The unix utility make wants you to declaratively build a dependency tree, so that subsequently each node of this tree can be created using known rules.
This view always went against my intuition. I don't view my program as a file that depends on its output directory and on a bunch of .o files, which depend on .cpp files, which depend on headers. Instead, I view it as a bunch of source files which have to be compiled and linked into the the program, and the fact that sometimes some of them don't have to be recompiled is an optimization the compiler should be concerned about. In other words, I view making a program as an imperative task:
compile all sources into .obj files
link .obj files into the output file
So now that I'm porting my stuff to FreeBSD, I decided to re-create the imperative make I've been using under Windows. Instead of porting my selectivebuild.exe, I decided to use perl.
Final makefile:
.ifndef DEBUG
OUTDIR = Release
GCC_OPTS += -O3
.else
OUTDIR = Debug
GCC_OPTS += -g
.endif
GCC_OPTS += -Wno-multichar
INCL = -I/usr/include/c++/4.2 -I.. -I.
SOURCES = byte_io.cpp mt_alloc.cpp cstring.cpp ... lots more cpp files
default:
@../compile_sources.pl $(OUTDIR) $(SOURCES) $(INCL) -O"$(GCC_OPTS)"
@../link_sources.pl $(OUTDIR) core.lib $(SOURCES)
clean:
rm -r Debug Release
This can be invoked with "make" or "make DEBUG=". The obj files and the resulting .lib file go into the output directory, which is either Debug or Release.
So... how do we implement compile_sources.pl and link_sources.pl? Well, the concept is simple. For compile_sources, I run makedepend with option -f- (output to stdout), and parse the resulting dependencies. I filter down the original array of sources to a ilst of out-of-date sources. I then iterate over that array and call gcc with options passed via -O.
For link_sources, I compare the modification dates of .obj files to the output file. If one of the objects is newer than the output file, I link. Based on the extension of the output file, I either call "ar" (for .lib) or "gcc" (otherwise).
A function objfile_of($outdir,$source) is used to map source files to obj files.
First, the utility file, filtercpp.pl:
#!/usr/bin/perl
use File::Basename;
# extract @includes
use Getopt::Long;
Getopt::Long::Configure ('bundling');
Getopt::Long::Configure ('no_ignore_case');
@includes;
@compile_opts;
GetOptions("I=s" => \@includes,
"O=s" => \$compile_opts);
@includes = map("-I$_", @includes);
sub objfile_of {
my ($out_dir,$a) = @_;
$a || return "";
my ($base, $dir, $ext) = fileparse($a,'\..*');
return "${dir}$out_dir/${base}.o";
}
sub do_exec {
print "@_\n";
`@_`;
}
1;
Now compile_sources.pl:
#!/usr/bin/perl
## sets variables @includes, $compile_flags
require "../filtercpp.pl";
$out_dir = shift(@ARGV);
$out_dir || die;
@sources = @ARGV;
sub get_ood {
my ($out_dir, $sources) = @_;
my %ood = ();
open DEPENDS, "makedepend @{$sources} @includes -f- |" || return [];
while () {
chomp;
if ($_ !~ m/DO NOT DELETE/) {
my @vars = split(/:/,$_);
my $source = $vars[0];
$source =~ s/\.o$/.cpp/;
$source ne "" || next;
next if $ood{$source};
my $obj = &objfile_of($out_dir,$source);
my $obj_date = ((stat $obj)[9] || 0);
#print STDERR "examining $obj, modified on $obj_date, rest is $vars[1]\n";
for my $header (split('\s+',$vars[1])) {
my $header_date = (stat $header)[9] || 0;
if ($header_date > $obj_date) {
print STDERR "$header ($header_date) is newer than $obj ($obj_date)\n";
$ood{$source} = true;
last;
}
}
if (((stat $source)[9] || 0) > $obj_date) {
$ood{$source} = true;
}
}
}
close DEPENDS;
return keys(%ood);
}
sub compile_sources {
my ($out_dir,$sources) = @_;
my @objs = map(&objfile_of($out_dir,$_), @{$sources});
#print("outdir $out_dir, includes @includes, sources @{$sources}\n");
@ood_sources = get_ood($out_dir, $sources);
mkdir $out_dir;
if (@ood_sources) {
print "ood sources: @ood_sources\n";
for $source (@ood_sources) {
#print("compiling $source\n");
my $obj = &objfile_of($out_dir,$source);
#print("ood obj is $obj for source $source\n");
&do_exec("gcc $source @includes $compile_flags -c -o$obj");
}
}
}
&compile_sources($out_dir,\@sources);
Now link_sources.pl:
#!/usr/bin/perl
## sets global variables @includes, $compile_opts
require "../filtercpp.pl";
$out_dir = shift(@ARGV);
$out_dir || die;
$out = shift(@ARGV);
$out || die;
@sources = @ARGV;
sub link_sources {
my ($out_dir,$sources,$out) = @_;
my $out_date = (stat "$out_dir/$out")[9] || 0;
my @objs = map (&objfile_of($out_dir,$_), @{$sources});
for my $obj (@objs) {
my $obj_date = (stat $obj)[9] || 0;
if ($obj_date > $out_date) {
print("$obj is newer than $out\n");
if ($out =~ /.lib$/) {
&do_exec("ar cr $out_dir/core.lib @objs $compile_opts");
last;
} else {
&do_exec("gcc @objs -o $out_dir/$out $compile_opts");
last;
}
}
}
}
&link_sources($out_dir,\@ARGV,$out);
