Question

What is the difference between the two options below? Wouldn’t $< expand to only one requisite (by definition “the first one”) in both cases?

Option 1:

libcounter.a(lexer.o): lexer.o
  $(AR) $(ARFLAGS) $@ $<

libcounter.a(counter.o): counter.o
  $(AR) $(ARFLAGS) $@ $<

Option 2:

libcounter.a: lexer.o
  $(AR) $(ARFLAGS) $@ $<

libcounter.a: counter.o
  $(AR) $(ARFLAGS) $@ $<

ARFLAGS holds rv in both cases.

As a reference, the examples are taken from the book Managing Projects with GNU Make.

Answer 1

In the first option, make knows enough to look inside libcounter.a at the lexer.o object to see if it needs to update the library with the new object. In the second option, the timestamp on the library itself is compared with the object, which may lead to missed updates of the library – e.g. imagine 1) lexer.o gets built, then 2) counter.o gets built, then 3) libcounter.a gets new lexer.o added; now at this point, the timestamp on libcounter.a is newer than counter.o, so make wrongly concludes it does not need to add the new counter.o to the library.