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c++ - 98/msdos/dmc EOF behavior

reply Howard <hlenderk bcpl.net> writes:
/*
Hello,
I am fairly new to C and am using dmc on win98/msdos terminal.

I have been learning C using the book "The C Primer" (Hancock, Kreiger, Zamir)
I am having trouble understanding why EOF does not work in a condition check as 
the book assumes it will and wonder what is causing the failure: 98/dos, dmc,
my 
computer ??
I have come up with a work around but still the question nags.

Here is the program from the book I'm trying:
----- */
#include <stdio.h>
int main()
{
FILE *file;
int i;

printf("%d  %x \n",EOF, EOF);                  /* hmmm, EOF prints ffff ffff */

file = fopen("temp", "w");
for(i=0; i < 985; i += 123)
   { putw(i, file); }       /* with nothing: last output 03d8 ffff*/
fclose(file);

file = fopen("temp", "r");
// while((i = getw(file)) != EOF)     //DOESN'T work, loop always runs on */
while((i = getw(file)) != 0xffff)     /* this does catch it */
   { printf("%6d  %4x  hi-0 \n", i, i); }
fclose(file);
}
/* ===== output ======
D:\C\primer\17_file_io>  dmc -A -r -wx 17_4.c
link 17_4,,,user32+kernel32/noi;

D:\C\primer\17_file_io>  17_4
D:\C\primer\17_file_io>  dmc -A -r -wx 17_4x1.c
link 17_4x1,,,user32+kernel32/noi;


D:\C\primer\17_file_io>  17_4x1
-1  ffffffff
      0     0  hi-0
    123    7b  hi-0
    246    f6  hi-0
    369   171  hi-0
    492   1ec  hi-0
    615   267  hi-0
    738   2e2  hi-0
    861   35d  hi-0
    984   3d8  hi-0
                    -when loop runs on the output continues with:
  65535  ffff     ...which repeats but ((i = getw(file)) != EOF)
                    doesn't see it right.
-I've tried
!= (int)EOF   -no success

I'm also into feof section of the book which is working, but I still wonder...
Any ideas?
*/
Feb 03 2007
parent reply Bertel Brander <bertel post4.tele.dk> writes:
Howard skrev:

 /*
 Hello,
 I am fairly new to C and am using dmc on win98/msdos terminal.
 
 I have been learning C using the book "The C Primer" (Hancock, Kreiger, 
 Zamir)
 I am having trouble understanding why EOF does not work in a condition 
 check as the book assumes it will and wonder what is causing the 
 failure: 98/dos, dmc, my computer ??
 I have come up with a work around but still the question nags.
 
 Here is the program from the book I'm trying:
 ----- */
 #include <stdio.h>
 int main()
 {
 FILE *file;
 int i;
 
 printf("%d  %x \n",EOF, EOF);                  /* hmmm, EOF prints ffff 
 ffff */
 
 file = fopen("temp", "w");
 for(i=0; i < 985; i += 123)
   { putw(i, file); }       /* with nothing: last output 03d8 ffff*/
 fclose(file);
 
 file = fopen("temp", "r");
 // while((i = getw(file)) != EOF)     //DOESN'T work, loop always runs 
 on */
 while((i = getw(file)) != 0xffff)     /* this does catch it */
   { printf("%6d  %4x  hi-0 \n", i, i); }
 fclose(file);
 }
 /* ===== output ======
 D:\C\primer\17_file_io>  dmc -A -r -wx 17_4.c
 link 17_4,,,user32+kernel32/noi;
 
 D:\C\primer\17_file_io>  17_4
 D:\C\primer\17_file_io>  dmc -A -r -wx 17_4x1.c
 link 17_4x1,,,user32+kernel32/noi;
 
 
 D:\C\primer\17_file_io>  17_4x1
 -1  ffffffff
      0     0  hi-0
    123    7b  hi-0
    246    f6  hi-0
    369   171  hi-0
    492   1ec  hi-0
    615   267  hi-0
    738   2e2  hi-0
    861   35d  hi-0
    984   3d8  hi-0
                    -when loop runs on the output continues with:
  65535  ffff     ...which repeats but ((i = getw(file)) != EOF)
                    doesn't see it right.
 -I've tried
 != (int)EOF   -no success
 
 I'm also into feof section of the book which is working, but I still 
 wonder...
 Any ideas?


I think it's a bug!
setw does not return an int, but a short.
What it returns on end of file is 0xFFFF not 0xFFFFFFFF.
0xFFFFFFFF is -1, the same as EOF.

This seems to work:
    while((i = (short )getw(file)) != EOF)

That is, if you assume that getw returns a short, it will
work.

-- 
Just another homepage:
http://damb.dk
But it's mine - Bertel
Feb 03 2007
parent reply Howard <hlenderk bcpl.net> writes:
Bertel Brander wrote:

 Howard skrev:

 /*
 Hello,
 I am fairly new to C and am using dmc on win98/msdos terminal.

 I have been learning C using the book "The C Primer" (Hancock, 
 Kreiger, Zamir)
 I am having trouble understanding why EOF does not work in a condition 
 check as the book assumes it will and wonder what is causing the 
 failure: 98/dos, dmc, my computer ??
 I have come up with a work around but still the question nags.

 Here is the program from the book I'm trying:
 ----- */
 #include <stdio.h>
 int main()
 {
 FILE *file;
 int i;

 printf("%d  %x \n",EOF, EOF);                  /* hmmm, EOF prints 
 ffff ffff */

 file = fopen("temp", "w");
 for(i=0; i < 985; i += 123)
   { putw(i, file); }       /* with nothing: last output 03d8 ffff*/
 fclose(file);

 file = fopen("temp", "r");
 // while((i = getw(file)) != EOF)     //DOESN'T work, loop always runs 
 on */
 while((i = getw(file)) != 0xffff)     /* this does catch it */
   { printf("%6d  %4x  hi-0 \n", i, i); }
 fclose(file);
 }
 /* ===== output ======
 D:\C\primer\17_file_io>  dmc -A -r -wx 17_4.c
 link 17_4,,,user32+kernel32/noi;

 D:\C\primer\17_file_io>  17_4
 D:\C\primer\17_file_io>  dmc -A -r -wx 17_4x1.c
 link 17_4x1,,,user32+kernel32/noi;


 D:\C\primer\17_file_io>  17_4x1
 -1  ffffffff
      0     0  hi-0
    123    7b  hi-0
    246    f6  hi-0
    369   171  hi-0
    492   1ec  hi-0
    615   267  hi-0
    738   2e2  hi-0
    861   35d  hi-0
    984   3d8  hi-0
                    -when loop runs on the output continues with:
  65535  ffff     ...which repeats but ((i = getw(file)) != EOF)
                    doesn't see it right.
 -I've tried
 != (int)EOF   -no success

 I'm also into feof section of the book which is working, but I still 
 wonder...
 Any ideas?


 
 I think it's a bug!
 setw does not return an int, but a short.
 What it returns on end of file is 0xFFFF not 0xFFFFFFFF.
 0xFFFFFFFF is -1, the same as EOF.
 
 This seems to work:
    while((i = (short )getw(file)) != EOF)
 
 That is, if you assume that getw returns a short, it will
 work.
 


Yes that 'seems' to work... but I don't know...
(char )getw(file) will work to stop loop too (ff is -1 also)
but I sure don't want that output!

This has turned into an interesting exercise.

I note that putw is also creating shorts.  Here's the hex of temp file:
  0: 00 00 7b 00 f6 00 71 01  ec 01 67 02 e2 02 5d 03
10: d8 03
-----
If I change the putw loop to:
int i;
for(i=0x7ffd; i < 0x8003; i += 1)  { putw(i, file); }

And use:
while((i = (short)getw(file)) != EOF)

I get a signed short and EOF is caught properly:
  32765  7ffd    sizeof(getw(file)): 4
  32766  7ffe
  32767  7fff
-32768  ffff8000
-32767  ffff8001
-32766  ffff8002

hex on temp:
  0: fd 7f fe 7f ff 7f 00 80   01 80 02 80
-----
using:
while((i = getw(file)) != EOF)

I get an unsigned short and loop runs on:
  32765  7ffd    sizeof(getw(file)): 4
  32766  7ffe
  32767  7fff
  32768  8000
  32769  8001
  32770  8002
  65535  ffff
  65535  ffff

I'm trying to understand (my first mistake)
Is what's going on here because the default getw is unsigned short and
being an unsigned short the EOF is not expanded  from ffff to ffffffff ?

Then when it's cast is specified as a short it becomes a signed short
and thusly the ffff EOF signal is expanded (being a negative)?

Why doesn't EOF look for all three types? ff ffff ffffffff

I have tried to (int)putw but get no change to the temp file.

If this IS a bug, am I supposed to report it to someone?

Do I ask too many questions?
BTW, I tried all this in gcc and putw does write 4 byte words in temp file,
and getw retuns 4 byte words, and EOF, of course, is caught ok.
Sorry for my ignorance,
Howard
Feb 07 2007
parent reply Bertel Brander <bertel post4.tele.dk> writes:
Howard skrev:


 Why doesn't EOF look for all three types? ff ffff ffffffff


EOF is just a value, defined in stdio.h as:

#define EOF (-1)


 I have tried to (int)putw but get no change to the temp file.


GetW is defined in stdio.h as:
int __CLIB getw(FILE *FHdl);

So it should return 0xFFFFFFFF on EOF, it does not,
it returns 0xFFFF


 If this IS a bug, am I supposed to report it to someone?


It IS a BUG, hope that someone at Digital Mars comes around,
I don't know how to report errors otherwice.


 Do I ask too many questions?


No.

-- 
Just another homepage:
http://damb.dk
But it's mine - Bertel
Feb 07 2007
parent Howard <hlenderk bcpl.net> writes:
Bertel Brander wrote:

 Howard skrev:
 

 Why doesn't EOF look for all three types? ff ffff ffffffff


 
 EOF is just a value, defined in stdio.h as:
 
 #define EOF (-1)
 


Well I think I get it, I'm slow...
For now I'll just accept the dmc bug as:
  putw and getw handle a  short  and NOT an  int
    and
  getw returns an unsigned short   (ffff = 65535)
    so I need to use
  (short)getw
    which will cast data as a signed short  (ffff = ffffffff = -1)
    and so will match EOF

Hope I've got that concept right.
Much thanks for your help in clearing it up for me,
Howard
Feb 08 2007