File: autoconf.info,  Node: Timestamps and Make,  Prev: Single Suffix Rules,  Up: Portable Make

12.20 Timestamp Resolution and Make
===================================

Traditionally, file timestamps had 1-second resolution, and ‘make’ used
those timestamps to determine whether one file was newer than the other.
However, many modern file systems have timestamps with 1-nanosecond
resolution.  Some ‘make’ implementations look at the entire timestamp;
others ignore the fractional part, which can lead to incorrect results.
Normally this is not a problem, but in some extreme cases you may need
to use tricks like ‘sleep 1’ to work around timestamp truncation bugs.

   Traditionally, commands like ‘cp -p’ and ‘touch -r’ did not copy file
timestamps to their full resolutions (*note Limitations of Usual Tools:
touch.).  Hence you should be wary of rules like this:

     dest: src
             cp -p src dest

   as ‘dest’ often appears to be older than ‘src’ after the timestamp is
truncated, and this can cause ‘make’ to do needless rework the next time
it is invoked.  To work around this problem, you can use a timestamp
file, e.g.:

     dest-stamp: src
             cp -p src dest
             echo >dest-stamp

   Apart from timestamp resolution, there are also differences in
handling equal timestamps.  Although POSIX suggests that ‘make’ update a
target with the same timestamp as its newest prerequisite, only HP-UX
‘make’ is known to do so; GNU and other ‘make’ implementations do not
update, a behavior that POSIX also allows.

   The HP-UX behavior can cause spurious rebuilds and this in turn can
cause ‘make’ to fail if it tries to rebuild generated files in a
possibly read-only source tree with tools not present on the build
machine.  Conversely, if a source file is updated immediately after
running ‘make’ in a file system with low-resolution timestamps, the GNU
behavior can cause a later ‘make’ to neglect rebuilds and quietly
generate incorrect results.  To avoid both these problems, use GNU
‘make’ on a platform with high-resolution timestamps.