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(Common Shell Constructs): New node, documenting AS_DIRNAME.

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(Limitations of Usual Tools): Refer to it when discussing dirname.
Also, update discussion of POSIX standard to reflect latest draft.
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Paul Eggert 2001-09-18 18:46:03 +00:00
parent 307d40fa4f
commit 3c3fd5fe64
1 changed files with 23 additions and 11 deletions
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doc/autoconf.texi
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@ -320,6 +320,7 @@ Programming in M4sugar
* Redefined M4 Macros:: M4 builtins changed in M4sugar
* Forbidden Patterns:: Catching unexpanded macros
* Common Shell Constructs:: Working around shell portability problems
Writing Autoconf Macros
@ -6688,6 +6689,8 @@ M4sugar''.
@menu
* Redefined M4 Macros:: M4 builtins changed in M4sugar
* Forbidden Patterns:: Catching unexpanded macros
* Common Shell Constructs:: Working around shell portability problems
@end menu
@node Redefined M4 Macros
@ -6791,7 +6794,19 @@ Any token matching @var{pattern} is allowed, including if it matches an
@code{m4_pattern_forbid} pattern.
@end defmac
@node Common Shell Constructs
@subsection Common Shell Constructs
M4sugar provides portable alternatives for some common shell constructs
that unfortunately are not portable in practice.
@defmac AS_DIRNAME (@var{pathname})
@msindex DIRNAME
Return the directory portion of @var{pathname}, using the algorithm
required by @sc{posix}. @xref{Limitations of Usual Tools}, for more
details about what this returns and why it is more portable than the
@command{dirname} command.
@end defmac
@c=================================================== Writing Autoconf Macros.
@ -8746,17 +8761,18 @@ Some implementations, such as Tru64's, fail when comparing to
@item @command{dirname}
@c --------------------
@prindex @command{dirname}
Not all hosts have @command{dirname}, but it is reasonably easy to
emulate, e.g.:
Not all hosts have a working @command{dirname}, and you should instead
use @code{AS_DIRNAME} (@pxref{Common Shell Constructs}). For example.
@example
dir=`expr "x$file" : 'x\(.*\)/[^/]*' \|
'.' : '.'
dir=`dirname "$file"` # This is not portable.
dir=`AS_DIRNAME(["$file"])` # This is more portable.
@end example
@noindent
But there are a few subtilities, e.g., under UN*X, should @samp{//1}
give @samp{/}? Paul Eggert answers:
This handles a few subtleties in the standard way required by
@sc{posix}. For example, under UN*X, should @samp{dirname //1} give
@samp{/}? Paul Eggert answers:
@quotation
No, under some older flavors of Unix, leading @samp{//} is a special
@ -8766,15 +8782,11 @@ to @samp{/}; but leading @samp{//} is special. I think this tradition
started with Apollo Domain/OS, an OS that is still in use on some older
hosts.
POSIX.2 allows but does not require the special treatment for @samp{//}.
@sc{posix} allows but does not require the special treatment for @samp{//}.
It says that the behavior of dirname on path names of the form
@samp{//([^/]+/*)?} is implementation defined. In these cases, GNU
@command{dirname} returns @samp{/}, but it's more portable to return
@samp{//} as this works even on those older flavors of Unix.
I have heard rumors that this special treatment of @samp{//} may be
dropped in future versions of POSIX, but for now it's still the
standard.
@end quotation