[Bug nptl/27880] New: Please provide a pthread pid accessor

["hpa at zytor dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27880

            Bug ID: 27880
           Summary: Please provide a pthread pid accessor
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: hpa at zytor dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

There is currently no way to access the kernel tid for another pthread, except
via the very cumbersome thread_db interface. The intent is good, to provide
interface isolation, but sometimes having such cross-layer interfaces (e.g.
fileno()) are at the very least serve a "harm reduction" function.

As it is, there are enough reasons access to a tid is necessary that
programmers are employing multiple unsafe techniques:

1. Calling gettid() early in the thread main function and caching that value
indefinitely;
2. Mixing pthreads and raw calls to clone();
3. Extracting or even hard-coding the offset for tid in struct pthread (almost
never using any kind of hard version tying.)

Having an interface like:

pid_t pthread_gettid_np(pthread_t thread)

... would at least allow the interface to be kept in sync with the current
libpthread, and provides a path to returning an error if no meaningful tid is
available (e.g. in the case of M:N threads, or the underlying kernel thread no
longer existing.)

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