why's it crash?????

[stevey]

can anyone tell me why the following code crashes my computer on reaching the end of the file ?????
its just reading from a binary file with the data structure as shown. it prints -

etc

(null) EGGSHELL CREAM 02 006529 0350 900805 007.99
(null) EGGSHELL SUNRISE 03 006789 1250 900618 008.00
(null) EGGSHELL LILAC 02 006015 0400 900115 007.50
(null) EGGSHELL PINK 00 001250 1000 900725 007.00
(null) EGGSHELL OFF WHITE 02 008025 0500 900623 007.99
(null) EGGSHELL PALE GOLD 00 001023 0150 900426 007.00
(null) EGGSHELL SUPERWHITE 04 008957 1500 900418 007.99
(null) EGGSHELL MAGENTA 00 002394 0500 900428 007.89
(null) EGGSHELL LEMON 00 007500 0500 900621 007.00
end of file

then crashes.
all the data is ok except the first field , why is this printed as null ???

#include <stdio.h>
#include <conio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <time.h>
#include <isam.h>
#include <common.h>




FILE *fp_stock_master;
FILE *fp_out;


RECORD_DATA *stock_rec;

int main()
{

if((fp_stock_master=fopen("A:STCKMAST.DAT","rb"))= =NULL)
{
printf("ERROR OPENING STOCK MASTER FILE\n");
exit(1);
}
else
printf("OPENING STOCK MASTER FILE\n");



do
{
if(fread(stock_rec,sizeof(RECORD_DATA),1,fp_stock_ master)!=1)
{
if(feof(fp_stock_master))
{ printf("end of file");}
else if(!feof(fp_stock_master)) /*else reading error*/
{printf("\nERROR READING CUSTOMER MASTER FILE - PROGRAM TERMINATED");exit(1); }
break;
}


if(!feof(fp_stock_master) )

printf("%6s %19s %02d %06ld %04d %06s %06.2f\n",stock_rec->key,stock_rec->desc,
stock_rec->supp_code,stock_rec->free_stock,stock_rec->min_stock,
stock_rec->movement_date,stock_rec->price);


}
while(!feof(fp_stock_master));

fclose(fp_stock_master);
return 1;
}

/*
typedef struct
{
char key[7];
char desc[20];
short supp_code;
long free_stock;
short min_stock;
char movement_date[7];
float price;
} RECORD_DATA;

this struct is in a header file */

[Salem]

> RECORD_DATA *stock_rec;
Because this isn't pointing at any valid memory - you need to allocate memory.

[Garfield]

I don't have enormous amounts of time right now to carefully look through your code, but I can give you a couple of things that might help.

The reason that it prints "(null)" is, that is the way that most recent compilers print a pointer when, you guessed it, the pointer is NULL. Restep through your code and see where you want to assign a value to the pointer. That could be the problem of messed up assigning.

The main reason for a lot of runtime errors (crashing) is also pointer related. Are you trying to initialize a pointer that doesn't exist? That happens a lot with what you are trying to do.

I'll take a better look at in the next couple days and I'll get back to you (if somebody doesn't beat me to it).

--Garfield the Great

[Garfield]

> > RECORD_DATA *stock_rec;
Because this isn't pointing at any valid memory - you need to allocate memory. <

Arg!!! Salem beat me to it. Well, anyway, Salem is right. Like I said, you are trying to assign to an invalud pointer.

--Garfield the Slow

[Cyber Kitten]

I have a question!

The RECORD_DATA structure (for the stock master file(INDEXED)) is declared in the header file "ISAM.H", therefore can't you just write in your code:

RECORD_DATA *stock_rec

Will this not access the structure in the header file without need to allocate memory?
If not how do you do this?

There is a function to read the stock file included in the header file, the prototype being:

static RECORD_DATA *search_file(char *product_code);

To use this would you declare it in your code with the other functions and then use it as follows:

RECORD_DATA *stock_rec;

if((stock_rec = search_file(part_number)) != NULL)
{
/*record found*/
}

Would this work?

A very confused Kitten

[quzah]

You're confusing 'declaring' a structure instance with 'defining' one.

Define:

struct mystruct 
{
   int x;
   float y;
   char *z;
};

Declare:

struct mystruct *mPtr;

Ok, so in the first part, we've declared a structure and it's contents. Now when we want to use one, we have to declare an instalce of it. In this case, we've elected to declare 'mPtr' as a pointer to a 'mystruct' structure.

When you declare a pointer, it points to some random space out, and you have to make it useable, by giving it a value.

Note that a pointer has no real space of it's own (it actually has a bit of space, but ignore that for now). You have to allocate space.

To allocate a block of memory for it to point at, we commonly use 'malloc'.

mPtr = malloc( sizeof( struct mystruct ) );

Now then, mPtr will end up "pointing at" an allocate block of memory who's size is "sizeof( struct mystruct )".

This means the allocated space is the size of a normal 'struct mystruct' instance.

You can now use this block of memory.

Quzah.

[stevey]

thanx folks - problem solved - bad habit i've got allocating(or not)
memory.
sorry, i tried to reply straight away to Salem, but my modem won't work - i think its cos i crashed my OS so much !!!

what confused me i think is cos i got away with it before for a previous problem, its interesting that the rest of the fields are ok,
and it only crashes when reaching eof - i actually had a version of my prog which didn't crash, but the 1st field was null.

so kitten
i take it youre doing 4240 p3 ??
as mentioned above i didn't allocate memory for my solution to this and got away with it !!! i guess this is cos i only accessed the price info, you notice on the bit of my file that i copied, the price was read ok !!!!!!

anyway, probably lost many marks for it, cos its obviously bad practice !!!

steve