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Are Linux threads the same as other implementations?

    No. They are better -- while mostly keeping the same API. As stated above, most multithreaded OSs define a thread separately from processes. Linus Torvalds has defined that a thread is a "context of execution" (COE). This means that only one process/thread table and one scheduler is needed. Also the scheduler has been optomized so that the switching time for threads vs. tasks varies little--about 1.7us (threads) and 1.8us (fork) on a 75MHz Pentium.

    Traditionally, a thread was just a CPU (and some other minimal state) state with the process containing the remains (data, stack, I/O, signals). This would lend itself to very fast switching but would cause basic problems (e.g. what do "fork()" or "execve()" calls mean when executed by a thread?).

    Consider Linux threads as a superset of this functionality: they still can switch fast and share process parts, but they can also identify what parts get shared and have no problems with execve() calls. There are four flags that determine the level of sharing:

    Clone Flag Values

    #define CLONE_VM 0x00000100
    #define CLONE_FS 0x00000200
    #define CLONE_FILES 0x00000400
    #define CLONE_SIGHAND 0x00000800
    #define CLONE_PID /* not fully implemented */

    There has been a lot of talk about "clone()". The system call (please note: "low level") clone() is an extension to fork(). In fact, clone(0) == fork(). But with the above #define's, any combination of the VM, filesystem, I/O, signal handlers and process ID may be shared.


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