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copied!<p>The underlying system call to create threads is <a href="http://man7.org/linux/man-pages/man2/clone.2.html" rel="nofollow noreferrer">clone(2)</a> (it is Linux specific). BTW, the list of Linux system calls is on <a href="http://man7.org/linux/man-pages/man2/syscalls.2.html" rel="nofollow noreferrer">syscalls(2)</a>, and you could use the <a href="http://man7.org/linux/man-pages/man1/strace.1.html" rel="nofollow noreferrer">strace(1)</a> command to understand the syscalls done by some process or command. Processes are usually created with <a href="http://man7.org/linux/man-pages/man2/fork.2.html" rel="nofollow noreferrer">fork(2)</a> (or <a href="http://man7.org/linux/man-pages/man2/vfork.2.html" rel="nofollow noreferrer">vfork(2)</a>, which is not much useful these days). However, you could (and some C standard libraries might do that) create them with some particular form of <code>clone</code>. I guess that the kernel is sharing some code to implement <code>clone</code>, <code>fork</code> etc... (since some functionalities, e.g. management of the <a href="https://en.wikipedia.org/wiki/Virtual_address_space" rel="nofollow noreferrer">virtual address space</a>, are common).</p> <p>Indeed, process creation (and also thread creation) is generally quite fast on most Unix systems (because they use <a href="http://en.wikipedia.org/wiki/Copy-on-write" rel="nofollow noreferrer">copy-on-write</a> machinery for the <a href="http://en.wikipedia.org/wiki/Virtual_memory" rel="nofollow noreferrer">virtual memory</a>), typically a small fraction of a millisecond. But you could have pathological cases (e.g. <a href="https://en.wikipedia.org/wiki/Thrashing_(computer_science)" rel="nofollow noreferrer">thrashing</a>) which makes that much longer.</p> <p>Since most <a href="https://en.wikipedia.org/wiki/C_standard_library" rel="nofollow noreferrer">C standard library</a> implementations are <a href="https://en.wikipedia.org/wiki/Free_software" rel="nofollow noreferrer">free software</a> on Linux, you could study the source code of the one on your system (often <a href="https://www.gnu.org/software/libc/" rel="nofollow noreferrer">GNU glibc</a>, but sometimes <a href="http://musl-libc.org/" rel="nofollow noreferrer">musl-libc</a> or something else).</p>

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