Installation process

To install a DEB package, dpkg will first ask dpkg-deb to extract the control file to do some sanity checks, e.g. the architecture type:

if (pkg->available.architecture && *pkg->available.architecture &&
      strcmp(pkg->available.architecture,"all") &&
      strcmp(pkg->available.architecture,architecture))
    forcibleerr(fc_architecture,
                _("package architecture (%s) does not match system (%s)"),
                pkg->available.architecture,architecture);

or the dependencies to avoid conflicts:

  for (dsearch= pkg->available.depends; dsearch; dsearch= dsearch->next) {
    switch (dsearch->type) {
    case dep_conflicts:
      /* Look for things we conflict with. */
      check_conflict(dsearch, pkg, pfilename);
      break;
    case dep_breaks:
      /* Look for things we break. */
      check_breaks(dsearch, pkg, pfilename);
      break;
    case dep_provides:
      /* Look for things that conflict with what we provide. */
      if (dsearch->list->ed->installed.valid) {
        for (psearch= dsearch->list->ed->installed.depended;
             psearch;
             psearch= psearch->nextrev) {
          if (psearch->up->type != dep_conflicts) continue;
          check_conflict(psearch->up, pkg, pfilename);
        }
      }
      break;
    case dep_suggests:
    case dep_recommends:
    case dep_depends:
    case dep_replaces:
    case dep_enhances:
      /* Ignore these here. */
      break;
    case dep_predepends:
      if (!depisok(dsearch,&depprobwhy,0,1)) {
        varbufaddc(&depprobwhy,0);
        fprintf(stderr, _("dpkg: regarding %s containing %s, pre-dependency problem:\n%s"),
                pfilename, pkg->name, depprobwhy.buf);
        if (!force_depends(dsearch->list))
          ohshit(_("pre-dependency problem - not installing %.250s"),pkg->name);
        fprintf(stderr, _("dpkg: warning - ignoring pre-dependency problem !\n"));
      }
    }
  }

Then the data is extracted and installed. Final step is installing the maintainer scripts (pre- and post- install).

This is only the big picture. For more informations, read proccessarc.c from the src/ directory of dpkg .

Now, let's see how to create a DEB package from a higher point of view.