Newsgroups: comp.os.msdos.djgpp,comp.lang.c
From: floyd AT icefog DOT polarnet DOT com (Floyd L. Davidson)
Subject: Re: Functions in struct's... possible?  How?
Message-ID: <5tj1tt$fsu@icefog.polarnet.com>
Date: 21 Aug 1997 19:43:57 -0800
References: <33FCDA5C DOT 2353659F AT execulink DOT com>
Organization: _________
Reply-To: floyd AT polarnet DOT com
Lines: 63
To: djgpp AT delorie DOT com
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[posted and emailed]
Jeff Weeks  <pweeks AT execulink DOT com> wrote:
>I'd like to have a function in a struct that I've created.  I realise I
>can't just stick a functions in there, so I used a pointer to a
>function.  Here's what my struct looks like:
>
>typedef struct {
>  VesaInfo card;           // information about the video card
>  VesaModeInfo mode;       // information about the current mode
>  int bytes_per_line;      // the number of bytes per scanline
>  int size;                // the size in bytes of the whole screen
>  char *address;           // the address of actual video memory
>  void (*blit)(char *);    // a routine to copy a virtual screen to memory
>} Driver;
>
>And then I define an instance and set blit to something:
>
>Driver driver;
>driver.blit = lfb_blit;
>
>But when I try to call the actual function, it crashes.  I know the
>lfb_blit() function is not at fault (I've tested it separately), so the
>problem is in calling the function from the struct.  I do it as follows:
>
>char *virt = (char *)malloc(640*480*2);
>driver.blit(virt);

Here is a tested example that demonstrates one way to do it:

#include <stdio.h>

typedef struct	{
  void (*foo)(char *);
} BAR;

BAR bar;

#define foobar	(*bar.foo)

void func1(char *);
void func2(char *);

int main(void)
{
  bar.foo = func1;
  foobar("The first function");

  bar.foo = func2;
  foobar("The second function");

  return 0;
}

void func1(char *s) { printf("func1: %s\n", s); }
void func2(char *s) { printf("func2: %s\n", s); }

/* eof */

  Floyd

-- 
Floyd L. Davidson                                floyd AT polarnet DOT com
Salcha, Alaska                               or: floyd AT tanana DOT polarnet DOT com