Explain typedef for function used in qsort library

Question

I am using qsort library function to sort an array of structure elements, while searching on the Internet I found a resource: INFO: Sorting Structures with the C qsort() Functio

Answer 1

Syntax:

typedef  int (*compfn)  (const void*, const void*);
  ^      ^       ^            ^          ^
  | return type  |               arguments type
  |             new type name 
  defining new type

compfn is a new user defined type defined by typedef keyword,

So, you have exactly typedefded int (*)(const void*, const void*); to comfn using the syntax I described above.

A declaration:

 compfn  fun; // same as: int (*fun)  (const void*, const void*);

means fun is a function pointer that takes two arguments of const void* types and returns int.

Suppose you have a function like:

int xyz  (const void*, const void*);    

then you can assign xyz address to fun.

fun = &xyz; 

At calling qsort():

In expression (compfn)compare, you are typecasting a function compare to (compfn) type function.

A doubt:

shouldn't the call be (*compfn).

No, its type name not function name.

Note: if you just writing int (*compfn) (const void*, const void*); without typedef then comfn will be a pointer to a function that returns int and take two arguments of type const void*