Message-Id: <199803211409.QAA20152@ankara.duzen.com.tr>
From: "S. M. Halloran" <mitch AT duzen DOT com DOT tr>
Organization: User RFC 822- and 1123-compliant
To: zappaz AT xtra DOT co DOT nz, djgpp AT delorie DOT com
Date: Sat, 21 Mar 1998 16:13:37 +0200
MIME-Version: 1.0
Content-type: text/plain; charset=US-ASCII
Content-transfer-encoding: 7BIT
Subject: Re: Why why why??? (help me with this code....)
In-reply-to: <6f0c4b$a9p$1@wolfman.xtra.co.nz>
Precedence: bulk

On 22 Mar 98 zappaz AT xtra DOT co DOT nz was found to have commented thusly:

> I'm using DJGPP on Win95 platform and this little procedure is giving
> me a real headache. All it does is return a frequency table (array of
> longs) of how often each byte appears in the file "namein". If I
> include the line marked with ' > ' then the following while loop never
> gets executed. ? Also, if I include the line marked with ' >>> ' the
> program crashes, even if the ' > ' line _is_ included. ? ? Compiles
> OK.
> 
> 
> 
> long *frequencyTable(char *namein) {
>       FILE *infile;
>       int c;
>       long *freq;
        ^^^^^^^^^^^
          This is your problem right here

>       memset(freq,0,2);
> 
>       infile = fopen(namein,"r");
>       printf("Opening %s successful.\n", namein);
> >    for (int i=0;i<256;i++) freq[i]=0;
>       while((c=getc(infile))!=EOF)
>          {  
>             printf("got \"%c\"\n",c);  // not really neccesary
> >>>      freq[c]++;
>          }
>      fclose(infile);
>      return freq;
> }
> 
> 
> Also, I've often noticed that adding a printf statement in a loop
> makes a big difference to how that loop runs (usually the difference
> between running or not running at all). Is this just something strange
> with DJGPP or is there something I don't know?
> 

Back to the School of C Programming for you, I think.  You have failed to 
initialize the pointer variable 'freq' properly.

Either use malloc() to assign the pointer to a block of (dynamic) memory:

          long *freq;   /* as you have it */

          /* somewhere before you reference 'freq' at all */
          if ((freq = malloc(256 * sizeof(long))) == NULL)
                /* report an error, maybe return from function,
                      then buy more memory */;

          /* more code */

       free(freq); /* don't forget to free before returning from func */ 

Probably it is perhaps better, in your case, to declare 'freq' as an array 
of 256 long integers:

       long freq[256];   /* a definition of an 'automatic' variable */

Also in your for loop marked with '>', you set all the values in the array 
with '0'.  I think it is better form to put as a suffix on long constants 
the character 'L', although I don't believe it is required to generate a 
compiler error if you do not do so.

Mitch Halloran
Research (Bio)chemist
Duzen Laboratories Group
Ankara   TURKEY
mitch AT duzen DOT com DOT tr

other job title:  Sequoia's (dob 12-20-95) daddy