这是因为size_t可以是int类型以外的任何类型(可能是struct类型)。其思想是将它的工作与底层类型解耦。

来自友好的维基百科:

stdlib.h和stddef.h头文件定义了一种名为size_t的数据类型，用于表示对象的大小。接受大小的标准库函数期望它们是size_t类型，而sizeof操作符的计算结果为size_t。 size_t的实际类型取决于平台;一个常见的错误是假定size_t与unsigned int相同，这可能导致编程错误，特别是在64位体系结构变得越来越普遍的情况下。

另外，检查为什么size_t很重要

Size_t是用来表示大小的类型(顾名思义)。它依赖于平台(甚至潜在的实现)，并且应该仅用于此目的。显然，size_t表示一个大小，是无符号的。许多stdlib函数，包括malloc、sizeof和各种字符串操作函数都使用size_t作为数据类型。

int在默认情况下是有符号的，尽管它的大小也依赖于平台，但在大多数现代机器上它将是一个固定的32位(尽管size_t在64位体系结构上是64位，但int在这些体系结构上仍然是32位)。

总结一下:使用size_t表示对象的大小，在其他情况下使用int(或long)。

SIZE_T的定义可以在下面找到: https://msdn.microsoft.com/en-us/library/cc441980.aspx和https://msdn.microsoft.com/en-us/library/cc230394.aspx

在这里粘贴所需的信息:

SIZE_T是一个ULONG_PTR，表示指针可以指向的最大字节数。

该类型的声明如下:

typedef ULONG_PTR SIZE_T;

ULONG_PTR是用于指针精度的无符号长类型。在将指针强制转换为长类型以执行指针算术时使用。

该类型的声明如下:

typedef unsigned __int3264 ULONG_PTR;

The size_t type is defined as the unsigned integral type of the sizeof operator. In the real world, you will often see int defined as 32 bits (for backward compatibility) but size_t defined as 64 bits (so you can declare arrays and structures more than 4 GiB in size) on 64-bit platforms. If a long int is also 64-bits, this is called the LP64 convention; if long int is 32 bits but long long int and pointers are 64 bits, that’s LLP64. You also might get the reverse, a program that uses 64-bit instructions for speed, but 32-bit pointers to save memory. Also, int is signed and size_t is unsigned.

There were historically a number of other platforms where addresses were wider or shorter than the native size of int. In fact, in the ’70s and early ’80s, this was more common than not: all the popular 8-bit microcomputers had 8-bit registers and 16-bit addresses, and the transition between 16 and 32 bits also produced many machines that had addresses wider than their registers. I occasionally still see questions here about Borland Turbo C for MS-DOS, whose Huge memory mode had 20-bit addresses stored in 32 bits on a 16-bit CPU (but which could support the 32-bit instruction set of the 80386); the Motorola 68000 had a 16-bit ALU with 32-bit registers and addresses; there were IBM mainframes with 15-bit, 24-bit or 31-bit addresses. You also still see different ALU and address-bus sizes in embedded systems.

任何时候int小于size_t，并且您试图将一个非常大的文件或对象的大小或偏移量存储在unsigned int中，都有可能溢出并导致错误。对于int型，也有可能得到一个负数。如果整型或无符号整型更宽，程序将正常运行，但会浪费内存。

如果想要可移植性，通常应该使用正确的类型。很多人会建议你使用有符号数学而不是无符号数学(以避免像1U < -3这样讨厌的、微妙的错误)。为此，标准库将<stddef.h>中的ptrdiff_t定义为从另一个指针减去一个指针的结果的有符号类型。

也就是说，一种解决方法可能是根据INT_MAX和0或INT_MIN适当检查所有地址和偏移量，并打开编译器关于比较有符号量和无符号量的警告，以防您遗漏任何警告。在C语言中，你应该总是，总是检查数组访问是否溢出。