IDL is an interactive program that runs across numerous operating systems and hardware platforms. The IDL user enjoys a large amount of portability across these platforms because IDL provides access to system abilities at a relatively high level of abstraction.

There are reasons to combine external code with IDL:

• Many sites have an existing investment in other code that they would prefer to use from IDL rather than incurring the cost of rewriting it in the IDL language.
• It is often best to use the reference implementation of a software package rather than re-implement it in another language, risk adding incorrect behaviors to it, and incur the ongoing maintenance costs of supporting it.
• IDL may be largely suitable for a given task, requiring only the addition of an operation that cannot be performed efficiently in the IDL language.

A programmer who is considering adding compiled code to IDL should understand the following caveats:

• We attempt to keep the interfaces described in this document stable, and we endeavor to minimize gratuitous change. However, we reserve the right to make any changes required by the future evolution of the system. Code linked with IDL is more likely to require updates and changes to work with new releases of IDL than programs written in the IDL language.
• The act of linking compiled code to IDL is inherently less portable than use of IDL at the user level.
• Troubleshooting and debugging such applications can be very difficult. With standard IDL, malfunctions in the program are can be reported and we attempt to fix them promptly. A program that combines IDL with other code makes it difficult to unambiguously determine where the problem lies. The level of support we can provide in such troubleshooting is minimal. The programmer is responsible for locating the source of the difficulty. If the problem is in IDL, a simple program demonstrating the problem must be provided to our Technical Support Group before we can address the issue.