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From:	"Rod Pemberton" <dont_use_email AT xnohavenotit DOT cnm>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Question about different behavior of fclose/fread on plain dos
	and windows.
Date:	Tue, 03 Dec 2013 10:17:21 -0500
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On Mon, 02 Dec 2013 21:40:10 -0500, Juan Manuel Guerrero  
<juan DOT guerrero AT gmx DOT de> wrote:

> Please inspect the code snippet below:
>
> #include <stdio.h>
>
> int main(void)
> {
>    char buf_w[] = "pipapo";
>    char buf_r[sizeof(buf_w)] = "";
>    FILE *fw = fopen("file.txt", "w");
>    FILE *fr = fopen("file.txt", "r");
>    size_t bytes;
>
>    bytes = fwrite(buf_w, sizeof(char), sizeof buf_w, fw);
>    printf("fwrite(fw) %s (%d): %s\n", bytes != sizeof buf_w ? "KO" :  
> "OK", bytes, buf_w);
>    bytes = fread(buf_r, sizeof(char), sizeof buf_r, fr);
>    printf("fread(fr) %s (%d): %s\n", bytes != 0 ? "KO" : "OK", bytes,  
> buf_r);
>
>    fclose(fw);
>    fseek(fr, 0, SEEK_SET);
>    bytes = fread(buf_r, sizeof(char), sizeof buf_r, fr);  /* This cuases  
> difficulties. */
>    printf("fread(fr) %s (%d): %s\n", bytes != sizeof buf_r ? "KO" :   
> "OK", bytes, buf_r);
>
>    return 0;
> }
>
>

You'll likely have to change the code for DOS to something like this:

#include <stdio.h>

int main(void)
{
   char buf_w[] = "pipapo";
   char buf_r[sizeof(buf_w)] = "";
   FILE *fw; /* don't open here */
   FILE *fr; /* don't open here */
   size_t bytes;

   /* delete file if it exists */
   if(fopen("file.txt","r")!=NULL)
   {
     remove("file.txt");
   }

   /* open after file deletion is confirmed */
   fw=fopen("file.txt", "w");
   fr=fopen("file.txt", "r");

   bytes = fwrite(buf_w, sizeof(char), sizeof buf_w, fw);
   printf("fwrite(fw) %s (%d): %s\n", bytes != sizeof buf_w ? "KO" : "OK",  
bytes, buf_w);
   bytes = fread(buf_r, sizeof(char), sizeof buf_r, fr);
   printf("fread(fr) %s (%d): %s\n", bytes != 0 ? "KO" : "OK", bytes,  
buf_r);

   fclose(fw);
   fseek(fr, 0, SEEK_SET);

   /* reopen read file */
   freopen("file.txt", "r", fr);

   bytes = fread(buf_r, sizeof(char), sizeof buf_r, fr);  /* This cuases  
difficulties. */
   printf("fread(fr) %s (%d): %s\n", bytes != sizeof buf_r ? "KO" : "OK",  
bytes, buf_r);

   return 0;
}


That seems to work correctly for MS-DOS v7.10 and Windows SE
DOS console window.  Sorry, I just realized I didn't check
v2.04 for that.  I'd have to reboot and loose this reply to
you to check. Let me know if you want me to test that.

I tried it both with and without "file.txt".  Some solutions
I attempted didn't update "file.txt" with a new "pipapo", but
were reading the old "pipapo" in "file.txt", i.e., the previously
saved text.  So, I also edited "file.txt" changed "pipapo" to
"zzzzzz" to verify "pipapo" was actually being updated when the
program executed.  Of course, the version above just deletes
"file.txt" each time.

One of the solutions I looked at which appeared promising at
first was changed fr's "r" mode to "a+", but it had the
"zzzzzz" problem.  I'm guessing there is a possibility that an
"a+" mode for fr and the freopen() might be all that's needed.
So, that might be worth trying too.  It'd be easier solution
if it works.

Hope that helps.


Rod Pemberton

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