Strange bug has me worried

[steveo2002]

Hi, I have got this incredibly strange bug happening with some code I've written and I fear there could be something fundamentaly wrong with (either me or) my system.

Here's the code (it's short, I promise):

======================================

#include <stdio.h>
#define FILE_NAME &quot;sdtest.dat&quot;

int main()
{
FILE *fp;
long idx = 0;

fp = fopen(FILE_NAME, &quot;r&quot;

if (fp)
{
fread(&idx, sizeof(long), 1, fp);
printf(&quot;old: %d\n&quot;,idx);
idx++;
fclose(fp);
}

fp = fopen(FILE_NAME, &quot;w&quot;

printf(&quot;new: %d\n&quot;,idx);
fwrite(&idx, sizeof(long), 1, fp);

fclose(fp);
return 0;
}

======================================

The first time I run it I get the following output:

old: 0
new: 1

which increments for each subsequent run, eg.

old: 1
new: 2

then

old: 2
new: 3

etc

BUT, once I have run it enough times for it to get to

old: 25
new: 26

the next run produces

old: 0
new: 1 !!!

The program can not read the file contents when it contains 26! It reads in the file contents of x&quot;1A 00 00 00&quot; as x&quot;00 00 00 00&quot; (verified with debugging).

I have tried this same code on a Unix system at work and it worked as you would expect, what on earth could be causing this? Any insight/advice would be very much appreiated, thanks.

Setup
-----
CPU: AMD Athlon 1GHz
OS: Win 98SE
Compiler: VC++ 6.0

 

[MKuiper]

I suppose 26 is the Ctrl-Z code, which was used as end-of-file marker in the earlier days. If this is the cuase of the problem, you should use other i-o functions.
Marcel

 

[Leibnitz]

if you use &quot;fprintf&quot; and &quot;fscanf&quot; instead of &quot;fread&quot; and &quot;fwrite&quot;,it will work.

#include <stdio.h>
#include <stdlib.h>
#define FILE_NAME &quot;sdtest.dat&quot;
void main( void )
{
FILE *fp;
long idx = 0;
fp = fopen( FILE_NAME, &quot;r&quot; );
if( fp )
{
//Set pointer to beginning of file
fseek( fp, 0L, SEEK_SET );
//Read data back from file:
fscanf( fp, &quot;%ld&quot;, &idx );
printf( &quot;old:%ld\n&quot;, idx );
idx++;
fclose( fp );
}
fp = fopen( FILE_NAME, &quot;w&quot; );
if( fp )
{
printf( &quot;new:%ld\n&quot;, idx );
fprintf( fp,&quot;%ld&quot;, idx );
fclose( fp );
}
}

 

[Leibnitz]

you wont need to include <stdlib.h>,i have forgot to remove it.

 

[steveo2002]

Thanks for the responses, this problem was also happening when reading complex file strutures (where I couldn't get around it by reading/writing each variable as a character string).

But did a bit of hunting the web and found that like you suggested, when 0x1A is read it is treated as eof by code compiled with a Visual C++ compiler, IF the file was opened in text mode.

So there was the solution, to open it in binary mode (&quot;rb&quot; instead of &quot;r&quot. I guess my compiler defaults to opening in text mode.

Thanks again, I can sleep easy now (until next time).

 

[OffByOne]

You should probably open the file using the &quot;rb&quot; and &quot;wb&quot; modes. This opens the file in binary format, as opposed to text format. In text format, certain &quot;character&quot; sequences are converted, while other single &quot;characters&quot;, such as 0x1a (Ctrl-Z), are interpreted differently, in this case, EOF, as was previously mentioned.

OffByOne