>Number: 150959 >Category: misc >Synopsis: Stub pthread_once in libc should call _libc_once >Confidential: no >Severity: non-critical >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Sun Sep 26 07:10:05 UTC 2010 >Closed-Date: >Last-Modified: >Originator: Kenton Varda >Release: 8-STABLE >Organization: >Environment: FreeBSD umaro.evlan.org 8.1-STABLE FreeBSD 8.1-STABLE #0: Thu Aug 5 03:29:49 PDT 2010 ken...@umaro.evlan.org:/usr/obj/usr/src/sys/GENERIC amd64 >Description: libc.so defines a "stub" version of pthread_once(). Unfortunately, the stub version simply returns zero (success) without ever calling the init function, which does not satisfy the contract of pthread_once.
The effect of this is that if code which uses pthread_once is accidentally linked without libpthread, the code's attempts to initialize data structures using pthread_once will silently fail, probably leading to unexpected and hard-to-debug crashes or other problems. When debugging this code, the developer may figure out easily enough that pthread_once() is not functioning, but is unlikely to realize that this is because he has not linked against libpthread -- most developers would expect to have seen a linker error if this were the problem. This also makes it difficult for developers to write libraries which are thread-safe without requiring a threading implementation. Many libraries have no need to spawn threads of their own, but do need to be safe to use in a multi-threaded program. These libraries may thus need to use pthread_once to initialize internal data structures. Unfortunately, this currently means that any program using such a library must link against libpthread whether or not the program actually uses threads. It would be convenient if such code could simply rely on the libc pthread stubs instead. >How-To-Repeat: 1. Write a small program that uses pthread_once(). 2. Compile and link without libpthread. 3. Observe that linking succeeds, but at runtime the init function is never called. >Fix: The basic fix is trivial: Have the pthread_once stub (defined in lib/libc/gen/_pthread_stubs.c) call _libc_once (defined in lib/libc/gen/_once_stub.c) instead of just returning zero. It appears that this was tried before, but reverted: http://svn.freebsd.org/viewvc/base?view=revision&revision=199614 >From the revision description, it seems that libstdc++ has unfortunately >decided to rely on the pthread_once stub's incorrect behavior as a way to >detect whether libpthread has been linked. This seems like a very poor choice >on libstdc++'s part; it should be fixed to avoid depending on this behavior. Another possibility would be to return a non-zero error code. This would at least let the caller know that pthread_once() did not correctly perform its documented behavior. >Release-Note: >Audit-Trail: >Unformatted: _______________________________________________ freebsd-bugs@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-bugs To unsubscribe, send any mail to "freebsd-bugs-unsubscr...@freebsd.org"
