diff options
Diffstat (limited to 'linuxthreads/FAQ.html')
| -rw-r--r-- | linuxthreads/FAQ.html | 986 |
1 files changed, 986 insertions, 0 deletions
diff --git a/linuxthreads/FAQ.html b/linuxthreads/FAQ.html new file mode 100644 index 0000000..45d2387 --- /dev/null +++ b/linuxthreads/FAQ.html @@ -0,0 +1,986 @@ +<HTML> +<HEAD> +<TITLE>LinuxThreads Frequently Asked Questions</TITLE> +</HEAD> +<BODY> +<H1 ALIGN=center>LinuxThreads Frequently Asked Questions <BR> + (with answers)</H1> + +<HR><P> + +<A HREF="#A">A. The big picture</A><BR> +<A HREF="#B">B. Getting more information</A><BR> +<A HREF="#C">C. Issues related to the C library</A><BR> +<A HREF="#D">D. Problems, weird behaviors, potential bugs</A><BR> +<A HREF="#E">E. Missing functions, wrong types, etc</A><BR> +<A HREF="#F">F. C++ issues</A><BR> +<A HREF="#G">G. Debugging LinuxThreads programs</A><BR> +<A HREF="#H">H. Compiling multithreaded code; errno madness</A><BR> +<A HREF="#I">I. X-Windows and other libraries</A><BR> +<A HREF="#J">J. Signals and threads</A><BR> +<A HREF="#K">K. Internals of LinuxThreads</A><P> + +<HR> +<P> + +<H2><A NAME="A">A. The big picture</A></H2> + +<H4><A NAME="A.1">A.1: What is LinuxThreads?</A></H4> + +LinuxThreads is a Linux library for multi-threaded programming. +It implements the Posix 1003.1c API (Application Programming +Interface) for threads. It runs on any Linux system with kernel 2.0.0 +or more recent, and a suitable C library (see section <A HREF="B">B</A>). +<P> + +<H4><A NAME="A.2">A.2: What are threads?</A></H4> + +A thread is a sequential flow of control through a program. +Multi-threaded programming is, thus, a form of parallel programming +where several threads of control are executing concurrently in the +program. All threads execute in the same memory space, and can +therefore work concurrently on shared data.<P> + +Multi-threaded programming differs from Unix-style multi-processing in +that all threads share the same memory space (and a few other system +resources, such as file descriptors), instead of running in their own +memory space as is the case with Unix processes.<P> + +Threads are useful for two reasons. First, they allow a program to +exploit multi-processor machines: the threads can run in parallel on +several processors, allowing a single program to divide its work +between several processors, thus running faster than a single-threaded +program, which runs on only one processor at a time. Second, some +programs are best expressed as several threads of control that +communicate together, rather than as one big monolithic sequential +program. Examples include server programs, overlapping asynchronous +I/O, and graphical user interfaces.<P> + +<H4><A NAME="A.3">A.3: What is POSIX 1003.1c?</A></H4> + +It's an API for multi-threaded programming standardized by IEEE as +part of the POSIX standards. Most Unix vendors have endorsed the +POSIX 1003.1c standard. Implementations of the 1003.1c API are +already available under Sun Solaris 2.5, Digital Unix 4.0, +Silicon Graphics IRIX 6, and should soon be available from other +vendors such as IBM and HP. More generally, the 1003.1c API is +replacing relatively quickly the proprietary threads library that were +developed previously under Unix, such as Mach cthreads, Solaris +threads, and IRIX sprocs. Thus, multithreaded programs using the +1003.1c API are likely to run unchanged on a wide variety of Unix +platforms.<P> + +<H4><A NAME="A.4">A.4: What is the status of LinuxThreads?</A></H4> + +In short, it's not completely finished (hence the version numbers in +0.<I>x</I>), but what is done is pretty mature. +LinuxThreads implements almost all of Posix 1003.1c, as well as a few +extensions. The only part of LinuxThreads that does not conform yet +to Posix is signal handling (see section <A HREF="#J">J</A>). Apart +from the signal stuff, all the Posix 1003.1c base functionality is +provided and conforms to the standard (to the best of my knowledge). +The signal stuff is hard to get right, at least without special kernel +support, and while I'm definitely looking at ways to implement the +Posix behavior for signals, this might take a long time before it's +completed.<P> + +<H4><A NAME="A.5">A.5: How stable is LinuxThreads?</A></H4> + +The basic functionality (thread creation and termination, mutexes, +conditions, semaphores) is very stable. Several industrial-strength +programs, such as the AOL multithreaded Web server, use LinuxThreads +and seem quite happy about it. There are some rough edges in +the LinuxThreads / C library interface, at least with libc 5, but most +of these rough edges are fixed in glibc 2, which should soon become +the standard C library for Linux distributions (see section <A +HREF="#C">C</A>). <P> + +<HR> +<P> + +<H2><A NAME="B">B. Getting more information</A></H2> + +<H4><A NAME="B.1">B.1: What are good books and other sources of +information on POSIX threads?</A></H4> + +The FAQ for comp.programming.threads lists several books: +<A HREF="http://www.serpentine.com/~bos/threads-faq/">http://www.serpentine.com/~bos/threads-faq/</A>.<P> + +There are also some online tutorials. Follow the links from the +LinuxThreads web page: +<A HREF="http://pauillac.inria.fr/~xleroy/linuxthreads">http://pauillac.inria.fr/~xleroy/linuxthreads</A>.<P> + +<H4><A NAME="B.2">B.2: I'd like to be informed of future developments on +LinuxThreads. Is there a mailing list for this purpose?</A></H4> + +I post LinuxThreads-related announcements on the newsgroup +<A HREF="news:comp.os.linux.announce">comp.os.linux.announce</A>, +and also on the mailing list +<code>linux-threads@magenet.com</code>. +You can subscribe to the latter by writing +<A HREF="mailto:majordomo@magenet.com">majordomo@magenet.com</A>.<P> + +<H4><A NAME="B.3">B.3: What are good places for discussing +LinuxThreads?</A></H4> + +For questions about programming with POSIX threads in general, use +the newsgroup +<A HREF="news:comp.programming.threads">comp.programming.threads</A>. +Be sure you read the +<A HREF="http://www.serpentine.com/~bos/threads-faq/">FAQ</A> +for this group before you post.<P> + +For Linux-specific questions, use +<A +HREF="news:comp.os.linux.development.apps">comp.os.linux.development.apps</A> +and <A +HREF="news:comp.os.linux.development.kernel">comp.os.linux.development.kernel</A>. +The latter is especially appropriate for questions relative to the +interface between the kernel and LinuxThreads.<P> + +Very specific LinuxThreads questions, and in particular everything +that looks like a potential bug in LinuxThreads, should be mailed +directly to me (<code>Xavier.Leroy@inria.fr</code>). Before mailing +me, make sure that your question is not answered in this FAQ.<P> + +<H4><A NAME="B.4">B.4: I'd like to read the POSIX 1003.1c standard. Is +it available online?</A></H4> + +Unfortunately, no. POSIX standards are copyrighted by IEEE, and +IEEE does not distribute them freely. You can buy paper copies from +IEEE, but the price is fairly high ($120 or so). If you disagree with +this policy and you're an IEEE member, be sure to let them know.<P> + +On the other hand, you probably don't want to read the standard. It's +very hard to read, written in standard-ese, and targeted to +implementors who already know threads inside-out. A good book on +POSIX threads provides the same information in a much more readable form. +I can personally recommend Dave Butenhof's book, <CITE>Programming +with POSIX threads</CITE> (Addison-Wesley). Butenhof was part of the +POSIX committee and also designed the Digital Unix implementations of +POSIX threads, and it shows.<P> + +Another good source of information is the X/Open Group Single Unix +specification which is available both +<A HREF="http://www.rdg.opengroup.org/onlinepubs/7908799/index.html">on-line</A> +and as a +<A HREF="http://www.UNIX-systems.org/gosolo2/">book and CD/ROM</A>. +That specification includes pretty much all the POSIX standards, +including 1003.1c, with some extensions and clarifications.<P> + +<HR> +<P> + +<H2><A NAME="C">C. Issues related to the C library</A></H2> + +<H4><A NAME="C.1">C.1: Which version of the C library should I use +with LinuxThreads?</A></H4> + +Most current Linux distributions come with libc version 5, maintained +by H.J.Lu. For LinuxThreads to work properly, you must use either +libc 5.2.18 or libc 5.4.12 or later. Avoid 5.3.12 and 5.4.7: these +have problems with the per-thread errno variable. +<P> + +Unfortunately, many popular Linux distributions (e.g. RedHat 4.2) come +with libc 5.3.12 preinstalled -- the one that does not work with +LinuxThreads. Fortunately, you can often find pre-packaged binaries +of more recent versions of libc for these distributions. In the case +of RedHat 4, there is a RPM package for libc-5.4 in the "contrib" +area of RedHat FTP sites. +<P> + +<H4><A NAME="C.2">C.2: What about glibc 2, a.k.a. libc 6?</A></H4> + +It's the next generation libc for Linux, developed by Ulrich +Drepper and other FSF collaborators. glibc 2 offers much better +support for threads than libc 5. Indeed, thread support was +planned from the very early stages of glibc 2, while it's a +last-minute addition to libc 5. glibc 2 actually comes with a +specially adapted version of LinuxThreads, which you can drop in the +glibc 2 sources as an add-on package. + +<H4><A NAME="C.3">C.3: So, should I switch to glibc 2, or stay with a +recent libc 5?</A></H4> + +Depends how you plan to do it. Switching an already installed +system from libc 5 to glibc 2 is not completely straightforward. +See the <A HREF="http://sunsite.unc.edu/LDP/HOWTO/Glibc2-HOWTO.html">Glibc2 +HOWTO</A> for more information. +But (re-)installing a Linux distribution based on glibc 2 is easy. +One such distribution available now is RedHat 5.0. Debian and other +Linux distributors will also provide glibc 2-based distributions in the +near future. +<P> + +<H4><A NAME="C.4">C.4: Where can I find glibc 2 and the version of +LinuxThreads that goes with it?</A></H4> + +On <code>prep.ai.mit.edu</code> and its many, many mirrors around the world. +See <A +HREF="http://www.gnu.org/order/ftp.html">http://www.gnu.org/order/ftp.html</A> +for a list of mirrors.<P> + +<HR> +<P> + +<H2><A NAME="D">D. Problems, weird behaviors, potential bugs</A></H2> + +<H4><A NAME="D.1">D.1: When I compile LinuxThreads, I run into problems in +file <code>libc_r/dirent.c</code></A></H4> + +You probably mean: +<PRE> + libc_r/dirent.c:94: structure has no member named `dd_lock' +</PRE> +I haven't actually seen this problem, but several users reported it. +My understanding is that something is wrong in the include files of +your Linux installation (<code>/usr/include/*</code>). Make sure +you're using a supported version of the C library. (See section <A +HREF="#B">B</A>).<P> + +<H4><A NAME="D.2">D.2: When I compile LinuxThreads, I run into problems with +<CODE>/usr/include/sched.h</CODE>: there are several occurrences of +<CODE>_p</CODE> that the C compiler does not understand</A></H4> + +Yes, <CODE>/usr/include/sched.h</CODE> that comes with libc 5.3.12 is broken. +Replace it with the <code>sched.h</code> file contained in the +LinuxThreads distribution. But really you should not be using libc +5.3.12 with LinuxThreads! (See question <A HREF="#C.1">C.1</A>.)<P> + +<H4><A NAME="D.3">D.3: My program does <CODE>fdopen()</CODE> on a file +descriptor opened on a pipe. When I link it with LinuxThreads, +<CODE>fdopen()</CODE> always returns NULL!</A></H4> + +You're using one of the buggy versions of libc (5.3.12, 5.4.7., etc). +See question <A HREF="#C.1">C.1</A> above.<P> + +<H4><A NAME="D.4">D.4: My program crashes the first time it calls +<CODE>pthread_create()</CODE> !</A></H4> + +You wouldn't be using glibc 2.0, by any chance? That's a known bug +with glibc 2.0. Please upgrade to 2.0.1 or later.<P> + +<H4><A NAME="D.5">D.5: When I'm running a program that creates N +threads, <code>top</code> or <code>ps</code> +display N+2 processes that are running my program. What do all these +processes correspond to?</A></H4> + +Due to the general "one process per thread" model, there's one process +for the initial thread and N processes for the threads it created +using <CODE>pthread_create</CODE>. That leaves one process +unaccounted for. That extra process corresponds to the "thread +manager" thread, a thread created internally by LinuxThreads to handle +thread creation and thread termination. This extra thread is asleep +most of the time. + +<H4><A NAME="D.6">D.6: Scheduling seems to be very unfair when there +is strong contention on a mutex: instead of giving the mutex to each +thread in turn, it seems that it's almost always the same thread that +gets the mutex. Isn't this completely broken behavior?</A></H4> + +What happens is the following: when a thread unlocks a mutex, all +other threads that were waiting on the mutex are sent a signal which +makes them runnable. However, the kernel scheduler may or may not +restart them immediately. If the thread that unlocked the mutex +tries to lock it again immediately afterwards, it is likely that it +will succeed, because the threads haven't yet restarted. This results +in an apparently very unfair behavior, when the same thread repeatedly +locks and unlocks the mutex, while other threads can't lock the mutex.<P> + +This is perfectly acceptable behavior with respect to the POSIX +standard: for the default scheduling policy, POSIX makes no guarantees +of fairness, such as "the thread waiting for the mutex for the longest +time always acquires it first". This allows implementations of +mutexes to remain simple and efficient. Properly written +multithreaded code avoids that kind of heavy contention on mutexes, +and does not run into fairness problems. If you need scheduling +guarantees, you should consider using the real-time scheduling +policies <code>SCHED_RR</code> and <code>SCHED_FIFO</code>, which have +precisely defined scheduling behaviors. <P> + +<H4><A NAME="D.7">D.7: I have a simple test program with two threads +that do nothing but <CODE>printf()</CODE> in tight loops, and from the +printout it seems that only one thread is running, the other doesn't +print anything!</A></H4> + +If you wait long enough, you should see the second thread kick in. +But still, you're right, one thread prevents the other one from +running for long periods of time. The reason is explained in +question <A HREF="#D.6">D.6</A> above: <CODE>printf()</CODE> performs +locking on <CODE>stdout</CODE>, and thus your two threads contend very +heavily for the mutex associated with <CODE>stdout</CODE>. But if you +do some real work between two calls to <CODE>printf()</CODE>, you'll +see that scheduling becomes much smoother. <P> + +<H4><A NAME="D.8">D.8: I've looked at <code><pthread.h></code> +and there seems to be a gross error in the <code>pthread_cleanup_push</code> +macro: it opens a block with <code>{</code> but does not close it! +Surely you forgot a <code>}</code> at the end of the macro, right? +</A></H4> + +Nope. That's the way it should be. The closing brace is provided by +the <code>pthread_cleanup_pop</code> macro. The POSIX standard +requires <code>pthread_cleanup_push</code> and +<code>pthread_cleanup_pop</code> to be used in matching pairs, at the +same level of brace nesting. This allows +<code>pthread_cleanup_push</code> to open a block in order to +stack-allocate some data structure, and +<code>pthread_cleanup_pop</code> to close that block. It's ugly, but +it's the standard way of implementing cleanup handlers.<P> + +<HR> +<P> + +<H2><A NAME="E">E. Missing functions, wrong types, etc</A></H2> + +<H4><A NAME="E.1">E.1: Where is <CODE>pthread_yield()</CODE> ? How +comes LinuxThreads does not implement it?</A></H4> + +Because it's not part of the (final) POSIX 1003.1c standard. +Several drafts of the standard contained <CODE>pthread_yield()</CODE>, +but then the POSIX guys discovered it was redundant with +<CODE>sched_yield()</CODE> and dropped it. So, just use +<CODE>sched_yield()</CODE> instead. + +<H4><A NAME="E.2">E.2: I've found some type errors in +<code><pthread.h></code>. +For instance, the second argument to <CODE>pthread_create()</CODE> +should be a <CODE>pthread_attr_t</CODE>, not a +<CODE>pthread_attr_t *</CODE>. Also, didn't you forget to declare +<CODE>pthread_attr_default</CODE>?</A></H4> + +No, I didn't. What you're describing is draft 4 of the POSIX +standard, which is used in OSF DCE threads. LinuxThreads conforms to the +final standard. Even though the functions have the same names as in +draft 4 and DCE, their calling conventions are slightly different. In +particular, attributes are passed by reference, not by value, and +default attributes are denoted by the NULL pointer. Since draft 4/DCE +will eventually disappear, you'd better port your program to use the +standard interface.<P> + +<H4><A NAME="E.3">E.3: I'm porting an application from Solaris and I +have to rename all thread functions from <code>thr_blah</code> to +<CODE>pthread_blah</CODE>. This is very annoying. Why did you change +all the function names?</A></H4> + +POSIX did it. The <code>thr_*</code> functions correspond to Solaris +threads, an older thread interface that you'll find only under +Solaris. The <CODE>pthread_*</CODE> functions correspond to POSIX +threads, an international standard available for many, many platforms. +Even Solaris 2.5 and later support the POSIX threads interface. So, +do yourself a favor and rewrite your code to use POSIX threads: this +way, it will run unchanged under Linux, Solaris, and quite a lot of +other platforms.<P> + +<H4><A NAME="E.4">E.4: How can I suspend and resume a thread from +another thread? Solaris has the <CODE>thr_suspend()</CODE> and +<CODE>thr_resume()</CODE> functions to do that; why don't you?</A></H4> + +The POSIX standard provides <B>no</B> mechanism by which a thread A can +suspend the execution of another thread B, without cooperation from B. +The only way to implement a suspend/restart mechanism is to have B +check periodically some global variable for a suspend request +and then suspend itself on a condition variable, which another thread +can signal later to restart B.<P> + +Notice that <CODE>thr_suspend()</CODE> is inherently dangerous and +prone to race conditions. For one thing, there is no control on where +the target thread stops: it can very well be stopped in the middle of +a critical section, while holding mutexes. Also, there is no +guarantee on when the target thread will actually stop. For these +reasons, you'd be much better off using mutexes and conditions +instead. The only situations that really require the ability to +suspend a thread are debuggers and some kind of garbage collectors.<P> + +If you really must suspend a thread in LinuxThreads, you can send it a +<CODE>SIGSTOP</CODE> signal with <CODE>pthread_kill</CODE>. Send +<CODE>SIGCONT</CODE> for restarting it. +Beware, this is specific to LinuxThreads and entirely non-portable. +Indeed, a truly conforming POSIX threads implementation will stop all +threads when one thread receives the <CODE>SIGSTOP</CODE> signal! +One day, LinuxThreads will implement that behavior, and the +non-portable hack with <CODE>SIGSTOP</CODE> won't work anymore.<P> + +<H4><A NAME="E.5">E.5: LinuxThreads does not implement +<CODE>pthread_attr_setstacksize()</CODE> nor +<CODE>pthread_attr_setstackaddr()</CODE>. Why? </A></H4> + +These two functions are part of optional components of the POSIX +standard, meaning that portable applications should test for the +"feature test" macros <CODE>_POSIX_THREAD_ATTR_STACKSIZE</CODE> and +<CODE>_POSIX_THREAD_ATTR_STACKADDR</CODE> (respectively) before using these +functions.<P> + +<CODE>pthread_attr_setstacksize()</CODE> lets the programmer specify +the maximum stack size for a thread. In LinuxThreads, stacks start +small (4k) and grow on demand to a fairly large limit (2M), which +cannot be modified on a per-thread basis for architectural reasons. +Hence there is really no need to specify any stack size yourself: the +system does the right thing all by itself. Besides, there is no +portable way to estimate the stack requirements of a thread, so +setting the stack size is pretty useless anyway.<P> + +<CODE>pthread_attr_setstackaddr()</CODE> is even more questionable: it +lets users specify the stack location for a thread. Again, +LinuxThreads takes care of that for you. Why you would ever need to +set the stack address escapes me.<P> + +<H4><A NAME="E.6">E.6: LinuxThreads does not support the +<CODE>PTHREAD_SCOPE_PROCESS</CODE> value of the "contentionscope" +attribute. Why? </A></H4> + +With a "one-to-one" model, as in LinuxThreads (one kernel execution +context per thread), there is only one scheduler for all processes and +all threads on the system. So, there is no way to obtain the behavior of +<CODE>PTHREAD_SCOPE_PROCESS</CODE>. + +<H4><A NAME="E.7">E.7: LinuxThreads does not implement process-shared +mutexes, conditions, and semaphores. Why?</A></H4> + +This is another optional component of the POSIX standard. Portable +applications should test <CODE>_POSIX_THREAD_PROCESS_SHARED</CODE> +before using this facility. +<P> +The goal of this extension is to allow different processes (with +different address spaces) to synchronize through mutexes, conditions +or semaphores allocated in shared memory (either SVR4 shared memory +segments or <CODE>mmap()</CODE>ed files). +<P> +The reason why this does not work in LinuxThreads is that mutexes, +conditions, and semaphores are not self-contained: their waiting +queues contain pointers to linked lists of thread descriptors, and +these pointers are meaningful only in one address space. +<P> +Matt Messier and I spent a significant amount of time trying to design a +suitable mechanism for sharing waiting queues between processes. We +came up with several solutions that combined two of the following +three desirable features, but none that combines all three: +<UL> +<LI>allow sharing between processes having different UIDs +<LI>supports cancellation +<LI>supports <CODE>pthread_cond_timedwait</CODE> +</UL> +We concluded that kernel support is required to share mutexes, +conditions and semaphores between processes. That's one place where +Linus Torvalds's intuition that "all we need in the kernel is +<CODE>clone()</CODE>" fails. +<P> +Until suitable kernel support is available, you'd better use +traditional interprocess communications to synchronize different +processes: System V semaphores and message queues, or pipes, or sockets. +<P> + +<HR> +<P> + +<H2><A NAME="F">F. C++ issues</A></H2> + +<H4><A NAME="F.1">F.1: Are there C++ wrappers for LinuxThreads?</A></H4> + +Douglas Schmidt's ACE library contains, among a lot of other +things, C++ wrappers for LinuxThreads and quite a number of other +thread libraries. Check out +<A HREF="http://www.cs.wustl.edu/~schmidt/ACE.html">http://www.cs.wustl.edu/~schmidt/ACE.html</A><P> + +<H4><A NAME="F.2">F.2: I'm trying to use LinuxThreads from a C++ +program, and the compiler complains about the third argument to +<CODE>pthread_create()</CODE> !</A></H4> + +You're probably trying to pass a class member function or some +other C++ thing as third argument to <CODE>pthread_create()</CODE>. +Recall that <CODE>pthread_create()</CODE> is a C function, and it must +be passed a C function as third argument.<P> + +<H4><A NAME="F.3">F.3: I'm trying to use LinuxThreads in conjunction +with libg++, and I'm having all sorts of trouble.</A></H4> + +From what I understand, thread support in libg++ is completely broken, +especially with respect to locking of iostreams. H.J.Lu wrote: +<BLOCKQUOTE> +If you want to use thread, I can only suggest egcs and glibc. You +can find egcs at +<A HREF="http://www.cygnus.com/egcs">http://www.cygnus.com/egcs</A>. +egcs has libsdtc++, which is MT safe under glibc 2. If you really +want to use the libg++, I have a libg++ add-on for egcs. +</BLOCKQUOTE> +<HR> +<P> + +<H2><A NAME="G">G. Debugging LinuxThreads programs</A></H2> + +<H4><A NAME="G.1">G.1: Can I debug LinuxThreads program using gdb?</A></H4> + +Essentially, no. gdb is basically not aware of the threads. It +will let you debug the main thread, and also inspect the global state, +but you won't have any control over the other threads. Worse, you +can't put any breakpoint anywhere in the code: if a thread other than +the main thread hits the breakpoint, it will just crash!<P> + +For running gdb on the main thread, you need to instruct gdb to ignore +the signals used by LinuxThreads. Just do: +<PRE> + handle SIGUSR1 nostop pass noprint + handle SIGUSR2 nostop pass noprint + +</PRE> + +<H4><A NAME="G.2">G.2: What about attaching to a running thread using +the <code>attach</code> command of gdb?</A></H4> + +For reasons I don't fully understand, this does not work.<P> + +<H4><A NAME="G.3">G.3: But I know gdb supports threads on some +platforms! Why not on Linux?</A></H4> + +You're correct that gdb has some built-in support for threads, in +particular the IRIX "sprocs" model, which is a "one thread = one +process" model fairly close to LinuxThreads. But gdb under IRIX uses +ioctls on <code>/proc</code> to control debugged processes, while +under Linux it uses the traditional <CODE>ptrace()</CODE>. The support +for threads is built in the <code>/proc</code> interface, but some +work remains to be done to have it in the <CODE>ptrace()</CODE> +interface. In summary, it should not be impossible to get gdb to work +with LinuxThreads, but it's definitely not trivial. + +<H4><A NAME="G.4">G.4: OK, I'll do post-mortem debugging, then. But +gdb cannot read core files generated by a multithreaded program! Or, +the core file is readable from gcc, but does not correspond to the +thread that crashed! What happens?</A></H4> + +Some versions of gdb do indeed have problems with post-mortem +debugging in general, but this is not specific to LinuxThreads. +Recent Linux distributions seem to have corrected this problem, +though.<P> + +Regarding the fact that the core file does not correspond to the +thread that crashed, the reason is that the kernel will not dump core +for a process that shares its memory with other processes, such as the +other threads of your program. So, the thread that crashes silently +disappears without generating a core file. Then, all other threads of +your program die on the same signal that killed the crashing thread. +(This is required behavior according to the POSIX standard.) The last +one that dies is no longer sharing its memory with anyone else, so the +kernel generates a core file for that thread. Unfortunately, that's +not the thread you are interested in. + +<H4><A NAME="G.5">G.5: How can I debug multithreaded programs, then?</A></H4> + +Assertions and <CODE>printf()</CODE> are your best friends. Try to debug +sequential parts in a single-threaded program first. Then, put +<CODE>printf()</CODE> statements all over the place to get execution traces. +Also, check invariants often with the <CODE>assert()</CODE> macro. In truth, +there is no other effective way (save for a full formal proof of your +program) to track down concurrency bugs. Debuggers are not really +effective for concurrency problems, because they disrupt program +execution too much.<P> + +<HR> +<P> + +<H2><A NAME="H">H. Compiling multithreaded code; errno madness</A></H2> + +<H4><A NAME="H.1">H.1: You say all multithreaded code must be compiled +with <CODE>_REENTRANT</CODE> defined. What difference does it make?</A></H4> + +It affects include files in three ways: +<UL> +<LI> The include files define prototypes for the reentrant variants of +some of the standard library functions, +e.g. <CODE>gethostbyname_r()</CODE> as a reentrant equivalent to +<CODE>gethostbyname()</CODE>.<P> + +<LI> If <CODE>_REENTRANT</CODE> is defined, some +<code><stdio.h></code> functions are no longer defined as macros, +e.g. <CODE>getc()</CODE> and <CODE>putc()</CODE>. In a multithreaded +program, stdio functions require additional locking, which the macros +don't perform, so we must call functions instead.<P> + +<LI> More importantly, <code><errno.h></code> redefines errno when +<CODE>_REENTRANT</CODE> is +defined, so that errno refers to the thread-specific errno location +rather than the global errno variable. This is achieved by the +following <code>#define</code> in <code><errno.h></code>: +<PRE> + #define errno (*(__errno_location())) +</PRE> +which causes each reference to errno to call the +<CODE>__errno_location()</CODE> function for obtaining the location +where error codes are stored. libc provides a default definition of +<CODE>__errno_location()</CODE> that always returns +<code>&errno</code> (the address of the global errno variable). Thus, +for programs not linked with LinuxThreads, defining +<CODE>_REENTRANT</CODE> makes no difference w.r.t. errno processing. +But LinuxThreads redefines <CODE>__errno_location()</CODE> to return a +location in the thread descriptor reserved for holding the current +value of errno for the calling thread. Thus, each thread operates on +a different errno location. +</UL> +<P> + +<H4><A NAME="H.2">H.2: Why is it so important that each thread has its +own errno variable? </A></H4> + +If all threads were to store error codes in the same, global errno +variable, then the value of errno after a system call or library +function returns would be unpredictable: between the time a system +call stores its error code in the global errno and your code inspects +errno to see which error occurred, another thread might have stored +another error code in the same errno location. <P> + +<H4><A NAME="H.3">H.3: What happens if I link LinuxThreads with code +not compiled with <CODE>-D_REENTRANT</CODE>?</A></H4> + +Lots of trouble. If the code uses <CODE>getc()</CODE> or +<CODE>putc()</CODE>, it will perform I/O without proper interlocking +of the stdio buffers; this can cause lost output, duplicate output, or +just crash other stdio functions. If the code consults errno, it will +get back the wrong error code. The following code fragment is a +typical example: +<PRE> + do { + r = read(fd, buf, n); + if (r == -1) { + if (errno == EINTR) /* an error we can handle */ + continue; + else { /* other errors are fatal */ + perror("read failed"); + exit(100); + } + } + } while (...); +</PRE> +Assume this code is not compiled with <CODE>-D_REENTRANT</CODE>, and +linked with LinuxThreads. At run-time, <CODE>read()</CODE> is +interrupted. Since the C library was compiled with +<CODE>-D_REENTRANT</CODE>, <CODE>read()</CODE> stores its error code +in the location pointed to by <CODE>__errno_location()</CODE>, which +is the thread-local errno variable. Then, the code above sees that +<CODE>read()</CODE> returns -1 and looks up errno. Since +<CODE>_REENTRANT</CODE> is not defined, the reference to errno +accesses the global errno variable, which is most likely 0. Hence the +code concludes that it cannot handle the error and stops.<P> + +<H4><A NAME="H.4">H.4: With LinuxThreads, I can no longer use the signals +<code>SIGUSR1</code> and <code>SIGUSR2</code> in my programs! Why? </A></H4> + +LinuxThreads needs two signals for its internal operation. +One is used to suspend and restart threads blocked on mutex, condition +or semaphore operations. The other is used for thread cancellation. +Since the only two signals not reserved for the Linux kernel are +<code>SIGUSR1</code> and <code>SIGUSR2</code>, LinuxThreads has no +other choice than using them. I know this is unfortunate, and hope +this problem will be addressed in future Linux kernels, either by +freeing some of the regular signals (unlikely), or by providing more +than 32 signals (as per the POSIX 1003.1b realtime extensions).<P> + +In the meantime, you can try to use kernel-reserved signals either in +your program or in LinuxThreads. For instance, +<code>SIGSTKFLT</code> and <code>SIGUNUSED</code> appear to be +unused in the current Linux kernels for the Intel x86 architecture. +To use these in LinuxThreads, the only file you need to change +is <code>internals.h</code>, more specifically the two lines: +<PRE> + #define PTHREAD_SIG_RESTART SIGUSR1 + #define PTHREAD_SIG_CANCEL SIGUSR2 +</PRE> +Replace them by e.g. +<PRE> + #define PTHREAD_SIG_RESTART SIGSTKFLT + #define PTHREAD_SIG_CANCEL SIGUNUSED +</PRE> +Warning: you're doing this at your own risks.<P> + +<H4><A NAME="H.5">H.5: Is the stack of one thread visible from the +other threads? Can I pass a pointer into my stack to other threads? +</A></H4> + +Yes, you can -- if you're very careful. The stacks are indeed visible +from all threads in the system. Some non-POSIX thread libraries seem +to map the stacks for all threads at the same virtual addresses and +change the memory mapping when they switch from one thread to +another. But this is not the case for LinuxThreads, as it would make +context switching between threads more expensive, and at any rate +might not conform to the POSIX standard.<P> + +So, you can take the address of an "auto" variable and pass it to +other threads via shared data structures. However, you need to make +absolutely sure that the function doing this will not return as long +as other threads need to access this address. It's the usual mistake +of returning the address of an "auto" variable, only made much worse +because of concurrency. It's much, much safer to systematically +heap-allocate all shared data structures. <P> + +<HR> +<P> + +<H2><A NAME="I">I. X-Windows and other libraries</A></H2> + +<H4><A NAME="I.1">I.1: My program uses both Xlib and LinuxThreads. +It stops very early with an "Xlib: unknown 0 error" message. What +does this mean? </A></H4> + +That's a prime example of the errno problem described in question <A +HREF="#H.2">H.2</A>. The binaries for Xlib you're using have not been +compiled with <CODE>-D_REENTRANT</CODE>. It happens Xlib contains a +piece of code very much like the one in question <A +HREF="#H.2">H.2</A>. So, your Xlib fetches the error code from the +wrong errno location and concludes that an error it cannot handle +occurred.<P> + +<H4><A NAME="I.2">I.2: So, what can I do to build a multithreaded X +Windows client? </A></H4> + +The best solution is to recompile the X libraries with multithreading +options set. They contain optional support for multithreading; it's +just that all binary distributions for Linux were built without this +support. See the file <code>README.Xfree3.3</code> in the LinuxThreads +distribution for patches and info on how to compile thread-safe X +libraries from the Xfree3.3 distribution. The Xfree3.3 sources are +readily available in most Linux distributions, e.g. as a source RPM +for RedHat. Be warned, however, that X Windows is a huge system, and +recompiling even just the libraries takes a lot of time and disk +space.<P> + +Another, less involving solution is to call X functions only from the +main thread of your program. Even if all threads have their own errno +location, the main thread uses the global errno variable for its errno +location. Thus, code not compiled with <code>-D_REENTRANT</code> +still "sees" the right error values if it executes in the main thread +only. <P> + +<H4><A NAME="I.2">This is a lot of work. Don't you have precompiled +thread-safe X libraries that you could distribute?</A></H4> + +No, I don't. Sorry. But you could approach the maintainers of +your Linux distribution to see if they would be willing to provide +thread-safe X libraries.<P> + +<H4><A NAME="I.3">I.3: Can I use library FOO in a multithreaded +program?</A></H4> + +Most libraries cannot be used "as is" in a multithreaded program. +For one thing, they are not necessarily thread-safe: calling +simultaneously two functions of the library from two threads might not +work, due to internal use of global variables and the like. Second, +the libraries must have been compiled with <CODE>-D_REENTRANT</CODE> to avoid +the errno problems explained in question <A HREF="#H.2">H.2</A>. +<P> + +<H4><A NAME="I.4">I.4: What if I make sure that only one thread calls +functions in these libraries?</A></H4> + +This avoids problems with the library not being thread-safe. But +you're still vulnerable to errno problems. At the very least, a +recompile of the library with <CODE>-D_REENTRANT</CODE> is needed. +<P> + +<H4><A NAME="I.5">I.5: What if I make sure that only the main thread +calls functions in these libraries?</A></H4> + +That might actually work. As explained in question <A HREF="#I.1">I.1</A>, +the main thread uses the global errno variable, and can therefore +execute code not compiled with <CODE>-D_REENTRANT</CODE>.<P> + +<H4><A NAME="I.6">I.6: SVGAlib doesn't work with LinuxThreads. Why? +</A></H4> + +Because both LinuxThreads and SVGAlib use the signals +<code>SIGUSR1</code> and <code>SIGUSR2</code>. One of the two should +be recompiled to use different signals. See question <A +HREF="#H.4">H.4</A>. +<P> + + +<HR> +<P> + +<H2><A NAME="J">J. Signals and threads</A></H2> + +<H4><A NAME="J.1">J.1: When it comes to signals, what is shared +between threads and what isn't?</A></H4> + +Signal handlers are shared between all threads: when a thread calls +<CODE>sigaction()</CODE>, it sets how the signal is handled not only +for itself, but for all other threads in the program as well.<P> + +On the other hand, signal masks are per-thread: each thread chooses +which signals it blocks independently of others. At thread creation +time, the newly created thread inherits the signal mask of the thread +calling <CODE>pthread_create()</CODE>. But afterwards, the new thread +can modify its signal mask independently of its creator thread.<P> + +<H4><A NAME="J.2">J.2: When I send a <CODE>SIGKILL</CODE> to a +particular thread using <CODE>pthread_kill</CODE>, all my threads are +killed!</A></H4> + +That's how it should be. The POSIX standard mandates that all threads +should terminate when the process (i.e. the collection of all threads +running the program) receives a signal whose effect is to +terminate the process (such as <CODE>SIGKILL</CODE> or <CODE>SIGINT</CODE> +when no handler is installed on that signal). This behavior makes a +lot of sense: when you type "ctrl-C" at the keyboard, or when a thread +crashes on a division by zero or a segmentation fault, you really want +all threads to stop immediately, not just the one that caused the +segmentation violation or that got the <CODE>SIGINT</CODE> signal. +(This assumes default behavior for those signals; see question +<A HREF="#J.3">J.3</A> if you install handlers for those signals.)<P> + +If you're trying to terminate a thread without bringing the whole +process down, use <code>pthread_cancel()</code>.<P> + +<H4><A NAME="J.3">J.3: I've installed a handler on a signal. Which +thread executes the handler when the signal is received?</A></H4> + +If the signal is generated by a thread during its execution (e.g. a +thread executes a division by zero and thus generates a +<CODE>SIGFPE</CODE> signal), then the handler is executed by that +thread. This also applies to signals generated by +<CODE>raise()</CODE>.<P> + +If the signal is sent to a particular thread using +<CODE>pthread_kill()</CODE>, then that thread executes the handler.<P> + +If the signal is sent via <CODE>kill()</CODE> or the tty interface +(e.g. by pressing ctrl-C), then the POSIX specs say that the handler +is executed by any thread in the process that does not currently block +the signal. In other terms, POSIX considers that the signal is sent +to the process (the collection of all threads) as a whole, and any +thread that is not blocking this signal can then handle it.<P> + +The latter case is where LinuxThreads departs from the POSIX specs. +In LinuxThreads, there is no real notion of ``the process as a whole'': +in the kernel, each thread is really a distinct process with a +distinct PID, and signals sent to the PID of a thread can only be +handled by that thread. As long as no thread is blocking the signal, +the behavior conforms to the standard: one (unspecified) thread of the +program handles the signal. But if the thread to which PID the signal +is sent blocks the signal, and some other thread does not block the +signal, then LinuxThreads will simply queue in +that thread and execute the handler only when that thread unblocks +the signal, instead of executing the handler immediately in the other +thread that does not block the signal.<P> + +This is to be viewed as a LinuxThreads bug, but I currently don't see +any way to implement the POSIX behavior without kernel support.<P> + +<H4><A NAME="J.3">J.3: How shall I go about mixing signals and threads +in my program? </A></H4> + +The less you mix them, the better. Notice that all +<CODE>pthread_*</CODE> functions are not async-signal safe, meaning +that you should not call them from signal handlers. This +recommendation is not to be taken lightly: your program can deadlock +if you call a <CODE>pthread_*</CODE> function from a signal handler! +<P> + +The only sensible things you can do from a signal handler is set a +global flag, or call <CODE>sem_post</CODE> on a semaphore, to record +the delivery of the signal. The remainder of the program can then +either poll the global flag, or use <CODE>sem_wait()</CODE> and +<CODE>sem_trywait()</CODE> on the semaphore.<P> + +Another option is to do nothing in the signal handler, and dedicate +one thread (preferably the initial thread) to wait synchronously for +signals, using <CODE>sigwait()</CODE>, and send messages to the other +threads accordingly. + +<H4><A NAME="J.4">J.4: When one thread is blocked in +<CODE>sigwait()</CODE>, other threads no longer receive the signals +<CODE>sigwait()</CODE> is waiting for! What happens? </A></H4> + +It's an unfortunate consequence of how LinuxThreads implements +<CODE>sigwait()</CODE>. Basically, it installs signal handlers on all +signals waited for, in order to record which signal was received. +Since signal handlers are shared with the other threads, this +temporarily deactivates any signal handlers you might have previously +installed on these signals.<P> + +Though surprising, this behavior actually seems to conform to the +POSIX standard. According to POSIX, <CODE>sigwait()</CODE> is +guaranteed to work as expected only if all other threads in the +program block the signals waited for (otherwise, the signals could be +delivered to other threads than the one doing <CODE>sigwait()</CODE>, +which would make <CODE>sigwait()</CODE> useless). In this particular +case, the problem described in this question does not appear.<P> + +One day, <CODE>sigwait()</CODE> will be implemented in the kernel, +along with others POSIX 1003.1b extensions, and <CODE>sigwait()</CODE> +will have a more natural behavior (as well as better performances).<P> + +<HR> +<P> + +<H2><A NAME="K">K. Internals of LinuxThreads</A></H2> + +<H4><A NAME="K.1">K.1: What is the implementation model for +LinuxThreads?</A></H4> + +LinuxThreads follows the so-called "one-to-one" model: each thread is +actually a separate process in the kernel. The kernel scheduler takes +care of scheduling the threads, just like it schedules regular +processes. The threads are created with the Linux +<code>clone()</code> system call, which is a generalization of +<code>fork()</code> allowing the new process to share the memory +space, file descriptors, and signal handlers of the parent.<P> + +Advantages of the "one-to-one" model include: +<UL> +<LI> minimal overhead on CPU-intensive multiprocessing (with +about one thread per processor); +<LI> minimal overhead on I/O operations; +<LI> a simple and robust implementation (the kernel scheduler does +most of the hard work for us). +</UL> +The main disadvantage is more expensive context switches on mutex and +condition operations, which must go through the kernel. This is +mitigated by the fact that context switches in the Linux kernel are +pretty efficient.<P> + +<H4><A NAME="K.2">K.2: Have you considered other implementation +models?</A></H4> + +There are basically two other models. The "many-to-one" model +relies on a user-level scheduler that context-switches between the +threads entirely in user code; viewed from the kernel, there is only +one process running. This model is completely out of the question for +me, since it does not take advantage of multiprocessors, and require +unholy magic to handle blocking I/O operations properly. There are +several user-level thread libraries available for Linux, but I found +all of them deficient in functionality, performance, and/or robustness. +<P> + +The "many-to-many" model combines both kernel-level and user-level +scheduling: several kernel-level threads run concurrently, each +executing a user-level scheduler that selects between user threads. +Most commercial Unix systems (Solaris, Digital Unix, IRIX) implement +POSIX threads this way. This model combines the advantages of both +the "many-to-one" and the "one-to-one" model, and is attractive +because it avoids the worst-case behaviors of both models -- +especially on kernels where context switches are expensive, such as +Digital Unix. Unfortunately, it is pretty complex to implement, and +requires kernel support which Linux does not provide. Linus Torvalds +and other Linux kernel developers have always been pushing the +"one-to-one" model in the name of overall simplicity, and are doing a +pretty good job of making kernel-level context switches between +threads efficient. LinuxThreads is just following the general +direction they set.<P> + +<H4><A NAME="K.3">K.3: I looked at the LinuxThreads sources, and I saw +quite a lot of spinlocks and busy-waiting loops to acquire these +spinlocks. Isn't this a big waste of CPU time?</A></H4> + +Look more carefully. Spinlocks are used internally to protect +LinuxThreads's data structures, but these locks are held for very +short periods of time: 10 instructions or so. The probability that a +thread has to loop busy-waiting on a taken spinlock for more than, +say, 100 cycles is very, very low. When a thread needs to wait on a +mutex, condition, or semaphore, it actually puts itself on a waiting +queue, then suspends on a signal, consuming no CPU time at all. The +thread will later be restarted by sending it a signal when the state +of the mutex, condition, or semaphore changes.<P> + +<HR> +<ADDRESS>Xavier.Leroy@inria.fr</ADDRESS> +</BODY> +</HTML> |