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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: declarator
Date: Mon, 29 Jun 2015 19:01:16 -0700
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"Bill Cunningham" <nospam@nspam.invalid> writes:
>     Ok here is just a quickie. Demonstrating the "function pointer." This is 
> sloppy because I did get these two warnings after compilation. And I know if 
> something was passed to the print wrapper function and the printf specifier 
> needed to be something other than %s there would be a problem. But this is 
> as I say just a quickie.

Compiler warnings often indicate *serious* problems.  That's not always
the case, but it is here.

> #include <stdio.h>
>
> int print(char *);
> int (*pf) (char *);

Ok, pf is a pointer-to-function.

> int main()
> {
>     pf = print("hello there");

The print() function return a result of type int.  You're trying to
assign that int value to a function pointer.

The name of a function, in an expression context, gives you the address
of (i.e., a pointer to) the function.  The name of a function followed
by parentheses *calls* the function, and gives you the value returned by
the function.

If you want the pointer object pf to point to the function print, you
need to do:

    pf = print;

which is equivalent to:

    pf = &print;

Once you've done that, you can use pf to call the function it points to:

    pf("hello there");

which is equivalent to:

    (*pf)("hello there");

>     return pf;

Now you're trying to return a pointer from main.  Why?

> }
>
> int print(char *input)
> {
>     printf("%s\n", input);
>     return 0;
> }
>
>     And the warnings:
>
> p.c: In function 'main':
> p.c:8:8: warning: assignment makes pointer from integer without a cast 
> [enabled by default]
>      pf = print("hello there");
>         ^
> p.c:9:5: warning: return makes integer from pointer without a cast [enabled 
> by default]
>      return pf;
>      ^
>
> I did not use -Wall either. as the compiler says "enabled by default. It 
> compiles and runs.

gcc has an unfortunate habit of issuing non-fatal warnings for code that
is unambiguously incorrect.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"