strcat and strncat problem on different machines

[BAKERWL at UWYO.EDU]

I have a small function in the r.le programs that works fine under
SunOS 4.1.3 on my Sparcstation, but fails under Solaris 2.* and on
other machines.  The function uses getchar and strcat to get user
digital input from the screen and dump it into an array.  Here is 
a typical statement to read digits and append them to the end of 
a character array called "num":

	while ((c=getchar()) && isdigit(c)) strcat(num, &c);

Earlier, Changyong Cao told me that this fails under Linux, since
strcat(num, &c) appends a bunch of strange characters instead of
just one.  He found that the following substitution works:

	while ((c=getchar()) && isdigit(c)) strncat(num, &c, 1);

While I will probably do this substitution and hope for the best,
I wonder if anyone knows why this happens or whether there is a 
more general solution.  Also, for some reason on some machines
my function only reads a couple of digital characters, then stops.
Anyone know why?

	Bill Baker
	bakerwl at uwyo.edu