NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/1394

Text File | 1996-08-05 | 2.1 KB | 61 lines

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Strcat Doesn't work for this?
Date: Sat, 13 Jan 96 13:54:10 GMT
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In article <[e-mail]>
           [e-mail] "** Craig Cook **" writes:

>Here is my excerpt of code in question:
>
>
>Putword(int w, char *imageChar)
>{
>        w = (w & 0xff);
>        strcat( imageChar, (const char *)w );
>
>}
>
>Why does it core dump?  It should work right?

You seem to have some misconceptions about what casts do. A cast takes the
*value* of its operand and tries to convert that value to the type specified
in the cast. So (double)45 will take the integer value 45 and convert it
to a double with the value 45 (or as close as possible). The C language
doesn't explicitly define what happens when you cast an int to a pointer (
except a constant expression with value 0 which is converted to a null
pointer) - that is up to your implementation. Many implementation allow
you create pointers to explicit memory locations using such a cast (e.g.
(char *)45 creates a pointer value that points to a char at memory location
45), however you can't use this in portable C code. Maybe you were looking
for something like the following:

void Putword(int w, char *imageChar)
{
        char *ptr = imageChar + strlen(imageChar);

        *ptr++ = w & 0xff;
        *ptr = '\0';
}

This actually results in undefined behaviour where char is equivalent to
signed char and can't represent that value (w & 0xff). unsigned char
may be a better choice in this code.

Usually if you find yourself resorting to pointer casts you are taking
the wrong approach.

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