How should i used the searchpath parameter in C?

the syntax is


char *searchpath(const char *file);


now in this what comes in between the brackets. what is const char *file and how to get the value or how will it return.

How should i used the searchpath parameter in C?
i didn't got it well but to search for a file by its extension just write down in the search title *.file extension for example *.exe or *.doc