Writes OK but doesn't Read very well?

[Sirjorj]

Hi Boys 'n Girls-
Kin someone point out the problem with this code?

#include "stdafx.h"
#include <stdio.h>

int main(int argc, char* argv[])
{
FILE *Afile;
int xx=123456,yy;
Afile=fopen("d:Test.txt","wb");
fwrite(&xx,4,1,Afile);
rewind (Afile);
fread(&yy,4,1,Afile);
printf("\n Wrote: %d\n Read: %d\n",xx,yy);
_fcloseall();
return 0;
}
Thanks,

[Andreas Masur]

Well...what is the problem? I simply assume - based on your subject - that the write command has not been flushed to the file at the time you are reading again...so you might want to add a 'fflush(Afile)' after your write statement...

[Philip Nicoletti]

you open with the mode : "wb"

I'm not really that familiar with the C open modes, but I think
that it should be : "r+b" or "w+b"

[Andreas Masur]

Originally posted by Philip Nicoletti
you open with the mode : "wb"

I'm not really that familiar with the C open modes, but I think
that it should be : "r+b" or "w+b"

No..."wb" is actually fine...you are referring to a different mode...there exist two modes starting with a 'r':

• "r" -> opens file for reading
  
• "r+" -> opens file for reading and changing

Thus, both "rb" and "r+b" are valid - though different - operations...same applies to writing...

[aferlitsch]

To read and write, you must open the file as either "r+" or "w+". The binary mode (no translation of carriage return/line feed) is probably irrelavent in this case (though "r+b" or "w+b" would be correct).

In your case, the "wb" only opened the file for writing and not reading.

Further, when opened this way, writes/reads are not neccessarily committed when you do a rewind(), so you need to do a flush. Here is some info from MSDN:

>>
When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for "update"). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.

>>
b
Open in binary (untranslated) mode; translations involving carriage-return and linefeed characters are suppressed.

Link:
http://msdn.microsoft.com/library/de...c_._wfopen.asp

[Andreas Masur]

Originally posted by aferlitsch
In your case, the "wb" only opened the file for writing and not reading.

Overlooked the obvious...

Well, I guess it is too late for me...

[Sirjorj]

No- its not too late!
Sorry folks I made a mistake in setting up an example situation of the real problem. The integer is actually being read in as an element of a larger char block.
A more correct example program would go like this:

#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[])
{
FILE *Afile;
int xx=123456,yy;
char aa[4];
Afile=fopen("d:Test.txt","w+b");
fwrite(&xx,4,1,Afile);
rewind (Afile);
fread(aa,4,1,Afile);
yy=(int)aa;
printf("\n Wrote: %d\n Read: %d\n",xx,yy);
_fcloseall();
return 0;
}
..and thanx for all the replies.

[wayside]

yy=(int)aa;

You are setting yy to be equal to a pointer to aa, not the bytes that are in aa. You would need to do something like

yy = (int) (*(int *)aa);

I can't remember off the top of my head the byte-ordering that Intel processors use, but you might run into byte ordering problems doing this. Why not just do:

fread(&yy,4,1,Afile);

[awni]

you can try memcpy(&yy, a, 4)
not sure the bytes will still mean what you want them to mean

awni

[Sirjorj]

Originally posted by wayside

Why not just do:

fread(&yy,4,1,Afile);

'Casue I don't know the file address for yy but I do know its location in a buffer that had been read.
So I must handle it from that location.

[Sirjorj]

[QUOTE]Originally posted by wayside

yy=(int)aa;

You are setting yy to be equal to a pointer to aa, not the bytes that are in aa. You would need to do something like

yy = (int) (*(int *)aa);

Bravo wayside!
Your code did the trick.
{Haven't tested awni's memcpy)

Thanks for the help.

[wayside]

Originally posted by Sirjorj
'Casue I don't know the file address for yy but I do know its location in a buffer that had been read.
So I must handle it from that location.

I don't understand what you mean by the "file address", this makes no sense to me.

By doing

fread(&yy,4,1,Afile);

you are using the yy variable, which you have declared as an int, a a 4 byte buffer, the same as you are using with your char aa[4] variable. They both take the same amount of space, by using &yy you are getting a pointer to it and can treat it like a 4 byte buffer, and read directly into it. You don't need to go through a char buffer first.

[Sirjorj]

[QUOTE]Originally posted by wayside
I don't understand what you mean by the "file address", this makes no sense to me.

Yes you are right with regard to the SAMPLE code I submitted, I could read the value directly.
BUT in my REAL program I would not be able to set fseek()...
so I must take the value from an available buffer as it is being processed.

Guess I should have explained the difference between my sample and the real-life problem.
But- tnx again.

P.S. the memcpy() also works OK.