Managing file interactions with make

Suggestions and warnings

The most common difficulties arise from make's specific meaning of dependency. If file x.c has a

   #include "defs.h"

line, then the object file x.o depends on defs.h; the source file x.c does not. If defs.h is changed, nothing is done to the file x.c while file x.o must be recreated.

To discover what make would do, the -n option is very useful. The command

   $ make -n

orders make to print out the commands that make would issue without actually taking the time to execute them. If a change to a file is absolutely certain to be mild in character (adding a comment to an include file, for example), the -t (touch) option can save a lot of time. Instead of issuing a large number of superfluous recompilations, make updates the modification times on the affected file. Thus, the command

   $ make -ts

(touch silently) causes the relevant files to appear up to date. Obvious care is necessary because this mode of operation subverts the intention of make and destroys all memory of the previous relationships.