C - struct problems - writing - c

Question

I'm making a program in C, and I'mm having some troubles with memory, I think.
So my problem is: I have 2 functions that return a struct. When I run only one function at a time I have no problem whatsoever. But when I run one after the other I always get an error when writting to the second struct.
Function struct item* ReadFileBIN(char *name) -- reads a binary file.
struct tables* getMesasInfo(char* Filename) -- reads a text file.
My code is this:
#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
int numberOfTables=0;
int numberOfItems=0;
//struct tables* mesas;
//struct item* Menu;
typedef struct item{
char nome[100];
int id;
float preco;
};
typedef struct tables{
int id;
int capacity;
bool inUse;
};
struct tables* getMesasInfo(char* Filename){
struct tables* mesas;
char *c;
int counter,numberOflines=0,temp=0;
char *filename=Filename;
FILE * G;
G = fopen(filename,"r");
if (G==NULL){
printf("Cannot open file.\n");
}
else{
while (!feof(G)){
fscanf(G, "%s", &c);
numberOflines++;
}
fclose(G);
}
/* Memory allocate for input array */
mesas = (struct tables *)malloc(numberOflines* sizeof(struct tables*));
counter=0;
G=fopen(filename,"r");
while (!feof(G)){
mesas[counter].id=counter;
fscanf(G, "%d", &mesas[counter].capacity);
mesas[counter].inUse= false;
counter++;
}
fclose(G);
numberOfTables = counter;
return mesas;
}
struct item* ReadFileBIN(char *name)
{
int total=0;
int counter;
FILE *ptr_myfile;
struct item my_record;
struct item* Menu;
ptr_myfile=fopen(name,"r");
if (!ptr_myfile)
{
printf("Unable to open file!");
}
while (!feof(ptr_myfile)){
fread(&my_record,sizeof(struct item),1,ptr_myfile);
total=total+1;
}
numberOfItems=total-1;
Menu = (struct item *)calloc(numberOfItems , sizeof(struct item));
fseek(ptr_myfile, sizeof(struct item), SEEK_END);
rewind(ptr_myfile);
for ( counter=1; counter < total ; counter++)
{
fread(&my_record,sizeof(struct item),1,ptr_myfile);
Menu[counter] = my_record;
printf("Nome: %s\n",Menu[counter].nome);
printf("ID: %d\n",Menu[counter].id);
printf("Preco: %f\n",Menu[counter].preco);
}
fclose(ptr_myfile);
return Menu;
}
int _tmain(int argc, _TCHAR* argv[])
{
struct item* tt = ReadFileBIN("menu.dat");
struct tables* t = getMesasInfo("Capacity.txt");
getchar();
}**
the error that im getting is :
"Unhandled exception at 0x00411700 in test.exe: 0xC0000005: Access violation writing location 0x00000000."
in "Menu[counter] = my_record;"
Thanks in advance.

Answer 1

You seem to allocate a memory block of the wrong size in getMesasInfo(): sizeof(struct tables*) gives you the size of the pointer, not that of the struct it is pointing to:
mesas = (struct tables *)malloc(numberOflines* sizeof(struct tables*));
So you can easily overwrite unallocated memory. The proper allocation should be
mesas = (struct tables *)malloc(numberOflines* sizeof(struct tables));
or, similar to how you allocate the other array in ReadFileBIN():
mesas = (struct tables *)calloc(numberOflines, sizeof(struct tables));
Moreover, I don't know whether it's intentional or not, but in ReadFileBIN() you are allocating (1) and reading (2) one less record than the total number of records:
numberOfItems=total-1; // 1
Menu = (struct item *)calloc(numberOfItems , sizeof(struct item)); // 1
fseek(ptr_myfile, sizeof(struct item), SEEK_END);
rewind(ptr_myfile);
for ( counter=1; counter < total ; counter++) // 2
...
Since the loop counter is started from 1 (instead of 0 as is normal in C), you effectively execute the loop total-1 times. That is, you read into elements 1 to total-1 of the array. However, the real elements of the array are indexed from 0 to total-2 so the very first element in the array is left uninitialized, and in the end you commit a buffer overflow error.

Answer 2

You have a problem in these lines:
numberOfItems=total-1;
Menu = (struct item *)calloc(numberOfItems , sizeof(struct item));
fseek(ptr_myfile, sizeof(struct item), SEEK_END);
rewind(ptr_myfile);
for ( counter=1; counter < total ; counter++)
First, you're setting numberOfItems to one less than the total. This is incorrect. You don't even need numberOfItems; since total has the number of lines in the file, the next line should really be Menu = (struct item*) calloc(total, sizeof(struct item));
Second, you're trying to use Menu as a one-based array in the for loop. C arrays are zero-based. You should have the for loop use for (counter = 0; counter < total; counter++).
Finally, as Peter pointed out, in the first function you're allocating the wrong size of object. You need to malloc numberOfLines*(sizeof(struct tables) (not sizeof(struct tables*).

Answer 3

To further illustrate Peter's point:
struct tables {
int id;
int capacity;
int inUse; /* bool is not a C type */
};
int main()
{
printf("sizeof: %d\n",sizeof(struct tables*));
printf("sizeof: %d\n",sizeof(struct tables));
}
Will output:
sizeof: 4
sizeof: 12