undefined reference to "function name"

[llewelly@edevnull.com]

Pavlos Parissis <surfking@x-treme.gr> writes:

> Hello,
> 
> >
> > On my i686-linux-pc-gnu system, I successfully compiled and linked
> >   your example with the following command line:
> >
> > gcc -g -v -Wall -L/usr/X11R6/lib -lX11 -lXtst X_fun.c

-l/usr/X11R6/lib tells gcc to look in /usr/X11R6/lib for libraries.

-lX11 tells gcc to link your code with libX11 .

-lXtst tells gcc to link your code with libXtst .

Please read http://gcc.gnu.org/onlinedocs/gcc_2.html#SEC14 to learn
  more about these flags.

(You can also find this information in the info pages that come with
  gcc.)

> >
> 
> ok i tried it and it worked, but why?
> I mean, before install RedHat 6.2 i had 5.1 and i compiled the same program
> without
> -L/usr/X11R6/lib -lX11 -lXtst in gcc command.

Odd. Code that uses libraries should *not* link unless gcc is told
  what libraries to link against, and where to look for those libraries.
[snip]