The program needs to know a magical system-dependent constant called FIBMAP that is defined in the C include file <linux/fs.h>. It assumes that the current directory contains a file, fibmap.ph, which will define $FIBMAP with the appropriate value. But if fibmap.ph does not exist, then the program creates it with the correct value.

Please don't ask what this program was for.

unless (eval {require "fibmap.ph"}) {
  open C, "> fibmap.c" or die "Couldn't write configurator: $!; aborti
+ng";
  print C while <DATA>;
  close C;
  system("cc -o fibmap fibmap.c") == 0 or die "Couldn't run C compiler
+.\n";
  system("./fibmap > fibmap.ph") == 0 or die "Couldn't run configurato
+r.\n";
  unless (eval {require "fibmap.ph"}) {
    die "Couldn't load configurator: $@.\n";
  }
  unlink "fibmap", "fibmap.c";
}
print "Configurator ran OK; FIBMAP is $FIBMAP\n";

# ...
# much code omitted here
# ...

__DATA__
#include <stdio.h>
#include <linux/fs.h>

int main(void)
{
  printf("$FIBMAP = %d;\n1;\n", FIBMAP);
  return 0;
}

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In reply to Run-time generation of magic constant by Dominus

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