一般来说，如果从0开始向上，总是使用无符号类型，以避免溢出将您带入负值的情况。这是非常重要的，因为如果你的数组边界刚好小于你的循环的最大值，但你的循环最大值刚好大于你的类型的最大值，你将绕负，你可能会遇到分割错误(SIGSEGV)。所以，一般来说，对于从0开始并向上的循环，永远不要使用int。使用unsigned符号。

Since nobody has yet mentioned it, the primary linguistic significance of size_t is that the sizeof operator returns a value of that type. Likewise, the primary significance of ptrdiff_t is that subtracting one pointer from another will yield a value of that type. Library functions that accept it do so because it will allow such functions to work with objects whose size exceeds UINT_MAX on systems where such objects could exist, without forcing callers to waste code passing a value larger than "unsigned int" on systems where the larger type would suffice for all possible objects.

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。

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

Size_t和int是不可互换的。例如，在64位Linux上size_t的大小是64位的(即sizeof(void*))，而int是32位的。

还要注意size_t是无符号的。如果你需要签名版本，那么在某些平台上有ssize_t，它与你的例子更相关。

作为一般规则，我建议在大多数情况下使用int，只有在特定需要时才使用size_t/ssize_t(例如mmap())。

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。