problem with fwrite

problem with fwrite

[Lei Yang]

Hi all,

This is not really a kernel issue, apologize if anyone thinks that this 
is not the right place to post it. But I am writing a kernel module and 
got stuck on fwrite, really hope someone could point out what stupid 
mistake I've made. I wrote a very simple code to test the idea, what I 
really want to do in fred() is to read from 'dest' and write to 'src'. 
It seems that upon running , fgetc doesn't get anything from in_stream, 
so the first char it gets is an EOF and it breaks. Just why fwrite 
didn't write anything to in_stream?

If this is not the right way to do it, what is ?

Appreciate any comments, even harsh ones.

TIA
Lei

// in test.c

#include <stdio.h>

fred(char *dest, size_t *destlen, char *src, size_t size)
{
     FILE *in_stream = tmpfile();
     FILE *out_stream = tmpfile();
     fwrite(src, 1, size, in_stream);

     int c, i;
     for(i = 0;;i++)
     {
   	c = fgetc(in_stream);
   	fprintf(stderr, "get char %c\n", c);
	if ( c == EOF) break;
   	fputc(c, out_stream);
      }

     *destlen = i;
	
     fseek(out_stream, 0, SEEK_SET); //rewind
     fread(dest, 1, *destlen, out_stream);
     fclose(in_stream);
     fclose(out_stream);
     fprintf(stderr, "buf = `%s', size = %d\n", dest, *destlen);
}

int main(void)
{
   static char source[] = "really hope this works ";

   char *bp = malloc (2048);
   size_t destlen;

   fred(bp, &destlen, source, strlen(source));

   fprintf(stderr, "buf = `%s', size = %d\n", bp, destlen);
   return 0;
}

Re: problem with fwrite

[Martin Zwickel]

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On Thu, 19 Aug 2004 09:42:41 -0400
Lei Yang <leiyang@nec-labs.com> bubbled:

> Hi all,
> 
> This is not really a kernel issue, apologize if anyone thinks that
> this is not the right place to post it. But I am writing a kernel
> module and got stuck on fwrite, really hope someone could point out
> what stupid mistake I've made. I wrote a very simple code to test the
> idea, what I really want to do in fred() is to read from 'dest' and
> write to 'src'. It seems that upon running , fgetc doesn't get
> anything from in_stream, so the first char it gets is an EOF and it
> breaks. Just why fwrite didn't write anything to in_stream?
> 
> If this is not the right way to do it, what is ?
> 
> Appreciate any comments, even harsh ones.

First: Wrong place!!!

> 
> TIA
> Lei
> 
> // in test.c
> 
> #include <stdio.h>
> 
> fred(char *dest, size_t *destlen, char *src, size_t size)
> {
>      FILE *in_stream = tmpfile();
>      FILE *out_stream = tmpfile();
>      fwrite(src, 1, size, in_stream);

Seek in_stream to 0 (rewind)! like:
       rewind(in_stream);

> 
>      int c, i;
>      for(i = 0;;i++)
>      {
>    	c = fgetc(in_stream);
>    	fprintf(stderr, "get char %c\n", c);

the fprintf of the read char should be after the EOF check!

> 	if ( c == EOF) break;
>    	fputc(c, out_stream);
>       }
> 
>      *destlen = i;
> 	
>      fseek(out_stream, 0, SEEK_SET); //rewind
>      fread(dest, 1, *destlen, out_stream);
>      fclose(in_stream);
>      fclose(out_stream);
>      fprintf(stderr, "buf = `%s', size = %d\n", dest, *destlen);
> }
> 
> int main(void)
> {
>    static char source[] = "really hope this works ";
> 
>    char *bp = malloc (2048);
>    size_t destlen;
> 
>    fred(bp, &destlen, source, strlen(source));
> 
>    fprintf(stderr, "buf = `%s', size = %d\n", bp, destlen);
>    return 0;
> }
> -
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-- 
MyExcuse:
Traffic jam on the Information Superhighway.

Martin Zwickel <martin.zwickel@technotrend.de>
Research & Development

TechnoTrend AG <http://www.technotrend.de>

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