Explanation of a macro

Applications Programming
1

Can some body explain this part in a header file for me?

#include <limits.h> 
 
#define BIGNUM unsigned long long
typedef BIGNUM (*hash_t) (char *str); 
 ......

I have hard time for the second part:

typedef BIGNUM (*hash_t) (char *str);

First, I could not find the definition of hash_t, which may be from standard library. Is it? If so, where is the header file for hash_t?
The second is the whole typedef.
Thanks a lot!

2

This is a function typedef -

typedef BIGNUM (*hash_t) (char *str);

The function returns unsigned long long , a pointer to signed char is the argument.

hash_t is now a function declaration rather than a pure datatype declaration.

The Function Pointer Tutorials - Syntax

As to what header file, I do not know. If you have a hash library (example pcre) check there. I am guessing you want to use this and expect it to be on your system already.
Try

grep 'hash_t' $(find /usr/include -type f -name '*.h')
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