de XX
IP \ \ \ \-
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TL
APE \(em The ANSI/POSIX Environment
AU
Howard Trickey
SH
Introduction
PP
When a large or frequently-updated program must be imported or exported
to and from Plan 9, the ANSI/POSIX environment known as APE can be useful.
APE combines the set of headers and object code libraries specified by
the ANSI C standard (ANSI X3.159-1989) with the POSIX operating system
interface standard (IEEE 1003.1-1990, ISO 9945-1).
Using APE will cause slower compilation and marginally slower execution speeds,
so if the importing or exporting happens only infrequently, due consideration
should be given to using the usual Plan 9 compilation environment instead.
Another factor to consider is that the Plan 9 header organization is
much simpler to remember and use.
PP
There are some aspectes of required POSIX behavior that are impossible or
very hard to simulate in Plan 9.  They are described below.
Experience has shown, however, that the simulation is adequate for the
vast majority of programs.  The much more common problem is that
many programs use functions or headers not defined by POSIX.
APE has some extensions to POSIX to help in this regard.
SH
Pcc
PP
The
CW pcc
command acts as a front end to the Plan 9 C compilers and loaders.
It runs an ANSI C preprecessor over source files, using the APE
headers to satisfy
CW "#include <\fIfile\fP>"
directives; then it runs a Plan 9 C compiler; finally, it may load
with APE libraries to produce an executable program.
The document
I "How to Use the Plan 9 C Compiler"
explains how environment variables are used by convention to
handle compilation for differing architectures.
The environment variable
CW $objtype
controls which Plan 9 compiler and loader are used by
CW pcc ,
as well as the location of header and library files.
For example, if
CW $objtype
is
CW mips ,
then
CW pcc
has
CW cpp
look for headers in
CW /mips/include/ape
followed by
CW /sys/include/ape ;
then
CW pcc
uses
CW vc
to create
CW .v
object files;
finally,
CW vl
is used to create an executable using libraries in
CW /mips/lib/ape .
SH
Symbols
PP
The C and POSIX standards require that certain symbols be
defined in headers.
They also require that certain other classes of symbols not
be defined in the headers, and specify certain other
symbols that may be defined in headers at the discretion
of the implementation.
POSIX defines
I "feature test macros" ,
which are preprocessor symbols beginning with an underscore
and then a capital letter;  if the program
CW #define s
a feature test macro before the inclusion of any headers,
then it is requesting that certain symbols be visible in the headers.
The most important feature test macro is
CW _POSIX_SOURCE :
when it is defined, exactly the symbols required by POSIX are
visible in the appropriate headers.
Consider
CW <signal.h>
for example:
ANSI defines some names that must be defined in
CW <signal.h> ,
but POSIX defines others, such as
CW sigset_t ,
which are not allowed according to ANSI.
The solution is to make the additional symbols visible only when
CW _POSIX_SOURCE
is defined.
PP
When you export a program, it helps to know whether the program fits
in one of the following categories:
IP 1.
Strictly conforming ANSI C program. It only uses features of the language,
libraries, and headers explicitly required by the C standard.  It does not
depend on unspecified, undefined, or implementation-dependent behavior,
and does not exceed any minimum implementation limit.
IP 2.
Strictly conforming POSIX program. Similar, but for the POSIX standard.
IP 3.
Some superset of POSIX, with extensions.  Each extension
is selected by a feature test macro, so it is clear which extensions
are being used.
PP
With APE, if headers are always included to declare any library functions
used, then the set of feature test macros defined by a program will
show which of the above categories the program is in.
To accomplish this, no symbol is defined in a header if it not required
by the C or POSIX standard, and those required by the POSIX standard
are protected by
CW "#ifdef _POSIX_SOURCE" .
For example,
CW <errno.h>
defines
CW EDOM ,
CW ERANGE ,
and
CW errno ,
as required by the C standard.
The C standard allows more names beginning with
CW E ,
but our header defines only those unless
CW _POSIX_SOURCE
is defined, in which case the 35 more symbols required by POSIX are defined.
This means that a program which uses
CW ENAMETOOLONG
cannot masquerade as a strictly conforming ANSI C program.
PP
CW Pcc
does not predefine any preprocessor symbols except those required by
the ANSI C standard:
CW __STDC__ ,
CW __LINE__ ,
CW __FILE__ ,
CW __DATE__ ,
and
CW __TIME__ .
Any others must be defined in the program itself or using
CW -D
on the command line.
SH
Extensions
PP
The pedagogy enforced by only putting required
names in the headers is useful for exporting programs,
but it gets in the way when importing programs.
The compromise is to allow additional symbols in headers,
additional headers, and additional library functions,
but only under control of extension feature test macros.
The following extensions are provided; unless otherwise
specified, the additional library functions are in the
default APE library.
XX
CW _LIBG_EXTENSION .
This allows the use of the Plan 9 graphics library.
The functions are as described in the Plan 9 manual (see
I graphics (2))
except that
CW div
had to be renamed
CW ptdiv .
Include the
CW <libg.h>
header to declare the needed types and functions,
and use
CW -lg
on the
CW pcc
command line to load the graphics functions.
XX
CW _LIMITS_EXTENSION .
POSIX does not require that names such as
CW PATH_MAX
and
CW OPEN_MAX
be defined in
CW <limits.h> ,
but many programs assume they are defined there.
If
CW _LIMITS_EXTENSION
is defined, those names will all be defined when
CW <limits.h>
is included.
XX
CW _BSDTIME_EXTENSION .
The header
CW <sys/time.h>
is not defined by POSIX.
This extension allows the inclusion of that header,
with its definitions of the
CW timeval
and
CW timezone
structures.
XX
CW _SELECT_EXTENSION .
This extension allows the inclusion of the
CW <select.h>
header, which defines the Berkeley
CW select
function and associated types and macros
for dealing with multiple input sources.
The
CW <sys/time.h>
header must be included before
CW <select.h> ,
so the
CW _BSDTIME_EXTENSION
is needed too.
XX
CW _PCONS_EXTENSION .
This defines a ``pseudo-tty'' capability.
If
CW _PCONS_EXTENSION
is defined,
CW <pcons.h>
may be included; it defines the functions
CW pcons(void)
and
CW pushtty(void).
CW Pcons
arranges for a Plan 9 file server to provide the
CW /dev/cons
and
CW /dev/consctl
files.
It also provides
CW /dev/mcons ,
which is the ``master end'' of the pseudo-tty:
characters written to
CW /dev/mcons
can be read from
CW /dev/cons
(or
CW /dev/tty ,
a synonym)
and vice versa.
POSIX tty line processing is supported:
the contents of a
CW termios
structure can be written to or read from the
CW /dev/pttyctl
file in the format
P1
"stty %8ux %8ux %8ux %8ux "
P2
(the numbers are the
CW c_iflag ,
CW c_oflag ,
CW c_cflag ,
and
CW c_lflag ),
followed by
CW NCCS
bytes of
CW c_cc
data.
The
CW pushtty
function uses
CW pcons
to arrange that the standard input, output, and error files
behave as ttys are supposed to according to POSIX.
(This should not have to be done explicitly, but the cost of supporting
CW tcgetattr
and
CW tcputattr
is sufficiently high that it seemed better to not make it the default.)
XX
CW _NET_EXTENSION .
This extension allows inclusion of
CW <libnet.h> ,
which defines the networking functions described in the Plan 9 manual page
I dial (2).
Programs using them must be loaded with
CW -lnet .
XX
CW _REGEXP_EXTENSION .
This extension allows inclusion of
CW <libregexp.h> ,
which defines the regular expression matching functions described
in the Plan 9 manual page
I regexp (2X).
Programs using them must be loaded with
CW -lregexp .
XX
CW _RESEARCH_SOURCE .
This extension enables a small library of functions from the Tenth Edition Unix
Research System (V10).
These functions and the types needed to use them are all defined in the
CW <libv.h>
header, and programs using them must be loaded with
CW -lv .
The provided functions are:
CW popen
and
CW pclose
(common extensions to ANSI C stdio; the
CW <stdio.h>
header must have been included);
CW telldir
and
CW seekdir
(common extensions to the ANSI C
CW readdir
suite;
CW <dirent.h>
must have been included);
CW lstat ,
CW symlink ,
and
CW readlink
(dummy functions for Plan 9, since there are no symbolic links);
CW srand ,
CW rand ,
CW nrand ,
CW lrand ,
and
CW frand
(better random number generators);
CW getopt
(program argument handler);
CW getpass ,
CW tty_echoon ,
CW tty_echooff
(for dealing with the common needs for mucking with terminal
characteristics);
CW min
and
CW max ;
CW nap ;
CW setfields ,
CW getfields ,
and
CW getmfields
(for parsing a line into fields);
and
CW ftw
(for walking a file tree).
See the Research Unix Programmer's Manual for a description
of these functions.
SH
Common Problems
PP
Some large systems, including X11, have been ported successfully
to Plan 9 using APE.
The problems encountered fall into three categories:
(1) non- ANSI C/POSIX features used; (2) inadequate simulation of POSIX functions;
and (3) compiler/loader bugs.
By far the majority of problems are in the first category.
PP
POSIX is just starting to be a target for programmers.
Most existing code is written to work with one or both of a BSD or a System V Unix.
System V is fairly close to POSIX, but there are some differences.
Also, many System V systems have imported some BSD features that are
not part of POSIX.
Here are some common non-POSIX things used in current programs, and what to
do about them.
XX
CW sys_errlist
and
CW sys_nerr
globals.  Use
CW strerror
instead.
XX
Third (environment) argument to
CW main .
Use the
CW environ
global instead.
XX
CW OPEN_MAX ,
CW PATH_MAX ,
etc., assumed in
CW <limits.h> .
Rewrite to call
CW sysconf
or define
CW _LIMITS_EXTENSION .
XX
CW isascii
assumed in
CW <ctype.h> .
Let the dead codeset go: leave the test out, or better still,
convert to use multibyte characters (c.f. runes in Plan 9).
Or define your own
CW isascii
macro.
XX
CW <strings.h> ,
CW index ,
and
CW rindex .
Use
CW <string.h> ,
CW strchr ,
and
CW strrchr
instead.
XX
CW mktemp .
Rewrite to use ANSI C
CW tempnam
or
CW tmpfile .
XX
CW <varargs.h> .
Rewrite to use
CW <stdarg.h> .
XX
CW popen
assumed in
CW <stdio.h> .
Use
CW _RESEARCH_SOURCE
extension, include
CW <libv.h> ,
and load with
CW -lv .
Similarly for
CW telldir ,
CW seekdir ,
CW lstat ,
CW srand ,
CW rand ,
CW nrand ,
CW lrand ,
CW frand ,
and
CW getopt .
XX
CW select .
Define
CW _BSDTIME_EXTENSION
and
CW _SELECT_EXTENSION
and include
CW <sys/times.h>
and
CW <select.h> .
XX
Socket or stream stuff.
Rewrite to use the Plan 9 networking functions, define
CW _NET_EXTENSION ,
include
CW <libnet.h>
and load with
CW -lnet .
PP
The second class of problems has to do with inadequacies in the Plan 9
simulation of POSIX functions.
As long as this list is, these shortcomings have rarely gotten in the way
(except, perhaps for the
CW link
problem).
XX
Functions for setting the userid, groupid, effective userid and effective groupid
do not do anything useful.  The concept is impossible to simulate in Plan 9.
CW Chown
also does nothing.
XX
CW execlp
and the related functions do not look at the
CW PATH
environment variable.  They just try the current directory and
CW /bin
if the pathname is not absolute.
XX
Advisory locking via
CW fcntl
is not implemented.
XX
CW isatty
is hard to do correctly.
The approximation used is to see if the file is the same as
CW /dev/cons .
Sometimes this cannot be determined, and sometimes this is not the right test.
XX
With
CW kill ,
the behavior mandated for negative arguments (killing process groups)
is not implemented.
XX
CW link
always fails.
XX
With
CW open ,
the
CW O_NOCTTY
option has no effect.
The concept of a ``controlling'' tty is foreign to Plan 9.
XX
CW setsid
does a
CW forkpgrp ,
which is only approximately the right behavior.
XX
The functions dealing with stacking signals,
CW sigpending ,
CW sigprocmask
and
CW sigsuspend ,
do not work.
XX
CW system
uses
CW rc
instead of
CW sh
to interpret the argument.
Use of
CW system
is discouraged for security reasons anyway.
XX
CW tcgetattr
and
CW tcsetattr
only work if
CW pcons
or
CW pushtty
has been executed
(see the discussion about the
CW _PCONS_EXTENSION ,
above ).
XX
CW umask
has no effect, as there is no such concept in Plan 9.
XX
CW utime
has no effect, because the time can't be changed in Plan 9.

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