From: "Mr. Veli Suorsa" <VJSuorsa AT Surfeu DOT Fi>
Sender: vjsuorsa AT Surfeu DOT Fi
To: djgpp AT delorie DOT com
Cc: Jack Klein <jackklein AT spamcop DOT net>, VJSuorsa AT Surfeu DOT Fi
Date: Sun, 8 Apr 2001 01:51:34 +0300
Subject: Re: Read with C++ <fstream.h>
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Reply-To: djgpp AT delorie DOT com

Hi!

Thanks for your reply!

>On Fri, 6 Apr 2001 23:50:17 +0300, "Mr. Veli Suorsa"
><VJSuorsa AT Surfeu DOT Fi> wrote in comp.os.msdos.djgpp:
>>
>> Can You help me to convert this ascii- file reader
>> (tested C program) to Gnu C++ using <fstream.h> library?
>> 
>> And how do you put these read lines to an array ( lines[MAXLINELEN][ROW]
)?
>> 
>> 
>> ---
>> /* Program read lines from file.
>>    (strread.c), Veli Suorsa, 06.04.2001 */
>> 
>> #include <stdio.h>
>> 
>> #define MAXLINELEN 110
>> 
>> int main( int argc, char *argv[] )
>> {
>>    char filename[30], strline[MAXLINELEN];
>>    int line = 0;
>>    FILE *fileptr;
>> 
>>    // Test if given parameter
>>    if ( argc != 2 )
>>    {
>>       printf( "\nGive data file name : " );
>>       gets( filename );
>
>Have you tested this by typing in a filename that is 40
>bytes, or 50 bytes, or 100 bytes?  gets() is still in the
>standard library because a long, long time ago it was used
>by some very sloppy programmers.  It is the most dangerous
>function in the entire C or C++ library because there is
>literally no way to use it safely.  If the data entered is
>larger than the buffer there is no way to prevent undefined
>behavior.

Don't be so crude.
I Compile and test this program with filenames under 30 bytes. But I understand
that "safety", you emphasize. What was that english hacker who break into Mr
Gates files and order him viagra?

>>    }
>>    else
>>    {
>>       // Copy parameter to filename
>>       strcpy( filename, argv[1] );
>
>Again, DOS with short file names allows path names to have
>64 characters, 63 visible characters plus one for the '\0'
>at the end.

OK, I will use longer variable...

>>    }
>> 
>>    // Test if can read file
>>    if ( ( fileptr = fopen( filename, "r" ) ) == NULL )
>>    {
>>       printf( "\nError: File: %s can't read.\n", filename );
>>       exit(0);
>
>You need to include <stdlib.h> for the prototype of exit.

Why? It works fine with <stdio.h>!

>>    }
>> 
>>    // List file
>>    printf( "\nFile %s list :\n", filename );
>> 
>>    // Read and list, while not end of file
>>    while ( !feof( fileptr ))
>>    {
>>       fgets( strline, MAXLINELEN, fileptr);
>
>This is the wrong way to read a file from beginning to end.
>The feof() function does not work the way you think it
>does.  It does not return true just because you already
>read the last byte of the file.
>It returns an error flag that is set only after you try to
>read past the end of the file.
>
>The proper way to do this is to combine the read and test
>at the same time. Replace all three lines above with this:
>
>      while (NULL != fgets(strline, sizeof strline, fileptr)
>      {

I will try this. "My way" works, too, indeed.

>>       printf( "%s", strline);
>>       // Line counter
>>       line++;
>>    }
>>    // Close file
>>    fclose( fileptr );
>> 
>>    printf( "\n*** Read: %d rows from file: %s.\n", line, filename );
>> 
>>    return 0;
>> }
>> ---
>
>For a safe replacement for gets() see
>http://home.att.net/~jackklein/ctips01.html#safe_gets

Thanks for this link.

>For more information on why you are using feof()
>incorrectly and gets() see these two topics from the FAQ
>for comp.lang.c:
>
>12.2 Why won't the code `` while(!feof(infp)) { fgets(buf, MAXLINE,
>infp); fputs(buf, outfp); } '' work?
>
>12.23 Why does everyone say not to use gets()?
>
>There is a link to the FAQ in my signature.
>
>Perhaps you should correct the C version before attempting
>to translate to C++.

C version works fine (with a little finishing touches)!

Can you help me with this C++ conversion?

And how do you put these read lines to an array ( lines[MAXLINELEN][ROW] )?


>-- 
>Jack Klein
>Home: http://JK-Technology.Com
>FAQs for
>comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
>comp.lang.c++ http://www.parashift.com/c++-faq-lite/
>alt.comp.lang.learn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq
>

With Best Regards!

Veli Suorsa
---
"People must believe to the future to be able to live!"
---------------------------------- J.V.Snellman, 1890.

Oulu, FINLAND
Mailto:VJSuorsa AT Surfeu DOT Fi
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