ language. 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. On a 32-bit system size_t will take 32 bits, on a 64-bit one 64 bits. In other words, a variable of size_t type can safely store a pointer. The exception is pointers to class functions, but this is a special case. Although size_t can store a pointer, it is better to use another unsigned integer type uintptr_t for that purpose (its name reflects its capability). The types size_t and uintptr_t are synonyms(同义词). size_t type is usually used for loop counters, array indexing, and address arithmetic.

The maximum possible value of size_t type is constant SIZE_MAX.

`ptrdiff_t` type

1、从这段内容可知: size_t length需要大于等于pointer length相同: "The type's size is chosen so that it can store the maximum size of a theoretically possible array of any type"

ptrdiff_t type is a base signed integer type of C/C++ language. The type's size is chosen so that it can store the maximum size of a theoretically possible array of any type. On a 32-bit system ptrdiff_t will take 32 bits, on a 64-bit one 64 bits. Like in size_t, ptrdiff_t can safely store a pointer except for a pointer to a class function. Also, ptrdiff_t is the type of the result of an expression where one pointer is subtracted from the other (ptr1-ptr2). ptrdiff_t type is usually used for loop counters, array indexing, size storage, and address arithmetic. ptrdiff_t type has its synonym intptr_t whose name indicates more clearly that it can store a pointer.

Portability of `size_t` and `ptrdiff_t`

NOTE:

1、这两个类型都是机器相关的，因此，它们都是implementation defined，因此，它们能够较好的portability

The types size_t and ptrdiff_t enable you to write well-portable code. The code created with the use of size_t and ptrdiff_t types is easily portable. The size of size_t and ptrdiff_t always coincide with the pointer's size. Because of this, it is these types which should be used as indexes for large arrays, for storage of pointers and, pointer arithmetic.

Linux `long`

Linux-application developers often use long type for these purposes. Within the framework of 32-bit and 64-bit data models accepted in Linux, this really works. long type's size coincides with the pointer's size. But this code is incompatible with Windows data model and, consequently, you cannot consider it easily portable. A more correct solution is to use types size_t and ptrdiff_t.

Windows `DWORD_PTR`, `SIZE_T`, `SSIZE_T`

As an alternative to size_t and ptrdiff_t, Windows-developers can use types DWORD_PTR, SIZE_T, SSIZE_T etc. However, it is still desirable to confine to size_t and ptrdiff_t types.

Safety of `ptrdiff_t` and `size_t` types in address arithmetic

Address arithmetic issues have occured frequently since the beginning of the adaptation of 64-bit systems. Most problems in porting 32-bit applications to 64-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 64-bit systems are not limited by this, but most errors relate to address arithmetic and operation with indexes.

Example: operation with indexes

Here is a simple example:

size_t n = ...;
for (unsigned i = 0; i < n; i++)
  a[i] = 0;

NOTE:

1、index i的类型和n的类型是不同的，并且index i的类型长度小于n的类型长度，因此，存在这样的一种可能性: i 永远都无法和n相等，i一直递增到overflow都比n小。

2、关于上述程序的，更加深入的讨论，参见:

CppCoreGuidelines ES.107: Don't use unsigned for subscripts, prefer gsl::index

它收录在了Array-index章节

3、上述code，是非常常见的

If we deal with the array consisting of more than UINT_MAX items, this code is incorrect. 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(其特性), 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 64-bit horse that can count" [1].

NOTE:

1、这种错误是非常隐晦的，一旦出现，非常难以排查

Example: mix signed and unsigned arithmetic

NOTE:

1、CppCoreGuidelines ES.100: Don't mix signed and unsigned arithmetic

Another example of one more "sleeping" error which occurs at a particular combination of the input data (values of A and B variable):

#include <stdio.h>
int main()
{
    int A = -2;
    unsigned B = 1;
    int array[5] = { 1, 2, 3, 4, 5 };
    int *ptr = array + 3;
    ptr = ptr + (A + B); //Error
    printf("%i\n", *ptr);
}
// gcc  test.c

NOTE:

1、64-bit上运行，会core dump

This code will be correctly performed in the 32-bit version, and print number "3". After compilation in 64-bit mode there will be a fail when executing the code. Let's examine the sequence of code execution and the cause of the error:

1、A variable of int type is cast into unsigned type;

2、A and B are summed. As a result, we get 0xFFFFFFFF value of unsigned type;

3、"ptr + 0xFFFFFFFFu" expression is calculated. The result depends on the pointer's size on the current platform. In the 32-bit program, the expression will be equal to "ptr - 1" and we will successfully print number 3. In the 64-bit program, 0xFFFFFFFFu value will be added to the pointer, and as a result, the pointer will be far beyond the array's limits.

NOTE:

一、上述内容需要结合具体来进行分析:

A + B，会将A由signed转换为unsigned，它和B相加，结果为0xFFFFFFFF

对于ptr + (A + B):

1、在32-bit机器上，pointer是signed 32-bit，因此，0xFFFFFFFF的值大于-1

2、在64-bit机器上，pointer是signed 64-bit，因此，0xFFFFFFFF的值等于 268,435,455。

显然，此时会发现access array out of bound memory error。

Such errors can be easily avoided by using size_t or ptrdiff_t types. In the first case, if the type of "i" variable is size_t, there will be no infinite loop. In the second case, if we use size_t or ptrdiff_t types for "A" and "B" variable, we will correctly print number "3".

Let's formulate a guideline: wherever you deal with pointers or arrays, you should use size_t and ptrdiff_t types.

To learn more about the errors you can avoid by using size_t and ptrdiff_t types, see the following articles:

Performance of code using `ptrdiff_t` and `size_t`

Besides code safety, the use of ptrdiff_t and size_t types in address arithmetic can give you an additional gain in performance. For example, using int type as an index, the former's capacity being different from that of the pointer, will lead to the binary code containing additional data conversion commands. We speak about 64-bit code where pointers' size is 64 bits, and int type's size remains 32 bits.

NOTE:

1、省去了data conversion command

Code refactoring with the purpose of moving to ptrdiff_t and size_t

NOTE:

未阅读

pvs-studio About size_t and ptrdiff_t

Introduction

size_t type

ptrdiff_t type

Portability of size_t and ptrdiff_t

Linux long

Windows DWORD_PTR, SIZE_T, SSIZE_T

Safety of ptrdiff_t and size_t types in address arithmetic