Conditional compilation based on functionality in Linux kernel headers

Question

Consider the case where I'm using some functionality from the Linux headers exported to user space, such as perf_event_open from <linux/perf_event.h>.

The functionality offered by this API has changed over time, as members have been added to the perf_event_attr, such as perf_event_attr.cap_user_time.

How can I write source that compiles and uses these new functionalities if they are available locally, but falls back gracefully if they aren't and doesn't use them?

In particular, how can I detect in the pre-processor whether this stuff is available?

I've used this perf_event_attr as an example, but my question is a general one because structure members, new structures, definitions and functions are added all the time.

Note that here I'm only considering the case where a process is compiled on the same system that it will run on: if you want to compile on one host and run on another you need a different set of tricks.

Answer 1

Use the macros from /usr/include/linux/version.h:

#include <linux/version.h>

int main() {
#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,16)
                                      // ^^^^^^ change for the proper version when `perf_event_attr.cap_user_time` was introduced
   // use old interface
#else
   // use new interface
   // use  perf_event_attr.cap_user_time
#endif
}