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| author | Roland McGrath <roland@gnu.org> | 2005-07-03 04:40:34 +0000 |
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| committer | Roland McGrath <roland@gnu.org> | 2005-07-03 04:40:34 +0000 |
| commit | 88e3fbcb5d6829f8980d6bb356b1c0cd7a38187b (patch) | |
| tree | 41b73f55c91439f87d82e5d32b9d8e46e82ce3c1 /linuxthreads/FAQ.html | |
| parent | d19b1b4f2ff49e085b55e8564b52ca17b028e335 (diff) | |
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diff --git a/linuxthreads/FAQ.html b/linuxthreads/FAQ.html deleted file mode 100644 index 21be33e..0000000 --- a/linuxthreads/FAQ.html +++ /dev/null @@ -1,1039 +0,0 @@ -<HTML> -<HEAD> -<TITLE>LinuxThreads Frequently Asked Questions</TITLE> -</HEAD> -<BODY> -<H1 ALIGN=center>LinuxThreads Frequently Asked Questions <BR> - (with answers)</H1> -<H2 ALIGN=center>[For LinuxThreads version 0.8]</H2> - -<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="C">C</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> - -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, -as well as a number of optional extensions, are provided and conform -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 used to be some rough edges in -the LinuxThreads / C library interface with libc 5, but glibc 2 -fixes all of those problems and is now the standard C library on major -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> - -<H4><A NAME="B.4">B.4: How should I report a possible bug in -LinuxThreads?</A></H4> - -If you're using glibc 2, the best way by far is to use the -<code>glibcbug</code> script to mail a bug report to the glibc -maintainers. <P> - -If you're using an older libc, or don't have the <code>glibcbug</code> -script on your machine, then e-mail me directly -(<code>Xavier.Leroy@inria.fr</code>). <P> - -In both cases, before sending the bug report, make sure that it is not -addressed already in this FAQ. Also, try to send a short program that -reproduces the weird behavior you observed. <P> - -<H4><A NAME="B.5">B.5: 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> - -The best choice by far is glibc 2, a.k.a. libc 6. It offers very good -support for multi-threading, and LinuxThreads has been closely -integrated with glibc 2. The glibc 2 distribution contains the -sources of a specially adapted version of LinuxThreads.<P> - -glibc 2 comes preinstalled as the default C library on several Linux -distributions, such as RedHat 5 and up, and Debian 2. -Those distributions include the version of LinuxThreads matching -glibc 2.<P> - -<H4><A NAME="C.2">C.2: My system has libc 5 preinstalled, not glibc -2. Can I still use LinuxThreads?</H4> - -Yes, but you're likely to run into some problems, as libc 5 only -offers minimal support for threads and contains some bugs that affect -multithreaded programs. <P> - -The versions of libc 5 that work best with LinuxThreads are -libc 5.2.18 on the one hand, and libc 5.4.12 or later on the other hand. -Avoid 5.3.12 and 5.4.7: these have problems with the per-thread errno -variable. <P> - -<H4><A NAME="C.3">C.3: So, should I switch to glibc 2, or stay with a -recent libc 5?</A></H4> - -I'd recommend you switch to glibc 2. Even for single-threaded -programs, glibc 2 is more solid and more standard-conformant than libc -5. And the shortcomings of libc 5 almost preclude any serious -multi-threaded programming.<P> - -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. Much easier is (re-)installing a -Linux distribution based on glibc 2, such as RedHat 6.<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> - -<H4><A NAME="C.5">C.5: Where can I find libc 5 and the version of -LinuxThreads that goes with it?</A></H4> - -For libc 5, see <A HREF="ftp://sunsite.unc.edu/pub/Linux/devel/GCC/"><code>ftp://sunsite.unc.edu/pub/Linux/devel/GCC/</code></A>.<P> - -For the libc 5 version of LinuxThreads, see -<A HREF="ftp://ftp.inria.fr/INRIA/Projects/cristal/Xavier.Leroy/linuxthreads/">ftp://ftp.inria.fr/INRIA/Projects/cristal/Xavier.Leroy/linuxthreads/</A>.<P> - -<H4><A NAME="C.6">C.6: How can I recompile the glibc 2 version of the -LinuxThreads sources?</A></H4> - -You must transfer the whole glibc sources, then drop the LinuxThreads -sources in the <code>linuxthreads/</code> subdirectory, then recompile -glibc as a whole. There are now too many inter-dependencies between -LinuxThreads and glibc 2 to allow separate re-compilation of LinuxThreads. -<P> - -<H4><A NAME="C.7">C.7: What is the correspondence between LinuxThreads -version numbers, libc version numbers, and RedHat version -numbers?</A></H4> - -Here is a summary. (Information on Linux distributions other than -RedHat are welcome.)<P> - -<TABLE> -<TR><TD>LinuxThreads </TD> <TD>C library</TD> <TD>RedHat</TD></TR> -<TR><TD>0.7, 0.71 (for libc 5)</TD> <TD>libc 5.x</TD> <TD>RH 4.2</TD></TR> -<TR><TD>0.7, 0.71 (for glibc 2)</TD> <TD>glibc 2.0.x</TD> <TD>RH 5.x</TD></TR> -<TR><TD>0.8</TD> <TD>glibc 2.1.1</TD> <TD>RH 6.0</TD></TR> -<TR><TD>0.8</TD> <TD>glibc 2.1.2</TD> <TD>not yet released</TD></TR> -</TABLE> -<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 libc 5 library. (See question <A -HREF="#C.2">C.2</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.2">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 creates a lot of threads, and after -a while <CODE>pthread_create()</CODE> no longer returns!</A></H4> - -This is known bug in the version of LinuxThreads that comes with glibc -2.1.1. An upgrade to 2.1.2 is recommended. <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> - -That behavior has mostly disappeared in recent releases of -LinuxThreads (version 0.8 and up). It was fairly common in older -releases, though. - -What happens in LinuxThreads 0.7 and before 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> - -In LinuxThreads 0.8 and up, <code>pthread_unlock</code> restarts only -one waiting thread, and pre-assign the mutex to that thread. Hence, -if the thread that unlocked the mutex tries to lock it again -immediately, it will block until other waiting threads have had a -chance to lock and unlock the mutex. This results in much fairer -scheduling.<P> - -Notice however that even the old "unfair" behavior is perfectly -acceptable 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". 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> - -Again, this behavior is characteristic of old releases of LinuxThreads -(0.7 and before); more recent versions (0.8 and up) should not exhibit -this behavior.<P> - -The reason for this behavior 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> - -<H4><A NAME="D.9">D.9: I tried to use real-time threads and my program -loops like crazy and freezes the whole machine!</A></H4> - -Versions of LinuxThreads prior to 0.8 are susceptible to ``livelocks'' -(one thread loops, consuming 100% of the CPU time) in conjunction with -real-time scheduling. Since real-time threads and processes have -higher priority than normal Linux processes, all other processes on -the machine, including the shell, the X server, etc, cannot run and -the machine appears frozen.<P> - -The problem is fixed in LinuxThreads 0.8.<P> - -<H4><A NAME="D.10">D.10: My application needs to create thousands of -threads, or maybe even more. Can I do this with -LinuxThreads?</A></H4> - -No. You're going to run into several hard limits: -<UL> -<LI>Each thread, from the kernel's standpoint, is one process. Stock -Linux kernels are limited to at most 512 processes for the super-user, -and half this number for regular users. This can be changed by -changing <code>NR_TASKS</code> in <code>include/linux/tasks.h</code> -and recompiling the kernel. On the x86 processors at least, -architectural constraints seem to limit <code>NR_TASKS</code> to 4090 -at most. -<LI>LinuxThreads contains a table of all active threads. This table -has room for 1024 threads at most. To increase this limit, you must -change <code>PTHREAD_THREADS_MAX</code> in the LinuxThreads sources -and recompile. -<LI>By default, each thread reserves 2M of virtual memory space for -its stack. This space is just reserved; actual memory is allocated -for the stack on demand. But still, on a 32-bit processor, the total -virtual memory space available for the stacks is on the order of 1G, -meaning that more than 500 threads will have a hard time fitting in. -You can overcome this limitation by moving to a 64-bit platform, or by -allocating smaller stacks yourself using the <code>setstackaddr</code> -attribute. -<LI>Finally, the Linux kernel contains many algorithms that run in -time proportional to the number of process table entries. Increasing -this number drastically will slow down the kernel operations -noticeably. -</UL> -(Other POSIX threads libraries have similar limitations, by the way.) -For all those reasons, you'd better restructure your application so -that it doesn't need more than, say, 100 threads. For instance, -in the case of a multithreaded server, instead of creating a new -thread for each connection, maintain a fixed-size pool of worker -threads that pick incoming connection requests from a queue.<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: Does LinuxThreads implement -<CODE>pthread_attr_setstacksize()</CODE> and -<CODE>pthread_attr_setstackaddr()</CODE>?</A></H4> - -These optional functions are provided in recent versions of -LinuxThreads (0.8 and up). Earlier releases did not provide these -optional components of the POSIX standard.<P> - -Even if <CODE>pthread_attr_setstacksize()</CODE> and -<CODE>pthread_attr_setstackaddr()</CODE> are now provided, we still -recommend that you do not use them unless you really have strong -reasons for doing so. The default stack allocation strategy for -LinuxThreads is nearly optimal: stacks start small (4k) and -automatically grow on demand to a fairly large limit (2M). -Moreover, there is no portable way to estimate the stack requirements -of a thread, so setting the stack size yourself makes your program -less reliable and non-portable.<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> - -Yes, but not with the stock gdb 4.17. You need a specially patched -version of gdb 4.17 developed by Eric Paire and colleages at The Open -Group, Grenoble. The patches against gdb 4.17 are available at -<A HREF="http://www.gr.opengroup.org/java/jdk/linux/debug.htm"><code>http://www.gr.opengroup.org/java/jdk/linux/debug.htm</code></A>. -Precompiled binaries of the patched gdb are available in RedHat's RPM -format at <A -HREF="http://odin.appliedtheory.com/"><code>http://odin.appliedtheory.com/</code></A>.<P> - -Some Linux distributions provide an already-patched version of gdb; -others don't. For instance, the gdb in RedHat 5.2 is thread-aware, -but apparently not the one in RedHat 6.0. Just ask (politely) the -makers of your Linux distributions to please make sure that they apply -the correct patches to gdb.<P> - -<H4><A NAME="G.2">G.2: Does it work with post-mortem debugging?</A></H4> - -Not very well. Generally, 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.3">G.3: Any other ways to 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 subtle 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> - -The short answer is: because the Linux kernel you're using does not -support realtime signals. <P> - -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.<P> - -On ``old'' kernels (2.0 and early 2.1 kernels), there are only 32 -signals available and the kernel reserves all of them but two: -<code>SIGUSR1</code> and <code>SIGUSR2</code>. So, LinuxThreads has -no choice but use those two signals.<P> - -On recent kernels (2.2 and up), more than 32 signals are provided in -the form of realtime signals. When run on one of those kernels, -LinuxThreads uses two reserved realtime signals for its internal -operation, thus leaving <code>SIGUSR1</code> and <code>SIGUSR2</code> -free for user code. (This works only with glibc, not with libc 5.) <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 use X libraries that have been compiled with -multithreading options set. Linux distributions that come with glibc -2 as the main C library generally provide thread-safe X libraries. -At least, that seems to be the case for RedHat 5 and later.<P> - -You can try to recompile yourself the X libraries with multithreading -options set. They contain optional support for multithreading; it's -just that the binaries provided by your Linux distribution 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 consider installing a Linux distribution -that comes with thread-safe X libraries, such as RedHat 6.<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>. 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> - -<HR> -<ADDRESS>Xavier.Leroy@inria.fr</ADDRESS> -</BODY> -</HTML> |