Difference between size_t and unsigned int?

Question 1

if it is use to represent non negative value so why we not using unsigned int instead of size_t

Because unsigned int is not the only unsigned integer type. size_t could be any of unsigned char, unsigned short, unsigned int, unsigned long or unsigned long long, depending on the implementation.

Second question is that size_t and unsigned int are interchangeable or not and if not then why?

They aren't interchangeable, for the reason explained above ^^.

And can anyone give me a good example of size_t and its brief working ?

I don't quite get what you mean by "its brief working". It works like any other unsigned type (in particular, like the type it's typedeffed to). You are encouraged to use size_t when you are describing the size of an object. In particular, the sizeof operator and various standard library functions, such as strlen(), return size_t.

Bonus: here's a good article about size_t (and the closely related ptrdiff_t type). It reasons very well why you should use it.

Question 2

There are 5 standard unsigned integer types in C:

unsigned char
unsigned short
unsigned int
unsigned long
unsigned long long

with various requirements for their sizes and ranges (briefly, each type's range is a subset of the next type's range, but some of them may have the same range).

size_t is a typedef (i.e., an alias) for some unsigned type, (probably one of the above but possibly an extended unsigned integer type, though that's unlikely). It's the type yielded by the sizeof operator.

On one system, it might make sense to use unsigned int to represent sizes; on another, it might make more sense to use unsigned long or unsigned long long. (size_t is unlikely to be either unsigned char or unsigned short, but that's permitted).

The purpose of size_t is to relieve the programmer from having to worry about which of the predefined types is used to represent sizes.

Code that assumes sizeof yields an unsigned int would not be portable. Code that assumes it yields a size_t is more likely to be portable.

Question 3

size_t has a specific restriction.

Quoting from http://www.cplusplus.com/reference/cstring/size_t/ :

Alias of one of the fundamental unsigned integer types.

It is a type able to represent the size of any object in bytes: size_t is the type returned by the sizeof operator and is widely used in the standard library to represent sizes and counts.

It is not interchangeable with unsigned int because the size of int is specified by the data model. For example LLP64 uses a 32-bit int and ILP64 uses a 64-bit int.

Question 4

Apart from the other answers it also documents the code and tells people that you are talking about size of objects in memory

Question 5

size_t is used to store sizes of data objects, and is guaranteed to be able to hold the size of any data object that the particular C implementation can create. This data type may be smaller (in number of bits), bigger or exactly the same as unsigned int.

Question 6

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 could 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. 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 .

Question 7

In simple words size_t is platform and as well as implementation dependent whereas unsigned int is only platform dependent.