(autoconf.info)Here-Documents


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10.2 Here-Documents
===================

Don't rely on `\' being preserved just because it has no special
meaning together with the next symbol.  In the native `sh' on OpenBSD
2.7 `\"' expands to `"' in here-documents with unquoted delimiter.  As
a general rule, if `\\' expands to `\' use `\\' to get `\'.

   With OpenBSD 2.7's `sh'

     $ cat <<EOF
     > \" \\
     > EOF
     " \

and with Bash:

     bash-2.04$ cat <<EOF
     > \" \\
     > EOF
     \" \

   Some shells mishandle large here-documents: for example, Solaris 10
`dtksh' and the UnixWare 7.1.1 Posix shell, which are derived from Korn
shell version M-12/28/93d, mishandle braced variable expansion that
crosses a 1024- or 4096-byte buffer boundary within a here-document.
Only the part of the variable name after the boundary is used.  For
example, `${variable}' could be replaced by the expansion of `${ble}'.
If the end of the variable name is aligned with the block boundary, the
shell reports an error, as if you used `${}'.  Instead of
`${variable-default}', the shell may expand `${riable-default}', or
even `${fault}'.  This bug can often be worked around by omitting the
braces: `$variable'.  The bug was fixed in `ksh93g' (1998-04-30) but as
of 2006 many operating systems were still shipping older versions with
the bug.

   Many shells (including the Bourne shell) implement here-documents
inefficiently.  In particular, some shells can be extremely inefficient
when a single statement contains many here-documents.  For instance if
your `configure.ac' includes something like:

     if <cross_compiling>; then
       assume this and that
     else
       check this
       check that
       check something else
       ...
       on and on forever
       ...
     fi

   A shell parses the whole `if'/`fi' construct, creating temporary
files for each here-document in it.  Some shells create links for such
here-documents on every `fork', so that the clean-up code they had
installed correctly removes them.  It is creating the links that can
take the shell forever.

   Moving the tests out of the `if'/`fi', or creating multiple
`if'/`fi' constructs, would improve the performance significantly.
Anyway, this kind of construct is not exactly the typical use of
Autoconf.  In fact, it's even not recommended, because M4 macros can't
look into shell conditionals, so we may fail to expand a macro when it
was expanded before in a conditional path, and the condition turned out
to be false at runtime, and we end up not executing the macro at all.


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