Portability of dash to legacy systems, such as AT&T Unix PC

Hi,

While attempting to run an GNU autoconf script on the AT&T Unix PC, I
came across the need to get a POSIX compliant shell.

Indeed, the system shell was not up to the task, as it lacked important
features, such as local positional parameters (the $1, $2, etc would get 
clobbered by calls to sub functions)

Bash worked, but was too big for this memory-starved machine (ran out of
memory during long pipes, as each stage duplicates process' memory
image).

So my choice fell on dash. However, a number of adaptations were
necessary to get it to compile; appended to the end of this mail is a
patch.

Here a summary of the needed changes:

- test for presence of header files which are missing on the UnixPC
getopt.h sys/resource.h termios.h (and others, which might be missing
elsewhere)

- local implementation, replacement or skipping of functions missing on
the UnixPC: tcgetpgrp tcsetpgrp setpgid vfork wait3 sigaction
sigprocmask sigsuspend raise lstat dup2 getgroups strstr stpncpy
strcasecmp strerror strndup strtoul vsnprintf

- lack of POSIX signal handling (sigsuspend, sigaction ...) => use
signal() if sigaction() is absent, and for the other calls, simply skip

- SIGCHLD is called SIGCLD on SystemV systems such as the UnixPC

- realloc() doesn't work on NULL pointers (whereas on more modern
platforms it just behaves as malloc() if passed a NULL pointer

- Fixed USE_GLIBC_STDIO (actually a misnomer, as non GNU C libraries 
supply stdio as well => USE_LIBC_STDIO):moving around of #ifdef's mostly
"notyet" and USE_GLIBC_STDIO itself, missing semicolons, use stdio
streams rather than fd filedescriptors

- As vsnprintf is unavailable on Unix (and possibly other legacy
platforms as well), partly replaced the out1fmt in showvars with outstr,
or else it would fail on variables with *huge* contents as they happen
in GNU autoconf scripts

- choice of an "appropriate" imtmax_t type if none is suggested by the
system libraries: n.B. the type has to be supported by printf, not just
the compiler

- Reworked the handling of isalnum, iscntrl, etc. functions/macros.
On the UnixPC these are actually preprocessor macros doing an array
lookup, whereas dash's system.c so far relied on them just being calls
to other functions (_isalnum instead of isalnum). If these are actually
defined to something else entirely (array lookup),the approach doesn't
work. Just replaced it with wrapper functions. Or maybe we should do
away with that lookup table containing function pointers, and instead
use a huge series of if(...) else if() statements, that don't care about
whether they call a macro or something else

- the signal_names list compilation got the wrong signal names when cross
compiling (from the build platform, rather than from the intended host
platform) => switched to getting them from host platform by calling the
cross compiler with -E, and then compiling the result with the build
platform's own compiler.
Avoid to write real-time signal names beyond end of array (are these
signal names used anyways?).
I also made strsignal use this list if both strsignl and sys_siglist are
unavailable on target platform.

- Redid calculation of SSIZE_MAX so that it works even if size_t is a
*signed* type, as on UnixPC.

- if sys/wait.h lacks some of the needed macros, just define *all* of
them (because the sys/wait.h supplied for UnixPC is for the wrong
endianness... d'oh)

Regards,

Alain

Message ID	aa0e9215-622a-17e0-327b-f0233ed32ad8@knaff.lu (mailing list archive)
State	Changes Requested
Delegated to:	Herbert Xu
Headers	show Return-Path: <dash-owner@kernel.org> From: Alain Knaff <alain@knaff.lu> Subject: [PATCH] Portability of dash to legacy systems, such as AT&T Unix PC To: dash@vger.kernel.org Message-ID: <aa0e9215-622a-17e0-327b-f0233ed32ad8@knaff.lu> Date: Sat, 11 Jun 2022 00:16:00 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk
Series	Portability of dash to legacy systems, such as AT&T Unix PC \| expand Portability of dash to legacy systems, such as AT&T Unix PC

Portability of dash to legacy systems, such as AT&T Unix PC

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