Solaris Dynamic Tracing Guide

Stable Translations

While a translator provides the ability to convert information into a stable data structure, it does not necessarily resolve all stability issues that can arise in translating data. For example, if the input expression for an xlate operation itself references Unstable data, the resulting D program is also Unstable because program stability is always computed as the minimum stability of the accumulated D program statements and expressions. Therefore, it is sometimes necessary to define a specific stable input expression for a translator in order to permit stable programs to be constructed. The D inline mechanism can be used to facilitate such stable translations.

The DTrace procfs.d library provides the curlwpsinfo and curpsinfo variables described earlier as stable translations. For example, the curlwpsinfo variable is actually an inline declared as follows:

inline lwpsinfo_t *curlwpsinfo = xlate <lwpsinfo_t *> (curthread);
#pragma D attributes Stable/Stable/Common curlwpsinfo

The curlwpsinfo variable is defined as an inlined translation from the curthread variable, a pointer to the kernel's Private data structure representing a thread, to the Stable lwpsinfo_t type. The D compiler processes this library file and caches the inline declaration, making curlwpsinfo appear as any other D variable. The #pragma statement following the declaration is used to explicitly reset the attributes of the curlwpsinfo identifier to Stable/Stable/Common, masking the reference to curthread in the inlined expression. This combination of D features permits D programmers to use curthread as the source of a translation in a safe fashion that can be updated by Sun coincident to corresponding changes in the Solaris implementation.