From:	"A. Sinan Unur" <asu1 AT cornell DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Functions in struct's... possible? How?
Date:	Thu, 21 Aug 1997 21:03:43 -0400
Organization:	Cornell University http://www.cornell.edu
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Jeff Weeks 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);
> 
> Can C not do this?  Is there no way to call a function from a struct?

it would be good to see what you actually compiled along with the any
warning messages you received (assuming you compiled with -Wall.) the
following code compiles and runs fine:

#include <stdlib.h>
#include <stdio.h>

/* following are just so that i could compile this */
#define VesaInfo int
#define VesaModeInfo int

typedef struct {
  VesaInfo card;
  VesaModeInfo mode;
  int bytes_per_line;
  int size;
  char *address;
  void (*blit)(char *);
} Driver;

void lfb_blit(char *c)
{
  *c = (char) 'a';
  return;
}

int main(void)
{
  Driver driver;

  char* virt = malloc(640*480*2);
  
  if ( NULL == virt)
  {
   printf("Cannot allocate memory.\n");
   exit(1);
  }
	
  driver.blit = lfb_blit;
  driver.blit(virt);
	
  return 0;
}

-- 
   Sinan
 
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