Problems reading structures in VisualC++.NET

[shetlandbob]

Hello again,

Has anyone come across general problems reading binary file data directly into structures, i.e.

fread ( &structure, sizeof(Structure), 1,inputResultsFile );

If I read each line individually
(using fread ( &int, sizeof(int),.... etc for multiple variables)
then it works, but as when I create a structure with all the correct variables and then try to read it then I get errors reading the values (its very inconsistent, it will read some of them ok, then it will read a corrupt value for no apparent reason??)

Any ideas/help/good luck messages appreciated!!!!

Cheers

 

[ArkM]

1. Suppose you open file in binary mode (fopen(name,"rb")),
then
2.

struct S
{
  short x;
  double y;
  char c;
} s;

// sizeof(S) == 24 (!)
// not 11 = sizeof(s.x)+sizeof(s.y)+sizeof(s.c)!

Let's remember about word alignment, (garbage) padding bytes etc...
If we have wrote s member by member, we can't read it as whole and vice versa.

 

[Salem]

And if you want it to be size == 11, then you'll need to do this

#pragma pack(1)
struct S
{
  short x;
  double y;
  char c;
} s;
#pragma pack()

It depends on how the structure was packed when it was written out to the file in the first place. Writing structures in binary form is inherently non-portable.

At best, it can only fix your alignment problems. If the file comes from another machine, you could still have endian problems (and there's no magic to fix that).


--

 

[Skute]

Try the following code, i wrote it to work out if saving the data of a class directly to a file was quickly possible.

#include <stdio.h>
#include <conio.h>

void PressAnyKey();

class test
{
	public:
		test() { number = 10; number2 = 40; }
		int getnumber()
		{
			printf("%i\n", number);
			return number;
		}
		void setnumber(int newnumber)
		{
			int atesta = 0;
			number = newnumber;
		}

	private:
		int number;
		int number2;
};

typedef unsigned char byte;

int main()
{
	test a;
	a.setnumber(99);
	a.getnumber();

	FILE* pFile = NULL;
	pFile = fopen("C:\\memtest1.dat", "wb");
	fwrite((void *)&a, 1, sizeof(a), pFile);
	fclose(pFile);
	pFile = NULL;

	a.setnumber(10304);
	a.getnumber();

	pFile = fopen("C:\\memtest1.dat", "rb");
	fread((void *)&a, 1, sizeof(a), pFile);
	fclose(pFile);
	pFile = NULL;

	a.getnumber();

	test b;
	b.setnumber(99);
	b.getnumber();

	pFile = fopen("C:\\memtest2.dat", "wb");
	fwrite((void *)&b, 1, sizeof(b), pFile);
	fclose(pFile);
	pFile = NULL;

	b.setnumber(10304);
	b.getnumber();

	pFile = fopen("C:\\memtest2.dat", "rb");
	fread((void *)&b, 1, sizeof(b), pFile);
	fclose(pFile);
	pFile = NULL;

	b.getnumber();

	PressAnyKey();
	return 0;
}

void PressAnyKey()
{
	printf("Press any key to continue...\n");
	getch();
}

Hope that helps you out

Skute

"There are 10 types of people in this World, those that understand binary, and those that don't!"