Hi.

I have a pointer to a function within a structure and I want it
initialized "automatically". For example,

/* The header file */
#ifndef SOP_H
#define SOP_H
struct sop
{
  int (*func)(struct sop *s, FILE *ifp);
  initialize_sop() {
    func = sop_func;
  }

/* The supporting program */
#include "sop.h"

int sop_func( sop *s, FILE *ifp )
{
/* stuff here */

john
(Have I heard of C++? Yeah. I want to learn how to do it in C.)

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Since there are no constructors with C, you will have to initialize with an
assignment or a function call.  If you consistently remember to do it at
declaration time, you should be ok.  There really is simply _no_ way to make
it happen automagically.
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: Hi.

: I have a pointer to a function within a structure and I want it
: initialized "automatically". For example,

: /* The header file */
: #ifndef SOP_H
: #define SOP_H
: struct sop
: {
:   int (*func)(struct sop *s, FILE *ifp);
:   initialize_sop() {
:     func = sop_func;
:   }
: };
: #endif /* SOP_H */

: /* The supporting program */
: #include "sop.h"

: int sop_func( sop *s, FILE *ifp )
: {
: /* stuff here */
: }

: I am getting an error message when I declare func. Any hints would be
: appreciated.

You can't do that in C.  The nearest you can come is something like
this (note that it's a very bad idea to initialise structures in
headers, and that sop is a declaration, not a definition).

#include <stdio.h>

struct Sop;

typedef int (*func_t)(struct Sop *);

int init_sop(struct Sop *);
int run_sop(struct Sop *);

struct Sop
{
  func_t running_sop;
  func_t initialize_sop;
  int digit;

  return 0;

: struct Sop
: {
:   func_t running_sop;
:   func_t initialize_sop;
:   int digit;
: } sop = { init_sop, run_sop, 0 };

Wrong.  Try

  } sop = { run_sop, init_sop, 0 };

That'll teach me to edit code and not re-test it.

Will

You can initialize a structure using an initializer; however, all of
the initializer elements must be constant expressions. This isn't
a problem in your case because the address of a function is an
``address constant''. So you can do:

        struct sop {
                int (*func)(struct sop *s, FILE *ifp);
        }
        struct sop sop_instance = { sop_func };

provided that sop_func has been suitably declared.

In traditional UNIX kernels, there is a static declaration which initializers
a whole array of structures with function pointers, much like:

        #include "foo.h"      /* foo driver */
        #include "bar.h"      /* bar driver */

        struct operations {
                int (*open)(/* .. args .. */);
                int (*close)(/* .. args .. */);
                /* ... other operations ... */
        } device_table[MAX_DEVICES] = {
                { foo_open, foo_close,  /* ... */ },
                { bar_open, bar_close,  /* ... */ },
        }

Each structure points to the basic operations of a device driver. The ``device
major number'' of a UNIX device has traditionally just been its index within
this array.  (Though these days, drivers are dynamically loaded).  Since C
originated as a language for implementing UNIX, it is useful to look at it for
hints of how some of the ways the language was intended to be used.