Systemtap attempts to resolve references to global symbols (probes,
functions, variables) that are not defined within the script by a
systematic search through the tapset library for scripts that define
those symbols. Tapset scripts are installed under the default
directory named /usr/share/systemtap/tapset. A user may give
additional directories with the -I DIR option. Systemtap
searches these directories for script (.stp) files.
The search process includes subdirectories that are specialized for a particular kernel version and/or architecture, and ones that name only larger kernel families. Naturally, the search is ordered from specific to general, as shown in Figure 7.
When a script file is found that defines one of the undefined symbols, that entire file is added to the probing session being analyzed. This search is repeated until no more references can become satisfied. Systemtap signals an error if any are still unresolved.
This mechanism enables several programming idioms. First, it allows
some global symbols to be defined only for applicable kernel
version/architecture pairs, and cause an error if their use is
attempted on an inapplicable host. Similarly, the same symbol can be
defined differently depending on kernels, in much the same way that
different kernel include/asm/ARCH/ files contain macros that
provide a porting layer.
Another use is to separate the default parameters of a tapset routine from its implementation. For example, consider a tapset that defines code for relating elapsed time intervals to process scheduling activities. The data collection code can be generic with respect to which time unit (jiffies, wall-clock seconds, cycle counts) it can use. It should have a default, but should not require additional run-time checks to let a user choose another. Figure 8 shows a way.
A tapset that exports only data may be as useful as ones that exports functions or probe point aliases (see below). Such global data can be computed and kept up-to-date using probes internal to the tapset. Any outside reference to the global variable would incidentally activate all the required probes.
