What is the reason for deprecating/obsoleting certain Linux time APIs?


Linux has a number of APIs that can be used to set system time: we can use time(), gettimeofday(),clock_gettime() to get the time, and we can usestime(), settimeofday(),clock_settime() to set the time.

However, I noticed that some of these APIs are marked as "deprecated" or "obsolete". I couldn’t find the reason behind this deprecation. Specifically:

Why is stime() deprecated (source) but not time()? gettimeofday() offers higher resolution than time(), but the former rather than the latter was declared "obsolete" (source).

What are the reasons for these decisions?


These APIs have been declared as "obsolete" for the usual reasons: they are old, not secure, not portable, have unspecified/undefined behavior, have tricky edge cases, better alternatives exist, etc.

Deprecation of APIs in Linux is generally done on a case-by-case basis; there is no general rule.

Why is stime() deprecated…

It’s a non-standard function from SVr4 that nobody uses (on Linux). You should use clock_settime instead in every case.

…but not time()?

time() is a super standard function available everywhere, as it’s the part of the C language standard. It will be used until the end of… time!

gettimeofday() offers higher resolution than time(), but the former rather than the latter was declared "obsolete"

gettimeofday() is a POSIX function, and POSIX standardized existing practices. After some time (POSIX Issue 7) clock_gettime() was invented with greater resolution and to be more portable. Because a better and safer interface exists, POSIX noted that gettimeofday() is obsolesced. I think the note is pretty self-explanatory to a developer:


Applications should use the clock_gettime() function instead of the obsolescent gettimeofday() function.

In general, to find out why a particular API was deprecated, research about particular project history. Source code repositories offer commit history, standards, like POSIX or the C standard, come with "rationale", a separate document or included with the document, that explain the decisions.

For example, you can find out about stime() deprecation in glibc from this commit:

* The obsolete function stime is no longer available to newly linked
  binaries and it has been removed from <time.h> header.  This function
  has been deprecated in favor of clock_settime.

Answered By – KamilCuk

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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