Transcript Slide Show Title

Using POSIX to Maximize Code
Portability in Embedded Systems
Peter van der Veen
QNX Software Systems
POSIX®
/pahz-icks/

POSIX, pronounced pahz-icks as in positive, not poh-six, or
other variations

POSIX is a registered trademark of the IEEE
►
Licensed through certification

An acronym for Portable Operating System Interface

POSIX is a family of standards developed by the Portable
Applications Standards Committee (PASC) of the IEEE
Computer Society
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What Is POSIX?


It’s about portability
►
Both programmers and application source code
►
Portability of the OS kernel itself and/or application binary code are
not objectives
POSIX is a set of books specifying APIs
►
It is neither a piece of code
►
Nor an operating system
►
It is a rich, proven API
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What Is an API?

Application Program Interface
 Written contract between system developers and application
developers
 It is not a piece of code, it is a piece of paper defining what the
two sets of developers are guaranteed to receive and are in
turn responsible for providing
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The Need for Standard APIs
Standardized functionality

Standardized “square” peg in
the round hole
► “Bits where change is not
interesting”
► Where the benefits of
commonality outweigh the
value of differences
► Where we can achieve
economies of scale, including
interoperability
Private product specific functionality
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Scalable API Portability
POSIX APIs
support
portability
across a
range of
devices
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POSIX 1003.1, 2004 Edition

Approved December 6, 2001

2004 edition incorporates two Technical Corrigenda

Developed by the Austin Group (see later)

Supersedes all the major POSIX standards except 1003.13 (realtime
profiles) and 1003.5 (Ada bindings)

A combined system interfaces (including all realtime POSIX) and
utilities specification as a single 4000 page standard

The core of DoD’s mandated Joint Technical Architecture (JTA) OS
Services, replacing 1003.1-1996 and its amendments

Technically identical to the Base specifications of the Single UNIX
Specification and ISO 9945
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Definitions
Commands
Rationale
System Interfaces
The Common Base
Specifications
IEEE Std. 1003.1,
ISO/IEC 9945
The Open Group Base
Specifications Issue 6
The Core of the Single UNIX
Specification v3
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The Austin Group

The Austin Group combines the formal standards process of
the IEEE and ISO, with the industry standards of The Open
Group and the community at large

Electronic participation

Participation in the group is free
(www.opengroup.org/austin)

The final standard in HTML is available for free download
from the World Wide Web (www.unix.org/version3)
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POSIX 1003.13-2003

Approved December 10, 2003

A set of POSIX realtime profiles

Supersedes POSIX.13-1998 updated to address
►
1003.1-2001 (which includes all of realtime POSIX)
►
Profiles of all 1003.5-series standards (Ada bindings to POSIX)
►
Field experience with 1003.13-1998
►
Input from the Linux, realtime and/or embedded Linux, and
traditional RTOS communities
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POSIX 1003.13 Profiles
POSIX-RT
Profile 54
POSIX-RT
Profile 53
Profile 54: 1003.1-2003 Base Multiprocess, Threads, File System
Units of Functionality
Units of Functionality
Units of Functionality
Units of Functionality
-
POSIX-RT
Profile 52
Profile 53:
Multi-process, Threads; File System
Profile 52:
Single Process, Threads, File System
POSIX-RT
Profile 51
Profile 51:
Single Process, Threads; No File System
Portable Operating System Interface
IEEE Standard POSIX 1003.13
Allows Portability of Applications
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POSIX 1003.13

POSIX 1003.13 is a subprofile standard of 1003.1-2001
►
It allows diverse realtime operating systems “clothed” with a
runtime library to comply
►
This standardizes the application-to-RTOS API, allowing
considerable application code portability between different RTOS
offerings, which portability had not been possible in the past
►
RTOS+wrapper offerings can be compared and competed directly
►
There are currently four profiles
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Using POSIX in Embedded
Systems
Today’s Environment

Typical company has multiple product lines
and limited interoperability

Vendors are locked into a single OS solution or

Applications need to be recoded or ported to deploy on
different product lines
►
Takes time, adds costs
►
Increases delays to product deployment
Application
Application
Application
Product Line A
OS 1
Product Line B
OS 2
Product Line C
OS 3
Recoding
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Standards & Portability

Standard APIs preserve software investments

Portability lowers time, cost, and risk associated
with integrating new technology across product lines

Common standard maximizes application base for
development environments

Developers familiar with standard become productive
more quickly
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POSIX Compliance
OS Comparison
POSIX Feature Matrix
PSE 51
PSE 52
PSE 53
PSE 54
POSIX
1003.1-2001
1003.1-90 Processes
-
-



1003.1-90 Pipes
-
-



1003.1-90 Files & Directories
-

-


1003.1-90 Users & Groups
-
-
-


1003.1b-93 Memory Protection
-
-



1003.1b-93 Hi Res. Clocks &
Timers





1003.1b-93 Realtime Signals




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1003.1b-93 Semaphores

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
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1003.1b-93 Shared Memory


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1003.1b-93 IPC Message
Passing





Feature
Check for Advanced Realtime
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POSIX Feature Matrix
PSE 51
PSE 52
PSE 53
PSE 54
POSIX
1003.1-2001
1003.1c-95 Threads





1003.1c-95 Thread
Safe Functions




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1003.1c-95 Thread
Attribute Stack Address

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
1003.1c-95 Thread
Attribute Stack Size

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
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
1003.1c-95 Thread
Process Shared
-
-

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1003.1c-95 Thread
Priority Scheduling





1003.1c-95 Thread
Priority Inheritance
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1003.1c-95 Thread
Priority Protection

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
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
POSIX2_SW_DEV
-
-
-


Feature
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POSIX Profiles – OS Compliance
POSIX Standard
QNX Neutrino
Linux
RTOS x
Specification Base
1003.1-2001
1003.1-1996*
PSE 51/PSE 52
Realtime (.1b)



Realtime Threads (.1c)


-
1003.1d-1999 Additional
Realtime

-
-

-
-
Configure,
GCC, perl, …
Configure,
GCC, perl, …
-
(Sporadic server scheduling,
execution timers, …)
1003.1j-200x Advanced
Realtime
(Barriers, spin-locks, …)
Best practices
(development)
* Newer versions of Linux kernel moving toward conformance with 2001 specification.
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Linux-POSIX (Base) Portability

Application portability between Linux and POSIX Base (2003)
can be easily accomplished
►

Both Linux and new POSIX specification share large POSIX feature
set
Linux developers can retain programming model
and existing APIs while porting applications to
POSIX, including embedded systems
Bottom line:

Porting applications between Linux and POSIX Base
implementations is relatively simple

Standard POSIX APIs are key
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POSIX Summary

Standard interface increases software portability for all
embedded systems

Some markets, such as military, moving toward using
POSIX as their base specification

OS compliance/conformance a matter of degree

►
Many RTOS offerings conform to PSE 51 or PSE 52, against the
1003.1-1996 specification
►
QNX Neutrino provides compliance with 1003.1-2004
►
Linux moving toward 2001.3-2004 with latest versions
Migration of legacy RTOS code to POSIX RTOS
increases software portability
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Examples
Migration, portability in the presence of unspecified
behavior, etc.
Unspecified/Implementation
Specific Behavior

Task at-hand requires knowledge of unspecified behavior or
of implementation-defined behavior
►
For best performance
► To enable feature that is difficult to achieve with mandated
interfaces

Codify
►
Best practices
► Information about different implementations

Conditional compilation
►
Appropriate macro definitions
► Appropriate configuration
 E.g. Autoconf
 May include “discovery”
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Application Porting Example –
Lynx Text Browser
Untar the source:
Go to the source directory.
Make a host directory:
Go to the host directory.
tar xz lynx2.8.4.tar.gz
cd lynx2-8-4
mkdir x86-pc-nto-qnx
cd x86-pc-nto-qnx
Make.
../configure
make
Install the browser:
make install
Configure the source:
Install.
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Unspecified Behavior – Directory
Scanning (disk usage – du)
while (entry = readdir(dirp)) {
struct stat stat;
char path[PATH_MAX+1];
strcpy(path, pathname);
strcat(path, entry->d_name);
stat(path, &stat);
if (S_ISDIR(stat.st_mode))
usage += du(path);
#ifdef HAVE_SYMLINKS
/* May be determined by Autoconf! */
else if (S_ISLNK) {
lstat(path, &stat);
if (S_ISDIR(stat.st_mode)) {
char linkpath[PATH_MAX+1];
int chars;
if ((chars = readlink(path, linkpath, PATH_MAX)) > 0) {
linkpath[chars] = '\0';
usage += du(linkpath);
}
}
#endif
}
}
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Summary

POSIX is a scalable standard with many different levels of
compliance and many optional feature sets

Program by contract

►
Define a profile(s) that matches your needs and stick to it
►
Select vendor implementations that match your profile
Follow best practices whenever possible
►
Avoid unspecified or undefined behavior
►
Code conditionally to abstract over differences
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Thank You!

Learn more about POSIX Certification:
http://posixcertified.ieee.org

Download the QNX white paper “Using POSIX in Embedded
Systems: http://www.qnx.com/info/posix_whitepaper

Get your free 30-day evaluation copy of QNX Momentics
development suite v6.3: http://www.qnx.com/products/eval/
Questions?
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