why is array name a pointer to the first element of the array?

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An array name is not itself a pointer, but decays into a pointer to the first element of the array in most contexts. It’s that way because the language defines it that way.

From C11 6.3.2.1 Lvalues, arrays, and function designators, paragraph 3:

Except when it is the operand of the sizeof operator, the _Alignof operator, or the unary & operator, or is a string literal used to initialize an array, an expression that has type “array of type” is converted to an expression with type “pointer to type” that points to the initial element of the array object and is not an lvalue.

You can learn more about this topic (and lots about the subtle behaviour involved) from the Arrays and Pointers section of the comp.lang.c FAQ.

Editorial aside: The same kind of behaviour takes place in C++, though the language specifies it a bit differently. For reference, from a C++11 draft I have here, 4.2 Array-to-pointer conversion, paragraph 1:

An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to an rvalue of type “pointer to T“. The result is a pointer to the first element of the array.

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