Porting SGI Audio Applications to Linux

Most, if not all, SGI audio applications make extensive use of the excellent audio and audiofile libraries supplied with IRIX. The audiofile library provides an API primarily designed to allow effortless loading and saving of AIFF audio files. The audio library is designed to allow straightforward audio input and output as well as control global audio settings. In order to make porting as painless as possible, replacement libraries had to be written for the Linux operating system.

The audiofile library was tackled first. Since this library simply has to perform file I/O based on the calls made, it was relatively straightforward to set up the necessary AIFF structures and to initialize, load and save these structures as necessary. However, because samples are read from and written to disk one sample frame at a time, this straightforward port of the audiofile library is relatively slow. In addition, only AIFF files are supported—compressed AIFF-C and WAV format files are not.

The audio library was a more demanding port, requiring extensive investigation into the facilities provided by the Open Sound System (OSS/Free) driver which is, by default, compiled into the Linux kernel. The basic process when using OSS involves opening /dev/dsp and either writing sample data directly to the device or reading from the device. In addition, opening /dev/mixer allows control over the global audio settings.

The Linux conversion basically sets up internal sample queues and provides facilities to transfer these sample queues to and from /dev/dsp. Most complications which arose were due to the many different sample formats (resolution and number of channels) supported by both the audio library and the OSS driver, thus requiring many different data conversions on input and output.

The resulting audio library on Linux is very much a “brute force” conversion and differs significantly from the SGI-based library, despite the almost identical API. The main difference is that the Linux audio library is not threaded whereas the SGI-based library constantly inputs and outputs sample frames from a cyclic queue in the background. As a result, the API user needs to be aware that samples must be constantly written to or read from the device to avoid audio glitches. Also, when finishing sample playback, blank samples must be written to the device to force the remaining sample queue to play before closing the device. The other main difference is that only one audio “port” may be open at any given time, due to the exclusive nature of opening /dev/dsp.

The default SGI compiler is quite different from gcc, which is the most commonly used compiler on Linux. More specifically, the SGI compiler is “relaxed” and not nearly as strict as gcc. This manifests itself in several ways, which must be considered when porting software from one platform to the other.

The most obvious difference is that explicit casting is often required on gcc to avoid warnings and errors which do not occur when using the SGI compiler. Two examples are shown here.

Default SGI compiler accepts:

int x = 3.2;
int *y = calloc (10,sizeof (int));

Linux gcc requires:

int x = (int) 3.2;
int *y = (int *) calloc (10,sizeof (int));

Another major difference between the SGI default compiler and gcc arises when combining C and C++ files where the C files cannot, for syntactic reasons or otherwise, be passed through the C++ compiler. When using the default SGI compiler, the command for compiling both C and C++ files is CC, so there is no need to explicitly specify linkage specification using the extern C construct. When using the gcc development environment, the command to compile C files is gcc and the command to compile C++ files is g++; thus, the linkage specification must be specified when referring to C-based functions within C++ files, or else linking will fail due to C++ name mangling.

One final major difference between SGI and Linux development environments is that of variable argument lists for Xt and Motif function calls. The Xt toolkit provides the developer with basic GUI constructs which may be used directly to create a user interface. In addition, Motif and LessTif use the Xt toolkit to provide a higher-level API for constructing user interfaces.

Within these toolkits are functions which have a variable number of arguments, much like the standard printf system call. Unlike printf, these functions require a null entry to terminate the argument list. However, in the SGI development environment, these null entries are optional and SGI developers frequently forget to terminate the argument list with such an entry. This does not cause a problem on SGI-based systems, but if this same code is then compiled in a Linux environment, the resulting executable will almost certainly core dump upon execution. The fix is simply to add the missing null entries to the relevant calls.

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