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On a typical 64-bit system, the size_t will be 64-bit, but unsigned int will be 32 bit. So we cannot use them interchangeably. One standard recommendation is that the size_t be at most as big as an unsigned long.
When writing C code you should always use size_t whenever dealing with memory ranges. The int type on the other hand is basically defined as the size of the (signed) integer value that the host machine can use to most efficiently perform integer arithmetic.
size_t is the unsigned integer type of the result of sizeof , _Alignof (since C11) and offsetof, depending on the data model. The bit width of size_t is not less than 16.
size_t is unsigned because negative sizes make no sense.
if it is use to represent non negative value so why we not using
unsigned intinstead ofsize_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_tandunsigned intare 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.
There are 5 standard unsigned integer types in C:
unsigned charunsigned shortunsigned intunsigned longunsigned long longwith 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.
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.
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.
Apart from the other answers it also documents the code and tells people that you are talking about size of objects in memory
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