Porting from IRIX to Linux
Linux is a superset of the POSIX (Portable Operating System Interface for X) specification. It runs on commodity PC hardware, MIPS systems, Suns, Macintoshes and even IBM mainframes. It's sufficiently UNIX-like that much UNIX code can be ported over with little change.
The Advanced Concepts Research Tool (ACRT) is a descendant of the ARPA Reconfigurable Simulator Initiative (ARSI). It's primarily IRIX but has NT versions for many of its modules. ACRT is a vehicle simulator that can be reconfigured into different vehicles. Raytheon supplies the land vehicle simulators, which currently are the M1A1 and M1A2 tanks, M2 armored personnel carrier, M113, M577, HMMWV and the Future Scout. Reconfigurability is done by using “Erector Set”® type hardware, soft panels for controls, serial line inputs for grips and a common set of core software.
The Erector Set hardware looks like a scaled-up version of its namesake. This allows the crew seats, controls and monitors that can be placed in a variety of positions to emulate the vehicle interiors.
We use the term soft panels to refer to touch-screen activated control panels that are used in place of the real buttons and switches in the actual vehicles. These are designed to place the controls as close as possible to the real vehicle. This minimizes the amount of hardware that is vehicle-specific (currently only grips).
Grips are either actual crew station hardware from the vehicle or copies with simplified interior wiring. These provide switches and analog outputs used to control some vehicle functions. The outputs from these are converted to voltages in the range of zero to five volts DC and fed through a BG Systems Cereal Box. The Cereal Box acts as an analog to the digital converter and feeds values through serial lines to the host computer. Finally, the core set of software used by all vehicles makes it easier to develop a new vehicle or modify existing vehicle-specific modules.
At startup, the system runs different software modules on different hosts. These different modules include things like ground motion (how does the vehicle move), weapons models (how does a weapon behave, where does the round go) and control panels for the crew to interact with. The system is kicked-off from a single host and then starts up process managers (host_mgr) on other hosts through the UNIX rsh command. These host_mgr processes then parse through configuration files to see what runs on their local host.
The system uses message passing to move information between the different modules. Between modules on the same host, the messages are buffered in a memory-mapped file as a form of shared memory. Between hosts, UDP sockets are used to pass information. A gateway system translates between Defense Interactive Simulation (DIS) messages and the internal vehicle messages. This allows the simulator to interact with other simulators using the DIS protocol (see Figure 1).
A) Demonstrate an image generator using Performer Linux (Mongoose)
What started this project was SGI's release of its Performer visualization/simulation tool for Linux (this version is called “Mongoose”). Since our simulator's image generator (the part of the system that draws the out-the-window and sensor views) is based on Performer, the largest part of the work for a Linux port was done for us. What remained was to port the code that used Performer, the libraries and tools common to all vehicles (known as the Tiger core), and enough of the vehicle so that it can interact with its environment (to move, see other vehicles and utilize special effects like weapons fire and detonation).
B) Dual-compile whenever possible to shrink codebase
To make it easier to maintain, we decided to maximize the amount of code that would compile under either IRIX or Linux. This shrinks the codebase to a more manageable size. The theory is that each module would have a directory containing source code and under it would be subdirectories for architecture-specific items (compiled object files, libraries and executables). A Makefile in with the source files uses the OSNAME environment variable to bounce any make commands to the correct subdirectory depending on the architecture of the host (see Figure 2).
C) Arrange files so that machines of different architectures use the same directories
To keep the configurations as similar as possible across platforms, the files are arranged so that machines of different architectures use the same directories. For distinctions that can be made at compile time, there are different directories for compiled libraries with which modules link and where the executables are kept. Using environment variables for the BIN and LIB directories that incorporate the architecture name can help with that. For instance, on the IRIX systems, the binary directory (stored in $ARSI_BIN) might be /apps/projects/ACRT/bin/IRIX64. On Linux systems, it might be /apps/projects/ACRT/bin/Linux. Since the path is stored in the same environment variable, scripts and Makefiles can use it without regard for which type of system they are running (see Figure 3). Environment variables are also available at runtime to allow code to make decisions that can't be made at compile time. By embedding environment variables in configuration files, we can use the same configuration file for different architectures.
Portability of source files was an issue for the include files and modules that dual-compiled IRIX/NT. For IRIX to use the files, they had to be in UNIX format (the CR/LF issue) instead of DOS format. Fortunately, MS Studio accepts files in UNIX format, so we settled on that. Adding Linux didn't change this equation since Linux handles both DOS and UNIX formats. Note, however, that shell script files must be in UNIX format.
D) Overcome incompatibilities
We first tried to use POSIX functionality. When that was not applicable, we opted for the greatest common denominator.
E) Simplify maintenance
To keep maintenance simpler, we tried to avoid convoluted #ifdef-ed regions of code, be careful about variables used and set in each branch of code and verify that the flow of control was not drastically different between branches. When possible, #ifdef-ed code was moved to a separate file to minimize the scope of its effect on variables.
Finally, different compilers are sensitive to different things. A condition that goes unremarked on one compiler may cause a warning on another. Investigate and understand your compiler's warnings. They may lead you to an error (in which case it's easier to fix at compile time than to track down at runtime). Warnings may show you where intent differed from implementation. Warnings can also hide errors. Picking out the serious message in the midst of a page of warnings about unused variables is harder than taking out the unused variables.