Question

Here’s some pseudocode for what I want my makefile to do:

if (A doesn't exist) or (B is newer than A):
    rm -rf A
    create an empty A
parallel_for X in (a large set of files):
    if (X is newer than A):
        update A using the contents of X

In the above pseudocode, A is an SQLite database, B is a C header file, and each of the files in the “large set of files” is a C source file.

Basically, if I only modify one of the C source files, I just want the database to be quickly updated rather than rebuilding the entire database from scratch.

Is this type of problem solvable directly in GNU make, or am I going to have to resort to using a script?

Thanks in advance!

Answer 1

Something like this ought to work:

A:: B
        -rm -f $@
        create_A $@

A:: $(all_X)
        update_A_from_Xes $@ $?

$? expands to the subset of $(all_X) that are newer than A (see the “Automatic Variables section of the GNU Make manual for more details). Therefore, update_A_from_Xes must update its first argument with respect to all of the subsequent arguments; it will only be invoked once.

The double colons tell Make that the commands to run to update A are different when it’s out of date with respect to B than when it’s out of date with respect to the Xes. I am not sure whether both sets of commands will get run in the case that it is out of date with respect to both; if they do both get run, the A:: B rules will get run first.