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Diffstat (limited to 'doc/INSTALL')
| -rw-r--r-- | doc/INSTALL | 29 |
1 files changed, 29 insertions, 0 deletions
diff --git a/doc/INSTALL b/doc/INSTALL index f0b3e87b..a71e5179 100644 --- a/doc/INSTALL +++ b/doc/INSTALL @@ -109,6 +109,35 @@ If you use the 'configure' program, be sure to edit config.mk _after_ you run 'configure', since 'configure' generates config.mk. +COMPILED-IN BUILD DATETIME +-------------------------- + +By default, the Netpbm build system builds the datetime that you built it +into libnetpbm, so the --version global command line option can display it. +It's actually just when you began building from a clean build tree; if you +modify code and rebuild, the build datetime does not change. + +This is problematic for any of various reasons you might want to compare two +versions of libnetpbm, or anything of which it is part, for equality. Two +libnetpbms that are identical except that they were built at different times +would compare as different. + +Furthermore, as version information, the modification datetime of the source +code is often more useful than the build datetime. + +For these reasons, it is possible to change this timestamping behavior by +setting the environment variable 'SOURCE_DATA_EPOCH'. That is supposed to be +the POSIX datetime value (decimal number of seconds since 1969, excluding leap +seconds) for when the source code was last modified. When you set this, +before doing 'make' for the first time in a clean build tree, the resulting +libnetpbm contains that information and not build datetime information, and +the --version global option causes a different message. + +The name and meaning of the environment variable is taken from a standard +described at https://reproducible-builds.org/specs/source-date-epoch/ on March +16, 2017. + + AUTOMATING THE BUILD -------------------- |