In what cases CLOCK_MONOTONIC might not be available
Is it reasonable to assume that CLOCK_MONOTONIC clock is available on all modern Linux servers?
Yes. It is reasonable to assume that.
From the wording of the gettime manual entry, we can infer that really old versions of glibc do not support CLOCK_MONOTONIC. (I am still trying to figure out how old ... but probably when glibc claimed POSIX 1003.1 compliance.)
CLOCK_MONOTONIC was specified (at least) in IEEE Std 1003.1, 2004 Edition, though it remains possible for a compliant libc implementation to not support CLOCK_MONOTONIC.
The Linux kernel source code has had support for a CLOCK_MONOTONIC clock at least since Linux 3.0 (2011).
From other sources, it also depends on how your system's glibc was built. (When built with "emulated timers", CLOCK_MONOTONIC is not supported.)
Here are some cases where it is not supported:
There can be problems with some old CPU chips; e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1499480
It is not supported for some (all?) versions of Cygwin.
It is not supported for some (all?) versions of uCLibc on ARM.
It is also possible for CLOCK_MONOTONIC to be supported but buggy:
- Linux clock_gettime(CLOCK_MONOTONIC) strange non-monotonic behavior
Is it reasonable to assume that CLOCK_MONOTONIC clock is available on all modern Linux servers?
I can only comment on this question. Yes, it's reasonable that all production-grade systems you use will have a monotonic clock that Linux knows how to access. This is also true of virtual and container servers.
Good engineering dictates you put a check in for this and error-out if an assumption is broken, but I would count on this at design-time.