Linux make: Need to rebuild a text file when makefile changes

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I've inherited a Linux C++ app and makefile. In the existing makefile, it had code such as the following:

APPVER=5.01
REL=B

$(APP): $(OBJS) $(MAKEFILE)
          echo $APPVER > .sw_ver.txt
          echo " " >> .sw_ver.txt
          echo $REL >> .sw_ver.txt
          $(LD) $(OBJS) $(LDFLAGS) -o $(APP)

At runtime, the app would open this text file and display its version.

In the spirit of Make, rebuilding that .sw_ver.txt every time file just felt wrong. So I broke it up like the following:

APPVER=5.01
REL=B

$(APP): $(OBJS) $(MAKEFILE) .sw_ver.txt
          $(LD) $(OBJS) $(LDFLAGS) -o $(APP)

.sw_ver.txt: $(MAKEFILE)
          echo $APPVER > .sw_ver.txt
          echo " " >> .sw_ver.txt
          echo $REL >> .sw_ver.txt

Steps:

  1. make -B: it performs the steps, creating the text file and updating the app.
  2. do some update the Makefile
  3. make: here it says there's no need to update the text file and prunes the rule.

I expected it to see the Makefile has changed, and therefore go update the text file. But it doesn't.

I tried adding a dummy file, but it says seems to get pruned.

What I'm trying to do is if only if the Makefile has changed (thinking that perhaps APPVER or REL have changed), then rebuild the .sw_ver.txt file.

Do you have any advice?

thanks

===== edit #1

hi, so I tried an alternative, to move the version info out to a file instead of trying to create the file during a make run. Since I inherited this stuff, I was trying to change as little as possible.

Just for trivia's sake, it would be interesting to know if there is an answer to the original question.

thanks again

file .sw_ver.txt

5.01 B

Makefile:

$(APP): $(OBJS) $(MAKEFILE) .sw_ver.txt
          $(LD) $(OBJS) $(LDFLAGS) -o $(APP)

.sw_ver.txt: $(MAKEFILE)

===== edit #2

I spoke too soon. This setup works if any of the OBJS are changed, but it does not work if Makefile itself is changed.

So, kind strangers... any thoughts? Thanks

makefile linux c++
1 Answers

Two things to clarify:

  1. MAKEFILE is not an implicit built-in variable. So if you don't set its value it contains nothing. (You can run make -p in a directory without a Makefile to see what variables are there. See https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html)
  2. Based on the reason above, since your $(MAKEFILE) is empty, when it expands, the .sw_ver.txt becomes a rule without prerequisites. Hence if .sw_ver.txt already exists, nothing will be done to regenerate it. What you get is actually this (note that your syntax $APPVER and $REL are actually wrong as well): 
    .sw_ver.txt:                      # $(MAKEFILE) is empty
      echo PPVER > .sw_ver.txt    # $(A) is empty
      echo " " >> .sw_ver.txt
      echo EL >> .sw_ver.txt      # $(R) is empty

So what you need to do is:

  1. Give your MAKEFILE variable a value. 
    MAKEFILE := Makefile
  2. Fix your syntax: 
    .sw_ver.txt: $(MAKEFILE)
    echo $(APPVER) > $@
    echo " " >> $@
    echo $(REL) >> $@

After you do this you should be able to regenerate .sw_ver.txt whenever you update your Makefile.

user_76869170
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