Hi, [including some random mailing list, so that the text isn't lost :) ] On Thu, 18 Jun 2009, Cristian Rodríguez wrote:
there are some packages that fail to build because they miss phtread symbols. I have always have the doubt about why there is the "-pthread" gcc flag... somewhere I read that's the correct thing to do, that using -lpthread is wrong, but without explanations. What should I use in these cases? I suppose use -lpthread would be the equivalent to what they did without --as-needed... but perhaps the behavior without --as-needed was already wrong.
matz is the right person to answer your question, as far as I know, the right way is using -lpthread instead of -pthread , is that correct ?
Nope, -pthread is better than -lpthread (see below). It's completely possible that something is broken with as-needed vs. pthreads. Those cases have to be identified and then worked around (export SUSE_AS_NEEDED=0), or better, analyzed for the root cause. I didn't get the complete thread, only this forward, so I don't know what the nature of the breakage is, so I can give only speculations: I would think that if something with pthread and as-needed breaks it doesn't get exposed as missing symbols or the like (so that apps don't even start) but rather mysterious breakages related to non-thread-aware functions being used even though -pthread was given as compile and link flag. [background] Regarding the explanation of -pthread/-lpthread: Unfortunately the initial threading support for UNIX was a bit like bolted onto the existing (non thread-aware) libc. For various reasons it was decided that users that aren't interested in threading are not supposed to pay the cost of _potential_ thread safety. This cost is mostly associated with locking some shared data structures for libc and the process output (e.g. output of printf() ). Now, given this criteria, there's not much choice. Basically the user has to announce to the system if or if not the application is thread aware, so that it (the system) can chose between thread-aware or non thread-aware functions. This announcement is done by the -pthread flag. The difference to the -lpthread flag is that it is not only a link time flag (saying that the libpthread library has to be put into the dependence libraries) but also a compile time flag. So for compiling .c to .o files for a thread aware program it's necessary to include -pthread into the compile flags. The effect of that option is system dependend. Usually it will result in the compiler defining some preprocessor symbols (_REENTRANT) so that system headers can choose between different implementations of various posix functions (where the API doesn't make a difference between thread-safe and -unsafe functions) and internal data structures. So, for memoizing the difference: -lpthread is a link time only flag, simply telling the linker to include libpthread into the application. -pthread (without the "l") is a link time _and compile time flag_. Because sometimes different code has to be emitted (for the runtime system including libc) for programs supposed to be thread-safe, this can't be announced only at the link phase, the compiler has to know about this. [more magic] Now, for linux there's a bit magic involved. To make the cost for non thread-safe programs really low some routines in libc _are not thread-safe_. That's fine, those programs aren't linked against libpthread, so they have announced to be not interested in threads. libpthread.so provides thread-aware variants of some functions that are also provided by libc itself. For instance open() and friends is such function. The libpthread variant includes some locking for shared data structures. (Apart from these overriding functions it also includes the usual pthread functions, pthread_*, e.g. pthread_spin_lock). How this all works is because ELF symbols resolution rules say so. If an application calls "open" and is linked against libpthread and libc (which both provide a version of "open", one thread-aware the other not) then the dynamic linker will chose the first it finds. Other things (gcc .spec files) make sure that libpthread is linked in _before_ libc when -pthread is given. This can't be ensured for -lpthread, the latter being simply a request for a random other library, as far as the system knows. So only -pthread, not -lpthread, makes really sure that libpthread is linked at the right place vs. libc, so that it can override the non thread-aware functions of it. [less magic] Having said all this, it usually is just fine to simply link against libpthread (via -lpthread) with linux, as long as you don't mention -lc before. This is because most headers don't care about the _REENTRANT preprocessor define, i.e. they don't change their content depending on the flag, and because ELF rules will still make the functions from libpthread be preferred. But for cross-platform compatibility it's important to use -pthread, _not_ -lpthread. And as it's important for others, and at least good under linux, you can just as well use it always if you need. Ciao, Michael.