Q:

Is the function strcmpi in the C standard libary of ISO?

I noticed that the difference between linux and windows.
strcmpi is in windows C standard libary implementation but it is not in GNU's C standard libary implementation.
Is the function strcmpi in the C standard libary defined by  ISO? 
How can I get the standard file? 
Thank you.

A:

The POSIX Standard (aka, UNIX) has strcasecmp() but it's not part of C99 / ISO-C.
Note: If you compare the above reference with e.g. the one for strcmp() you'll note that the latter explicitly references ISO-C, while the former does not. An useful feature of the OpenGroup references.
Edit: Since the locale-dependency was mentioned as a complication, see the above reference for strcasecmp_l() which allows explicitly specifying the locale to be used for the conversion. Windows has _strcmpi_l() for the purpose, again keeping with its own naming conventions.

