1. Using POSIX to Maximize Code Portability in Embedded Systems
Peter van der Veen
QNX Software Systems

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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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5. 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
There are substantial benefits from having standardized products , look at the
Size of the market for commodity products such as video recorder, dvd players, these have all benefited from standards
Private product specific functionality
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6. Scalable API Portability
POSIX APIs support portability across a range of devices
Today the internet is the world’s largest embedded system
Connectivity is key
Access your data from anywhere anytime
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POSIX , 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 (realtime profiles) and (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 and its amendments
Technically identical to the Base specifications of the Single UNIX Specification and ISO 9945
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8. The Common Base Specifications
Rationale
System Interfaces
Commands
Definitions
IEEE Std ,
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 (
The final standard in HTML is available for free download from the World Wide Web (
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POSIX
Approved December 10, 2003
A set of POSIX realtime profiles
Supersedes POSIX updated to address
(which includes all of realtime POSIX)
Profiles of all series standards (Ada bindings to POSIX)
Field experience with
Input from the Linux, realtime and/or embedded Linux, and traditional RTOS communities
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POSIX Profiles
POSIX-RT
Profile 54
Units of Functionality -
POSIX-RT
Profile 53
Units of Functionality -
Profile 54: Base Multi-process, Threads, File System
Profile 53: Multi-process, Threads; File System
POSIX-RT
Profile 52
Units of Functionality -
POSIX-RT Profile 51
Units of Functionality -
Profile 52: Single Process, Threads, File System
Profile 51: Single Process, Threads; No File System
Allows Portability of Applications
Portable Operating System Interface
IEEE Standard POSIX
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POSIX
POSIX is a subprofile standard of
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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13. Using POSIX in Embedded Systems

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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
Product Line C
OS 3
Application
Product Line A
OS 1
Product Line B
OS 2
Recoding
In today’s software environment, companies typically have multiple product lines with limited code portability. Proprietary or non-standard APIs make it necessary to use one OS solution across product lines – even if the capabilities of the OS don’t meet the requirements of the system. Functionality may need to be cut back to meet deadlines and cost limitations.
The alternative is to spend time recoding applications for use on next-generation platforms or different product lines – which takes time and money.
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15. 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
Using an RTOS that conforms to standard APIs can help to preserve software investments across product lines, which in turn can lower the time, cost, and risk of integrating new technology.
A common standard maximizes the use of your application code.
And finally, developers familiar with the standard can become productive immediately – removing the learning curve associated with any new OS platform.
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16. POSIX Compliance
OS Comparison

17. Check for Advanced Realtime
POSIX Feature Matrix  -
1003.1b-93 Memory Protection
1003.1b-93 IPC Message Passing
1003.1b-93 Shared Memory
1003.1b-93 Semaphores
1003.1b-93 Realtime Signals
1003.1b-93 Hi Res. Clocks & Timers
Users & Groups
Files & Directories
Pipes
Processes
POSIX
PSE 54
PSE 53
PSE 52
PSE 51
Feature
Steve to provide notes
Check for Advanced Realtime
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POSIX Feature Matrix  -
POSIX2_SW_DEV
1003.1c-95 Thread Priority Protection
1003.1c-95 Thread Priority Inheritance
1003.1c-95 Thread Priority Scheduling
1003.1c-95 Thread Process Shared
1003.1c-95 Thread Attribute Stack Size
1003.1c-95 Thread Attribute Stack Address
1003.1c-95 Thread Safe Functions
1003.1c-95 Threads
POSIX
PSE 54
PSE 53
PSE 52
PSE 51
Feature
Steve to provide notes.
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19. POSIX Profiles – OS Compliance-
Configure, GCC, perl, …
Best practices (development)
PSE 51/PSE 52 *
Specification Base 
1003.1j-200x Advanced Realtime
(Barriers, spin-locks, …)
1003.1d-1999 Additional Realtime
(Sporadic server scheduling, execution timers, …)
Realtime Threads (.1c)
Realtime (.1b)
RTOS x
Linux
QNX Neutrino
POSIX Standard
* Newer versions of Linux kernel moving toward conformance with 2001 specification.
Steve to provide notes.
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20. 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
Because of the shared POSIX feature set, it’s fairly easy to port applications between Linux and QNX Neutrino.
Linux developers benefit in that they can retain the programming model of their existing Linux APIs, thereby reusing existing development work. Companies can then make their OS choice based on the needs of their system – realtime performance, high reliability, scalability, hardware support, etc.
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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 specification
QNX Neutrino provides compliance with
Linux moving toward with latest versions
Migration of legacy RTOS code to POSIX RTOS increases software portability
Steve to provide notes.
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22. Migration, portability in the presence of unspecified behavior, etc.
Examples
Migration, portability in the presence of unspecified
behavior, etc.

23. 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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24. Application Porting Example – Lynx Text Browser
Untar the source: Go to the source directory.
tar xz lynx2.8.4.tar.gz cd lynx2-8-4
Make a host directory: Go to the host directory. 
mkdir x86-pc-nto-qnx cd x86-pc-nto-qnx
Configure the source: Make.
../configure
make
Install the browser: Install.
make install
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25. 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:
Download the QNX white paper “Using POSIX in Embedded Systems:
Get your free 30-day evaluation copy of QNX Momentics development suite v6.3:
Questions?
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