NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/14355

Internet Message Format | 1996-08-05 | 2.7 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Nee help with a string and temp string
Date: Sat, 13 Apr 96 16:05:39 GMT
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In article <[e-mail]>
           [e-mail] "Tarang Deshpande" writes:

>Randall J. Pfeifer wrote:
>>
>> I am writing a program that reads data, validates it, and then writes it to a
> new file.
>> ...
>>
>> My problem involves lines 2 through X.
>>   Reading the data is fine. However, when I evaluate the components and write
> them to the
>>   buffer the previous lines data still resides in the string.

>> void examineBcard()
>> { int result;
>>
>>   strncpy(Bcard,"B",1);
>>   strncat(Bcard,&line[1],1);
>>   strncat(Bcard, Space,3);
>>
>>   padright(Bcard,5,7,3,3);
>>   padright(Bcard,8,12,5,5);
>>   padright(Bcard,13,18,6,6);
>>   strncat(Bcard," ",1);...
>
>Note that in C the end of a string is marked by a null character or
>char(0).

Hold that thought.

  Your call to strncpy is equivilant to the following:

>Bcard [ 0 ] = 'B';
>Bcard [ 1 ] = '\0';

Wrong. strncpy() always writes exactly the number of characters you specify
to the target buffer. It does not guarantee that the result is a string i.e.
null character terminated and is an oddity in the standard string library.
In this case it simply corresponds to:

    Bcard[0] = 'B';

So the subsequent call to strncat which depends on its first argument
pointing to a null character terminated string isn't going to work properly.
You could fix this by changing strncpy() to simply strcpy() with the
first 2 arguments or you could simplify that group of three lines to:

    sprintf(Bcard, "B%c   ", line[1]);

or character-wise:

    Bcard[0] = 'B';
    Bcard[1] = line[1];
    Bcard[2] = ' ';
    Bcard[3] = ' ';
    Bcard[4] = ' ';
    Bcard[5] = '\0';

sprintf is far more flexible though, it can probably replace the calls to
padright as well and the resulting code will be much clearer. I've no idea
what the arguments to padright() specify (I can probably guess the first!)

>The call in no way affects Bcard [ x ] where x >= 2.  I suspect that
>the problem is in you padright function which does not put a trailing
>null character at the end. This is one solution.

It doesn't fix the immediate problem.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------