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2.2.12 Automatic Dependency Tracking
Dependency tracking is performed as a side-effect of compilation.
Each time the build system compiles a source file, it computes its
list of dependencies (in C these are the header files included by the
source being compiled). Later, any time make is run and a
dependency appears to have changed, the dependent files will be
rebuilt.
Automake generates code for automatic dependency tracking by default, unless the developer chooses to override it; for more information, see section Automatic dependency tracking.
When configure is executed, you can see it probing each
compiler for the dependency mechanism it supports (several mechanisms
can be used):
~/amhello-1.0 % ./configure --prefix /usr … checking dependency style of gcc... gcc3 …
Because dependencies are only computed as a side-effect of the
compilation, no dependency information exists the first time a package
is built. This is OK because all the files need to be built anyway:
make does not have to decide which files need to be rebuilt.
In fact, dependency tracking is completely useless for one-time builds
and there is a configure option to disable this:
Some compilers do not offer any practical way to derive the list of
dependencies as a side-effect of the compilation, requiring a separate
run (maybe of another tool) to compute these dependencies. The
performance penalty implied by these methods is important enough to
disable them by default. The option ‘--enable-dependency-tracking’
must be passed to configure to activate them.
See section Dependency Tracking in Automake, for some discussion about the different dependency tracking schemes used by Automake over the years.
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