Build Instructions
------------------

* Requirements
  ------------

  - A POSIX-compliant C development environment
  - GNU make version 3.81 or later
  - If cross-compiling: you have to know the correct value for some
non-autodetectable sysdeps for your target architecture. See the
"Cross-compilation" section below.

 This software will install on any operating system that implements
POSIX.1-2008, available at:
  https://pubs.opengroup.org/onlinepubs/9699919799/


* Standard usage
  --------------

  ./configure && make && sudo make install

 will work for most users.
 It will install the static libraries in /usr/lib/skalibs, the shared
libraries in /lib, and the sysdeps in /usr/lib/skalibs/sysdeps.

 Please note that static libraries in /usr/lib/skalibs *will not*
be found by a default linker invocation: you need -L/usr/lib/skalibs.
Other skarnet.org software automatically handles that case if the
default configuration is used, but if you change the configuration,
remember to use the appropriate --with-lib configure option.

 You can strip the libraries of their extra symbols via "make strip"
before the "make install" phase. It will shave a few bytes off them.


* Customization
  -------------

 You can customize the installation process via flags given to configure.
 See ./configure --help for a list of all available configure options.


* Environment variables
  ---------------------

 Controlling a build process via environment variables is a big and
dangerous hammer. You should try and pass flags to configure instead;
nevertheless, a few standard environment variables are recognized.

 If the CC environment variable is set, its value will override compiler
detection by configure. The --host=HOST option will still add a HOST-
prefix to the value of CC.

 The values of CFLAGS, CPPFLAGS and LDFLAGS will be appended to the
default flags set by configure. To override those defaults instead
of appending to them, use the CPPFLAGS, CFLAGS and LDFLAGS
_make variables_ instead of environment variables.


* Make variables
  --------------

 You can invoke make with a few variables for more configuration.

 CC, CFLAGS, CPPFLAGS, LDFLAGS, LDLIBS, AR, RANLIB, STRIP, INSTALL and
CROSS_COMPILE can all be overridden on the make command line. This is
an even bigger hammer than running ./configure with environment
variables, so it is advised to only do this when it is the only way of
obtaining the behaviour you want.
 
 DESTDIR can be given on the "make install" command line in order to
install to a staging directory.


* Shared libraries
  ----------------

 Software from skarnet.org is small enough that shared libraries are
generally not worth using. Static linking is simpler and incurs less
runtime overhead and less points of failure: but since skalibs only
provides libraries, both versions are built by default.
 Nevertheless, you can:
  * avoid building shared libraries: --disable-shared
  * avoid building static libraries: --disable-static

 If you are using a GNU/Linux system, be aware that the GNU libc
behaves badly with static linking and produces huge executables,
so if you plan on making static executables, you should consider
using another libc, which you also need to use when compiling
skalibs. musl is recommended: http://musl-libc.org/


* Cross-compilation
  -----------------

 skarnet.org centralizes all the difficulty of cross-compilation
in skalibs.
 The native skalibs build process runs some tests to gather "sysdeps",
i.e. system-dependent properties of the target, and stores those
into a sysdeps directory; software depending on skalibs is provided
the name of the sysdeps directory at build time, and can depend on
its contents - that's how skarnet.org packages are easily made
portable.
 However, when the host differs from the target - the cross-compilation
case - some of those build-time tests, i.e. the ones that require
code execution on the target, are invalid. So you must manually
provide configure with appropriate values for the list of sysdeps that
cannot be autodetected.
 For a sysdep named K with a value V, the correct option to give to
configure is: --with-sysdep-K=V
 Most of the time, sysdeps have a boolean value, so the correct V is
"yes" or "no".
 For instance, to say that the target has a working pseudorandom
number generator in /dev/urandom, you would give the following
option to configure: --with-sysdep-devurandom=yes
 If there are additional arguments to the sysdep, such as linker
flags, give them separated by commas. Example:
--with-sysdep-posixspawn=yes,-lrt

 ./configure --help lists all the sysdeps you need to manually
provide a value for when cross-compiling.

 Note that you can provide manual sysdeps values at any time,
for any existing sysdep, to bypass autodetection - and you can
even do it when building natively. It's just not recommended
(you should let skalibs autodetect everything it can), and
and it's only mandatory for a small subset of sysdeps in the
cross-compilation case.


* The slashpackage convention
  ---------------------------

 The slashpackage convention (http://cr.yp.to/slashpackage.html)
is a package installation scheme that provides a few guarantees
over other conventions such as the FHS, for instance fixed
absolute pathnames. skarnet.org packages support it: use the
--enable-slashpackage option to configure, or
--enable-slashpackage=DIR for a prefixed DIR/package tree.
This option will activate slashpackage support during the build
and set slashpackage-compatible installation directories. If
$version is the current skalibs version number:

 --bindir will be set to /package/prog/skalibs-$version/command
 --includedir will be set to /package/prog/skalibs-$version/include
 --libdir will be set to /package/prog/skalibs-$version/library
 --dynlibdir will be set to /package/prog/skalibs-$version/library.so
 --sysdepdir will be set to /package/prog/skalibs-$version/sysdeps

 Note that --datadir will be unchanged, because the data exported
by skalibs, i.e. the leap second table, is system-wide. You should
manually specify --datadir=... if you want to deviate from the default.

 --prefix is pretty much ignored when you use --enable-slashpackage:
it will only impact --datadir. You should probably not use both
--enable-slashpackage and --prefix.

 When using slashpackage, two additional Makefile targets are
available after "make install":
 - "make -L update" changes the default version of the software to the
freshly installed one. (This is useful when you have several installed
versions of the same software, which slashpackage supports.)
 - "make -L global-links" adds links from DIR/command and DIR/library.so
to the default version of the binaries and shared libraries.
 The "-L" option to make is necessary because targets are symbolic links,
and the default make behaviour is to check the pointed file's timestamp
and not the symlink's timestamp.


* Out-of-tree builds
  ------------------

 skarnet.org packages do not support out-of-tree builds. They
are small, so it does not cost much to duplicate the entire
source tree if parallel builds are needed.
