static variables

This is probably a naive question, but: there are quite a lot of static variables in the git code where it's really unnecessary. Is that just a historical artifact, or is there some reason to prefer them? I'm working on a patch that will introduce threading, so naturally I'm on the lookout for static variables. In general, can I get rid of static variables where it seems straightforward to do so?

As an example, here's an excerpt from symlnks.c. In addition to being static, if I'm reading this right, it appears that the 'removal' variable is used before it's initialized:

static struct removal_def {
  char path[PATH_MAX];
  int len;
} removal;

static void do_remove_scheduled_dirs(int new_len)
{
  while (removal.len > new_len) {
    removal.path[removal.len] = '\0';
    if (rmdir(removal.path))
      break;
    do {
      removal.len--;
    } while (removal.len > new_len &&
       removal.path[removal.len] != '/');
  }
  removal.len = new_len;
}

void schedule_dir_for_removal(const char *name, int len)
{
  int match_len, last_slash, i, previous_slash;

  match_len = last_slash = i =
    longest_path_match(name, len, removal.path, removal.len,
           &previous_slash);
  /* Find last slash inside 'name' */
  while (i < len) {
    if (name[i] == '/')
      last_slash = i;
    i++;
  }

  /*
   * If we are about to go down the directory tree, we check if
   * we must first go upwards the tree, such that we then can
   * remove possible empty directories as we go upwards.
   */
  if (match_len < last_slash && match_len < removal.len)
    do_remove_scheduled_dirs(match_len);
  /*
   * If we go deeper down the directory tree, we only need to
   * save the new path components as we go down.
   */
  if (match_len < last_slash) {
    memcpy(&removal.path[match_len], &name[match_len],
           last_slash - match_len);
    removal.len = last_slash;
  }
}

void remove_scheduled_dirs(void)
{
  do_remove_scheduled_dirs(0);
}

EOF

Thanks,

Stefan

Stefan Zager wrote:

Sometimes it's for convenience. Other times it's to work around C89's requirement that initializers can't include pointers to automatic variables, so when using parse_options, old commands tend to use statics for the variables initialized by options. (Since then, git has stopped following that so rigidly, which is probably a good thing.)

Worse, some functions have static buffers when they need a large buffer and want to avoid too much allocation churn. As a general rule, historically very little of git's code (mostly pack related) needed to be usable with threads, though of course it would be excellent to fix more code to be thread-safe.

statics are allocated from the .bss section, where they are zeroed automatically.

Plumbing this through the call stack instead of using a static sounds like a good idea. That would mean allocating the removal_def in unpack-trees.c::check_updates, I think (see v1.6.3-rc0~147^2~16, "unlink_entry(): introduce schedule_dir_for_removal()", 2009-02-09 for context). Then the loop could be divided into chunks that each use their own removal_def or something.

Sometimes when git needs parallelism and threads don't work, it uses fork + exec (aka run_command). Making the relevant functionality thread-safe is generally much nicer, though.

Thanks and hope that helps, Jonathan

Jonathan Nieder wrote:

See for example path.c::git_path and mkpath.

Threaded code uses a specialized thread-safe dialect of the usual C used in git. I wish I had better news to offer.