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AS_IF can emit a syntactically invalid shell if-then-else,
if CONDITION
then :
# ...
else
fi
when its IF-FALSE argument consists of macros that don’t produce any
shell code. This was a documented limitation in AS_IF, but it’s a bad
limitation to have, because macros that *used* to expand to shell
commands might start expanding to nothing in future releases. For
instance, this broke the libzmq configure script, which did
AC_PROG_CC
AX_CHECK_COMPILE_FLAG([-std=gnu11],
[CFLAGS+=" -std=gnu11"],
[AC_PROG_CC_C99])
Perfectly valid in 2.69, but in 2.70 AC_PROG_CC_C99 doesn’t produce
any shell code and the script crashes.
We had that limitation for good reason: we can’t just put ‘:’ at the
beginning of the else-clause, like we do for the then-clause, because
that would clobber $? and the IF-FALSE commands might want to inspect
it. (This doesn’t matter for the then-clause, because $? is always
zero at the beginning of a then-clause anyway.) The simplest and
least inefficient shell construct I can find that works in this
context is a shell function that does ‘return $?’. Due to awkward
M4sh initialization ordering constraints (AS_IF gets used before we
can safely use shell functions) an indirection through a shell
variable is necessary. The structure of a m4sh script is now
#! /bin/sh
## M4sh Initialization
as_nop=:
...
## M4sh Shell Functions
as_fn_nop () { return $?; }
as_nop=as_fn_nop
...
and AS_IF emits
if CONDITION
then :
# ...
else $as_nop
# ...
fi
The uses of AS_IF that appear before the beginning of the M4sh Shell
Functions section are all under our control and they don’t need to
look at $?.
If anyone has a better idea for how to make this work I will be glad
to hear it.
Fixes bug #110369.
* lib/m4sugar/m4sh.m4
(_AS_IF_ELSE): When $1 is nonempty, invoke _AS_EMPTY_ELSE_PREPARE.
Emit $as_nop at beginning of else clause.
(_AS_BOURNE_COMPATIBLE): Initialize as_nop to ‘:’.
(_AS_EMPTY_ELSE_PREPARE): New macro which emits a definition of
as_fn_nop and resets as_nop to as_fn_nop.
(AS_PREPARE, _AS_PREPARE): Invoke _AS_EMPTY_ELSE_PREPARE.
(_AS_UNSET_PREPARE): Tweak white space.
* tests/m4sh.at (AS_IF and AS_CASE): Test AS_IF’s IF-FALSE argument
being empty after macro expansion.
* doc/autoconf.texi (AS_IF): Remove warning about use with
‘run-if-false’ argument empty after macro expansion.
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