Question

Where can I find what is the maximum identifier length in C?

In which header file is that limit specified?

Answer 1

There is no header file to contain the identifier length limit; even if there were, how could it help you? You can’t change your identifier lengths at compile time based on a value in a header file anyway.

The C standard, section 5.2.4.1 says:

• 63 significant initial characters in an internal identifier or a macro name (each universal character name or extended source character is considered a single character)
• 31 significant initial characters in an external identifier (each universal character name specifying a short identifier of 0000FFFF or less is considered 6 characters, each universal character name specifying a short identifier of 00010000 or more is considered 10 characters, and each extended source character is considered the same number of characters as the corresponding universal character name, if any)

It also contains a footnote:

Implementations should avoid imposing fixed translation limits whenever possible.

So you should check your documentation to see if your compiler supports a greater number of significant characters in identifiers.