Pattern syntax %.3: man/libfoo.man in Automake with different base name - linux

Question

I wrote a library libfoo providing functions bar and baz.
I want the user to be able to find the same man-page (from mans/libfoo.man) when they call man libfoo, man bar and man baz (Similar to man fprintf, man sprintf all pointing to the same page.)
My current setup has the files mans/libfoo.man and Makefile.am
To 'tell' automake that I want to end up with the three man-pages I specified the dist_man3_MANS variable.
Makefile.am:
dist_man3_MANS = mans/libfoo.3 mans/bar.3 mans/baz.3
Coming from GNU make, I thought I could just write
%.3: mans/libfoo.man
ln -S libfoo.man $#
to create links temporarily and then let Automake install those accordingly, but Automake errors out with Makefile.am:115: warning: '%'-style pattern rules are a GNU make extension. I want to do it properly and take this warning seriously by not relying on GNU Make to be as portable as possible.
The Automake manual suggests to add a target
.man.3:
$(LN_S) $^ $#
but that just tells Automake that xx.man can be compiled to xx.3, requiring the base name to be the same. I don't want to carry around those xx.man files, so this approach does not work.
I could hack it in with putting a rule
dist_man3_MANS = mans/libfoo.3 mans/bar.3 mans/baz.3
$(dist_man3_MANS): mans/libfoo.man
$(LN_S) libfoo.man $#
but that seems like a dirty hack, because I am not giving it a recipe to compile .man to .3, but rather say: "Hey, you can create those files with this rule", which for this case may work coincidental.

Answer 1

I would follow the example from the Automake info page section Extending Automake Rules and do something along the lines of
LIBFOO_MAN_ALIASES = bar baz
install-data-hook:
set -e; \
cd $(DESTDIR)$(man3dir) && \
for manalias in $(LIBFOO_MAN_ALIASES); do \
$(LN_S) libfoo.3 $${manalias}.3; \
done
uninstall-hook:
cd $(DESTDIR)$(man3dir) && \
for manalias in $(LIBFOO_MAN_ALIASES); do \
rm -f $${manalias}.3; \
done
relying on AC_PROG_LN_S to make sure that $(LN_S) does something reasonable for the system (symlink, hardlink, copy) to create a file name which can be open(2)ed and read.
FTR, I have just taken a look at three different systems' man pages and found them using three different methods to make the fprintf(3) man page show the same man page as printf(3) does:
Debian 10 uses symlinks
Fedora 35 uses a /usr/share/man/man3/fprintf.3 file containing .so man3/printf.3 (while some other man pages use symlinks to achieve the same effect)
FreeBSD 13 uses hardlinks, and find /usr/share/man -type l does not find any symlinks on my relatively clean system. However, manually testing both symlinks and the .so man3/printf.3 method suggests that FreeBSD man(1) does not treat symlinks in any special way and therefore opens the symlinked man page, and it also interprets the .so command just like Fedora 35's man(1) does.
I do not know how portable each of those methods is. Each of these three methods could set up on make install by using an appropriate install-data-hook, but any man file which can be opened using open(2) appears to be work, and therefore $(LN_S) looks like a good bet.