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Bootstrap Contexts

In Mac OS X kernel programming, the term context has several meanings that appear similar on the surface, but differ subtly.

First, the term context can refer to a BSD process or Mach task. Switching from one process to another is often referred to as a context switch.

Second, context can refer to the part of the operating system in which your code resides. Examples of this include thread contexts, the interrupt context, the kernel context, an application’s context, a Carbon File Manager context, and so on. Even for this use of the term, the exact meaning depends, ironically, on the context in which the term is used.

Finally, context can refer to a bootstrap context. In Mach, the bootstrap task is assigned responsibility for looking up requests for Mach ports. As part of this effort, each Mach task is registered in one of two groups—either in the startup context or a user’s login context. (In theory, Mach can support any number of independent contexts, however the use of additional contexts is beyond the scope of this document.)

For the purposes of this chapter, the term context refers to a bootstrap context.

When Mac OS X first boots, there is only the top-level context, which is generally referred to as the startup context. All other contexts are subsets of this context. Basic system services that rely on Mach ports must be started in this context in order to work properly.

When a user logs in, the bootstrap task creates a new context called the login context. Programs run by the user are started in the login context. This allows the user to run a program that provides an alternate port lookup mechanism if desired, causing that user’s tasks to get a different port when the tasks look up a basic service. This has the effect of replacing that service with a user-defined version in a way that changes what the user’s tasks see, but does not affect any of the rest of the system.

To avoid wasting memory, currently the login context is destroyed when the user logs out (or shortly thereafter). This behavior may change in the future, however. In the current implementation, programs started by the user will no longer be able to look up Mach ports after logout. If a program does not need to do any port lookup, it will not be affected. Other programs will terminate, hang, or behave erratically.

For example, in Mac OS 10.1 and earlier, sshd continues to function when started from a user context. However, since it is unable to communicate with lookupd or netinfo, it stops accepting passwords. This is not a particularly useful behavior.

Other programs such as esound, however, continue to work correctly after logout when started from a user context. Other programs behave correctly in their default configuration but fail in other configurations—for example, when authentication support is enabled.

There are no hard and fast rules for which programs will continue to operate after their bootstrap context is destroyed. Only thorough testing can tell you whether any given program will misbehave if started from a user context, since even programs that do not appear to directly use Mach communication may still do so indirectly.

In Mac OS X 10.2, a great deal of effort has gone into making sure that programs that use only standard BSD services and functions do not use Mach lookups in a way that would fail if started from a user context. If you find an application that breaks when started from a Terminal.app window, please file a bug report.

Contents:

How Contexts Affect Users
How Contexts Affect Developers

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