what happens if
what happens if i am running a multithreaded app ( 3 threads ) and one thread calls fork() ? ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
Hi Erez,
On Mon, Mar 19, 2012 at 10:47:20AM +0200, Erez D wrote:
what happens if i am running a multithreaded app ( 3 threads ) and one
thread calls fork() ?
See pthread_atfork(3).
baruch
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Re: what happens if
FTFM: The child process is created with a single thread - the one that called fork(). 2012/3/19 Erez D erez0...@gmail.com what happens if i am running a multithreaded app ( 3 threads ) and one thread calls fork() ? ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il -- Ubi dubium, ibi libertas (where there is doubt, there is freedom) ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012, Erez D wrote about what happens if: what happens if i am running a multithreaded app ( 3 threads ) and one thread calls fork() ? On Linux, the new process will run ONLY a copy of the thread doing the fork(). The other threads are *not* copied to the child process. There is no way in Linux to ask to duplicate all of the process's threads, as in Solaris's forkall(2) system call. Even if such a system call existed, its usefulness would be dubious as explained in Posix's fork(2) manual: http://pubs.opengroup.org/onlinepubs/009695399/functions/fork.html (look for the section starting in The addition of the forkall() function to the standard was considered and rejected.) All this makes fork() more or less useless for multithreaded applications except for one idiom: fork() followed by exec(). -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |This space is for sale - inquire inside. http://nadav.harel.org.il | ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012 at 11:40 AM, Nadav Har'El n...@math.technion.ac.ilwrote: On Mon, Mar 19, 2012, Erez D wrote about what happens if: what happens if i am running a multithreaded app ( 3 threads ) and one thread calls fork() ? On Linux, the new process will run ONLY a copy of the thread doing the fork(). The other threads are *not* copied to the child process. There is no way in Linux to ask to duplicate all of the process's threads, as in Solaris's forkall(2) system call. Even if such a system call existed, its usefulness would be dubious as explained in Posix's fork(2) manual: http://pubs.opengroup.org/onlinepubs/009695399/functions/fork.html (look for the section starting in The addition of the forkall() function to the standard was considered and rejected.) All this makes fork() more or less useless for multithreaded applications except for one idiom: fork() followed by exec(). my reason for fork is only for exec() - to protect the first program from the other. the problem that may arise in forking a multithreaded is that one thread may lock a mutex and then another call for fork, which means that the child inherits the locked mutex. i do not know internaly what library function use this mutex, e.g. will close() need this mutex ? will dup() will ? in this case, trying to close a side of a pipe, duping it or even closing open fds (for security issues) may hang the child while the library waits for the mutex , and no other thread exists in the child to release it a only way to resolve it, is to fork to another process before generating the other threads, the second process will be left single threaded and so may fork() with no problems, but this way i can't use pipe(), and need to revert to named pipe which is ugly any pther ideas ? things i missed ? -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |This space is for sale - inquire inside. http://nadav.harel.org.il | ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012, Erez D wrote about Re: what happens if: my reason for fork is only for exec() - to protect the first program from the other. Then you should be all fine. the problem that may arise in forking a multithreaded is that one thread may lock a mutex and then another call for fork, which means that the child inherits the locked mutex. i do not know internaly what library function use this mutex, e.g. will close() need this mutex ? will dup() will ? in this case, trying to close a side of a pipe, duping it or even closing open fds (for security issues) may hang the child while the library waits for the mutex , and no other thread exists in the child to release it If you're talking about the C language, you won't have any of these problems. System calls like close() or dup() do not use any pthread capabilities like mutexes. If you're talking about a different programming language, I frankly have no idea what its fork() does. Maybe you need to write some C code that does fork()/exec() in one function call, and avoid this risk. Did actually notice any problem with your code, or is this question just theoretical? -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |If God had intended us to be vegetarians, http://nadav.harel.org.il |He wouldn't have made animals out of meat ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012 at 12:20 PM, Nadav Har'El n...@math.technion.ac.ilwrote: On Mon, Mar 19, 2012, Erez D wrote about Re: what happens if: my reason for fork is only for exec() - to protect the first program from the other. Then you should be all fine. the problem that may arise in forking a multithreaded is that one thread may lock a mutex and then another call for fork, which means that the child inherits the locked mutex. i do not know internaly what library function use this mutex, e.g. will close() need this mutex ? will dup() will ? in this case, trying to close a side of a pipe, duping it or even closing open fds (for security issues) may hang the child while the library waits for the mutex , and no other thread exists in the child to release it If you're talking about the C language, you won't have any of these problems. System calls like close() or dup() do not use any pthread capabilities like mutexes. If you're talking about a different programming language, I frankly have no idea what its fork() does. Maybe you need to write some C code that does fork()/exec() in one function call, and avoid this risk. i will be using C++. are you sure this is safe ? Did actually notice any problem with your code, or is this question just theoretical? theoretical -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |If God had intended us to be vegetarians, http://nadav.harel.org.il |He wouldn't have made animals out of meat ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012, Erez D wrote about Re: what happens if: If you're talking about the C language, you won't have any of these problems. System calls like close() or dup() do not use any pthread capabilities like mutexes. .. i will be using C++. are you sure this is safe ? I'm sure. But don't sue me if I'm wrong ;-) -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |Disclaimer: The opinions expressed above http://nadav.harel.org.il |are not my own. ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On Mon, Mar 19, 2012 at 12:25 PM, Nadav Har'El n...@math.technion.ac.ilwrote: On Mon, Mar 19, 2012, Erez D wrote about Re: what happens if: If you're talking about the C language, you won't have any of these problems. System calls like close() or dup() do not use any pthread capabilities like mutexes. .. i will be using C++. are you sure this is safe ? I'm sure. But don't sue me if I'm wrong ;-) I wont thanks -- Nadav Har'El|Monday, Mar 19 2012, n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |Disclaimer: The opinions expressed above http://nadav.harel.org.il |are not my own. ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: what happens if
On יום שני 19 מרץ 2012 12:22:15 Erez D wrote: On Mon, Mar 19, 2012 at 12:20 PM, Nadav Har'El n...@math.technion.ac.ilwrote: On Mon, Mar 19, 2012, Erez D wrote about Re: what happens if: my reason for fork is only for exec() - to protect the first program from the other. Then you should be all fine. Another reason is for interacting with libraries which are singleton. I had to work with a library to talk to some special HW. The library was designed to work with one device per process, and I needed to support many. I found myself forking and using pipe() to use the API from my main app. The problem is that that fork() was called from a thread. Read ahead. If you're talking about the C language, you won't have any of these problems. System calls like close() or dup() do not use any pthread capabilities like mutexes. If you're talking about a different programming language, I frankly have no idea what its fork() does. Maybe you need to write some C code that does fork()/exec() in one function call, and avoid this risk. i will be using C++. are you sure this is safe ? All theory I read before implementing this said this was a bad idea. The theory says that libc may maintain some mutex inside malloc() which is called from printf() for example. This means that even trivial things may kill your app. The theory says that in multithreaded applications as soon as you clone() (the system called used by pthread_create()) you should execvp. In my application (a lot of C++, running on linux 2.6.32, glibc 2.9 and glibc 2.11.1 on ARM) erverything worked perfectly against the theory, your mileage may vary. ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
