@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2010, 2011,
@c   2013-2014, 2021 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Parallel Installations
@section Parallel Installations

@cindex pkg-config
@cindex effective version

Guile provides strong API and ABI stability guarantees during stable
series, so that if a user writes a program against Guile version 2.2.3,
it will be compatible with some future version 2.2.7.  We say in this
case that 2.2 is the @dfn{effective version}, composed of the major and
minor versions, in this case 2 and 2.

Users may install multiple effective versions of Guile, with each
version's headers, libraries, and Scheme files under their own
directories.  This provides the necessary stability guarantee for users,
while also allowing Guile developers to evolve the language and its
implementation.

However, parallel installability does have a down-side, in that users
need to know which version of Guile to ask for, when they build against
Guile.  Guile solves this problem by installing a file to be read by the
@code{pkg-config} utility, a tool to query installed packages by name.
Guile encodes the version into its pkg-config name, so that users can
ask for @code{guile-2.2} or @code{guile-3.0}, as appropriate.

For effective version @value{EFFECTIVE-VERSION}, for example, you would
invoke @code{pkg-config --cflags --libs guile-@value{EFFECTIVE-VERSION}}
to get the compilation and linking flags necessary to link to version
@value{EFFECTIVE-VERSION} of Guile.  You would typically run
@code{pkg-config} during the configuration phase of your program and use
the obtained information in the Makefile.

Guile's @code{pkg-config} file,
@file{guile-@value{EFFECTIVE-VERSION}.pc}, defines additional useful
variables:

@table @code
@item sitedir
@cindex @code{sitedir}
The default directory where Guile looks for Scheme source and compiled
files (@pxref{Installing Site Packages, %site-dir}).  Run
@command{pkg-config guile-@value{EFFECTIVE-VERSION} --variable=sitedir}
to see its value.  @xref{Autoconf Macros, GUILE_SITE_DIR}, for more on
how to use it from Autoconf.

@item extensiondir
@cindex @code{extensiondir}
The default directory where Guile looks for extensions---i.e., shared
libraries providing additional features (@pxref{Foreign Extensions}).
Run @command{pkg-config guile-@value{EFFECTIVE-VERSION}
--variable=extensiondir} to see its value.

@item guile
@itemx guild
@cindex program name transformations, dealing with
The absolute file name of the @command{guile} and @command{guild}
commands@footnote{The @code{guile} and @code{guild} variables defined
starting from Guile version 2.0.12.}.  Run @command{pkg-config
guile-@value{EFFECTIVE-VERSION} --variable=guile} or
@code{--variable=guild} to see their value.

These variables allow users to deal with program name transformations
that may be specified when configuring Guile with
@code{--program-transform-name}, @code{--program-suffix}, or
@code{--program-prefix} (@pxref{Transformation Options,,, autoconf, GNU
Autoconf Manual}).
@end table

@noindent
See the @code{pkg-config} man page, for more information, or its web
site, @url{http://pkg-config.freedesktop.org/}.
@xref{Autoconf Support}, for more on checking for Guile from within a
@code{configure.ac} file.


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