RE: make -j and multiple targets

[Howard Chu]

> -----Original Message-----
> From: address@hidden [mailto:address@hidden Behalf Of
> Peter Gammie

> g'day,
>
> i'm using make 3.79.1.
>
> if i write a rule like so:

These rules don't accurately describe the dependencies that you're dealing
with.
If you analyzed your actual order of events, you wouldn't have this problem:

lexer.c: lexer.l
        flex -t lexer.l > lexer.c

lexer.h: lexer.c
        genHeader.pl lexer.l

As a side note, it seems odd to me that a script designed to generate a
header
file needs to append anything to the C source file. Surely anything that
needs
to be defined should have gone into the header file, and #include'd by the C
file.

> ----Makefile----
> ... overall target that depends on lexer.o ...
> ... rules that depend on lexer.h ....
>
> lexer.c lexer.h: lexer.l
>       flex -t lexer.l > lexer.c
>       genHeader.pl lexer.l # generates lexer.h and appends stuff
> to lexer.c
>
> lexer.o: lexer.c # and some other stuff
> ----
>
> and run make like so:
>
> % make -j2
>
> i get the rule for lexer.c/lexer.h firing twice, which makes the
> compilation bomb sometimes due to the two appends (race).
>
> now, this is probably well understood behaviour, but could you at least
> add a caveat to the info page? (specifically in "parallel"). i'd prefer
> the semantics to be something like the following:
>
> "apply the rule once, taking the timestamp of the lhs to be the oldest of
> the n lhs files, and of the rhs to be the youngest of the m source files."
>
> rather than firing the rule several times.
>
> being completely naive about build systems, however, i'm sure someone will
> point out why this is a bad idea.
>
> cheers
> peter

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc