5.8. Conditional compilation

One of the steps of parsing is a simple conditional preprocessing stage. The general form of this is similar to the ternary operator (Section Section 5.6.7, “Ternary operator”).

%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)

The CONDITION is a limited expression whose format is determined by its first keyword. The following is the general syntax.

%( <condition> %? <code> [ %: <code> ] %)

If the first part of a conditional expression is the identifier kernel_v or kernel_vr, the second part must be one of six standard numeric comparison operators “<”, “<=”, “==”, “!=”, “>”, or “>=”, and the third part must be a string literal that contains an RPM-style version-release value. The condition returns true if the version of the target kernel (as optionally overridden by the -r option) matches the given version string. The comparison is performed by the glibc function strverscmp.

kernel_v refers to the kernel version number only, such as “2.6.13".

kernel_vr refers to the kernel version number including the release code suffix, such as “2.6.13-1.322FC3smp”.

If the first part of the conditional expression is the identifier arch which refers to the processor architecture, then the second part is a string comparison operator ”==” or ”!=”, and the third part is a string literal for matching it. This comparison is a simple string equality or inequality. The currently supported architecture strings are i386, i686, x86_64, ia64, s390x and ppc64.

TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens, possibly including nested preprocessor conditionals, that are pasted into the input stream if the condition is true or false. For example, the following code induces a parse error unless the target kernel version is newer than 2.6.5.

%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence

The following code adapts to hypothetical kernel version drift.

probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13-1.8273FC3smp" %? "do_page_fault" %: UNSUPPORTED %)
%)) { /* ... */ }

%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)