Size_t是一种无符号整型数据类型，它只能赋0和大于0的整型值。它测量任何对象大小的字节，并由sizeof操作符返回。

Const是size_t的语法表示，但是没有Const你也可以运行程序。

const size_t number;

Size_t经常用于数组索引和循环计数。如果编译器是32位的，它将在unsigned int上工作。如果编译器是64位的，它也可以在unsigned long long int上工作。size_t的最大大小取决于编译器类型。

Size_t已经在<stdio.h>头文件中定义，但它也可以由 <stddef.h>， <stdlib.h>， <string.h>， <time.h>， <wchar.h>头文件。

示例(使用const)

#include <stdio.h>

int main()
{
    const size_t value = 200;
    size_t i;
    int arr[value];

    for (i = 0 ; i < value ; ++i)
    {
        arr[i] = i;
    }

    size_t size = sizeof(arr);
    printf("size = %zu\n", size);
}

输出:size = 800

示例(不含const)

#include <stdio.h>

int main()
{
    size_t value = 200;
    size_t i;
    int arr[value];

    for (i = 0; i < value; ++i)
    {
        arr[i] = i;
    }

    size_t size = sizeof(arr);
    printf("size = %zu\n", size);
}

输出:size = 800

Size_t或任何无符号类型可能被视为循环变量，因为循环变量通常大于或等于0。

当使用size_t对象时，必须确保在使用它的所有上下文中(包括算术)只需要非负值。例如，下面的程序肯定会给出意想不到的结果:

// C program to demonstrate that size_t or
// any unsigned int type should be used 
// carefully when used in a loop

#include<stdio.h>
int main()
{
const size_t N = 10;
int a[N];

// This is fine
for (size_t n = 0; n < N; ++n)
a[n] = n;

// But reverse cycles are tricky for unsigned 
// types as can lead to infinite loop
for (size_t n = N-1; n >= 0; --n)
printf("%d ", a[n]);
}

Output
Infinite loop and then segmentation fault

types.h的manpage说:

Size_t应该是无符号整数类型

从维基百科:

According to the 1999 ISO C standard (C99), size_t is an unsigned integer type of at least 16 bit (see sections 7.17 and 7.18.3). size_tis an unsigned data type defined by several C/C++ standards, e.g. the C99 ISO/IEC 9899 standard, that is defined in stddef.h.1 It can be further imported by inclusion of stdlib.h as this file internally sub includes stddef.h. This type is used to represent the size of an object. Library functions that take or return sizes expect them to be of type or have the return type of size_t. Further, the most frequently used compiler-based operator sizeof should evaluate to a constant value that is compatible with size_t.

作为暗示，size_t是保证保存任何数组下标的类型。

这是一个平台特定的类型定义。例如，在特定的机器上，它可能是unsigned int或unsigned long。您应该使用这个定义来提高代码的可移植性。

Size_t是无符号整数数据类型。在使用GNU C库的系统上，这将是unsigned int或unsigned long int。Size_t通常用于数组索引和循环计数。