How to allocate a new TLS area with clone system call
linux, pthreads, system-calls
Solution
I am working on a research project and for something I am experimenting with I want to create threads using the clone system call instead of using pthread_create
In the exceedingly unlikely scenario where your new thread never calls any libc functions (either directly, or by calling something else which calls libc; this also includes dynamic symbol resolution via PLT), then you can pass whatever TLS storage you desire as the the new_tls parameter to `clone`.
You should ignore all references to `set_thread_area` -- they only apply to 32-bit/ix86 case.
If you are planning to use libc in your newly-created thread, you should abandon your approach: `libc` expects TLS to be set up a certain way, and there is no way for you to arrange for such setup when you call `clone` directly. Your new thread will intermittently crash when `libc` discovers that you didn't set up TLS properly. Debugging such crashes is exceedingly difficult, and the only reliable solution is ... to use `pthread_create`.
Problem
Short version of question: What parameter do I need to pass to the `clone` system call on x86_64 Linux system if I want to allocate a new TLS area for the thread that I am creating. Long version: I am working on a research project and for something I am experimenting with I want to create threads using the `clone` system call instead of using `pthread_create`. However, I also want to be able to use thread local storage. I don't plan on creating many threads right now, so it would be fine for me to create a new TLS area for each thread that I create with the clone system call. I was looking at the man page for `clone` and it has the following information about the flag for the TLS parameter: ``` CLONE_SETTLS (since Linux 2.5.32) The newtls argument is the new TLS (Thread Local Storage) descriptor. (See set_thread_area(2).) ``` So I looked at the man page for `set_thread_area` and noticed the following which looked promising: ``` When set_thread_area() is passed an entry_number of -1, it uses a free TLS entry. If set_thread_area() finds a free TLS entry, the value of u_info->entry_number is set upon return to show which entry was changed. ``` However, after experimenting with this some it appears that `set_thread_area` is not implemented in my system (Ubunut 10.04 on an x86_64 platform). When I run the following code I get an error that says: `set_thread_area() failed: Function not implemented` ``` #include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <errno.h> #include <sys/syscall.h> #include <sys/types.h> #include <linux/unistd.h> #include <asm/ldt.h> int main() { struct user_desc u_info; u_info.entry_number = -1; int rc = syscall(SYS_set_thread_area,&u_info); if(rc < 0) { perror("set_thread_area() failed"); exit(-1); } printf("entry_number is %d",u_info.entry_number); } ``` I also saw that when I use strace the see what happens when `pthread_create` is called that I don't see any calls to `set_thread_area`. I have also been looking at the nptl pthread source code to try to understand what they do when creating threads. But I don't completely understand it yet and I think it is more complex than what I'm trying to do since I don't need something that is as robust at the pthread implementation. I'm assuming that the `set_thread_area` system call is for x86 and that there is a different mechanism used for `x86_64`. But for the moment I have not been able to figure out what it is so I'm hoping this question will help me get some ideas about what I need to look at.