> This is a major problem.  The usual way out is to create a
> declaration of your own but then it conflicts with a
> declaration of the same thing in a vendor provided include
> file.  I consider writing your own prototype to be
> equivalent to casting the whole damn call!  Alas, it is not
> always possible to avoid it.
> 
> Someone ought write a book on porting software and how to
> write portable software.

Several people have...after building a very system dependant product
on a 1/2 dozen very different Unixes I'm a huge fan of autoconf.

Autoconf basically generates a set of portability defines.  There are
many default tests provided, as well as several basic types of tests 
added.  The defines that autoconf generates are automatically generated
on a given system.  In our case of lseek() one might have a get of
autoconf tests that generate the following defines:

HAVE_LSEEK
NEED_LSEEK_PROTOTYPE
LSEEK_OFFSET_ARG_TYPE
LSEEK_IN_UNISTD_H
LSEEEK_IN_OSFCNS_H	/* SunOS 4 lossage */

Autoconf would have simple standard compile tests for most of these defines,
the 3rd define might be generated by a run time test.
The referencing code would then reference lseek by saying:

#ifdef HAVE_LSEEK
#  ifdef NEED_LSEEK_PROTOTYPE
LSEEK_OFFSET_ARG_TYPE lseek(int fd, LSEEK_OFFSET_ARG_TYPE off, int whence);
#  else
#    ifdef LSEEK_IN_UNISTD_H
#      include <unistd.h>
#    else if defined(LSEEK_IN_OSFCNS_H)
#      include <osfcns.h>
#    else
#      define x(y	/* puke and die, more portable than #error */
#    endif
#  endif
#else
	/* deal with it, define lseek(), whatever */
#endif

It's really less work than you would think.  The above can be isolated in 
a port.h header or whatever.  The big winnage is that after you've done
2-3 ports the rest start to go lots quicker since autoconf does the
tests automatically.  That also means that things like:

#ifdef sun

don't break when something changes in the following OS release.
--
Mark Gooderum
mark@good.com