Re: Unpleasant problem w/ foo-config style scripts (and possible solutions).

[Marius Vollmer]

Rob Browning <address@hidden> writes:

> I was wondering there's been any thought given to fixing the following
> problem which makes the foo-config style scripts unusable in some
> common situations.

Here is what I have written some time ago about this problem.  I
didn't pursue this further when I noticed that it has to work below
libtool... (since libtool pulls together linker options from .la
files, and these options need to be `canonicalized' as well.)

#! /bin/sh
# Written in 2000 by Marius Vollmer.
# Public Domain.

# canonicalize-prefixes -- reorder command line arguments and print
# the result to stdout.
# 
# The reordering is done to bring the -L and -I compiler options into
# a canonical order to avoid picking up wrong versions of libraries
# and header files.
# 
# 
# The Problem
# -----------
# 
# For the sake of discussing the utility of this hack, consider three
# packages: ME, FOO and BAR.  ME wants to use both FOO and BAR and
# expects FOO and BAR to be already installed in the system when ME is
# to be configured and build.  Further, both FOO and BAR come with
# little programs, foo-config and bar-config, that provide
# configuration information to other packages.
# 
# Now imagine we have this situation: both the FOO and BAR package
# come with the OS and have been installed in /usr.  But the user has
# also installed its own (newer) version of BAR in /home/fred.  He
# wants ME to build with this newer version, of course.
# 
# The configure script of ME uses foo-config and bar-config to find
# out how to link to libfoo (from FOO) and libbar (from BAR) and it
# correctly finds:
#
#   foo-config says to use:   -L/usr/lib -lfoo
#   bar-config says:          -L/home/fred/lib -lbar
#
# The ME package dutifully invokes the linker thus:
#
#   cc -o me me.o -L/usr/lib -lfoo -L/home/fred/lib -lbar
# 
# The problem with this is that the linker will pick up
# /usr/lib/libbar instead of /home/fred/lib/libbar because /usr/lib is
# first on the search path.
# 
# The only one who has enough information to get out of this dilemma
# is Fred himself, because he has configured his system and knows what
# prefixes he wants to use.
# 
# Thus, we need a way for Fred to tell the ME package that he wants
# the linker to invoked in this fashion
# 
#   cc -o me me.o -L/home/fred/lib -L/usr/lib -lfoo -lbar
# 
# because he knows that he wants the /home/fred prefix to shadow the
# /usr prefix.  (On the other hand, only ME knows that it wants to
# have -lfoo precede -lbar.)
# 
# 
# Solution
# --------
# 
# The ME package can solve this problem by passing its compiler and
# linker options thru this program, `canonicalize-prefixes'.  Fred can
# set the environment variable SYSTEM_PREFIXES to control what it
# does, or, when he has a standard system, accept the default.
# 
# Say, the configure script of ME has put the needed linker options
# into $FOO_LIBS and $BAR_LIBS, respectively.  It can then find the
# correct options for linking the `me' program like this
# 
#   ME_LIBS=`canonicalize-prefixes $FOO_LIBS $BAR_LIBS`
# 
# 
# Details
# -------
# 
# canonicalize-prefixes will move all "-L" and "-I" options to the
# front and maybe change their order.  All other options will be
# gathered at the end, in their original order.
# 
# The reordering of -L and -I options is determined by dividing their
# argument directories into two classes: system and non-system
# locations.  A system location is one that matches one of the
# prefixes in the SYSTEM_PREFIXES environment variable.  All other
# directories are non-system locations.  The SYSTEM_PREFIXES variable
# is a colon separated list of directory names.
# 
# In the final sequence, options with non-sytem locations will precede
# the ones with system locations.  The non-system options will be in
# their original order.  The system options will appear in the same
# order as their prefix appears in SYSTEM_PREFIXES.  For deciding
# which prefix belongs to a certain location when there are multiple
# possible choices, the longest prefix is chosen.  When multiple
# system options map to one prefix, the order among them is the same
# as in the original sequence.
# 
# "-I" and "-L" options will retain their relative order.
# 
# When SYSTEM_PREFIXES is not set, its default value is
#
#   SYSTEM_PREFIXES="$HOME:/usr/local:/usr"
#

pfxs=${SYSTEM_PREFIXES-$HOME:/usr/local:/usr}

locs=
rest=
prev=
for option; do
  if test -n "$prev"; then
    eval "locs=\"$locs $prev$option\""
    prev=
    continue
  fi

  case "$option" in
    -L) prev="-L";;
    -L*) locs="$locs $option";;
    -I) prev="-I";;
    -I*) locs="$locs $option";;
    *) rest="$rest$option " ;;
  esac
done

# XXX - use an O(n^2) algorithm, not O(n^3).

sys=
nonsys=
pos=0
IFS=":"; for pfx in $pfxs; do
  IFS=" "; for loc in $locs; do
    locpos=0
    bestpos=0
    bestlen=0
    issys=
    IFS=":"; for p in $pfxs; do
      if test -z "$p"; then continue; fi
      if echo $loc | grep -q "^-[IL]$p"; then
        len=`expr length $p`
        if test $len -gt $bestlen; then
          bestpos=$locpos
          bestlen=$len
        fi
      fi
      locpos=`expr $locpos + 1`
    done
    if test $bestlen = 0 -a $pos = 0; then
      nonsys="$nonsys$loc "
    else if test $bestpos = $pos; then
      if echo $sys | grep -v -q -e "$loc "; then
        sys="$sys$loc "
      fi
    fi fi
  done
  pos=`expr $pos + 1`
done

echo $nonsys$sys$rest