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copied!<p>You first should understand what is the role of the <a href="http://en.wikipedia.org/wiki/Linux_kernel" rel="nofollow noreferrer">linux kernel</a>, and that applications interact with the kernel <em>only</em> thru <a href="http://en.wikipedia.org/wiki/System_call" rel="nofollow noreferrer">system calls</a>.</p> <p>In effect, an application runs on the "virtual machine" provided by the kernel: it is running in the <a href="http://en.wikipedia.org/wiki/User_space" rel="nofollow noreferrer">user space</a> and can only do (at the lowest machine level) the set of machine instructions permitted in <a href="http://en.wikipedia.org/wiki/CPU_modes" rel="nofollow noreferrer">user CPU mode</a> augmented by the instruction (e.g. <code>SYSENTER</code> or <code>INT 0x80</code> ...) used to make system calls. So, from the user-level application point of view, a syscall is an atomic pseudo machine instruction.</p> <p>The <a href="http://tldp.org/HOWTO/Assembly-HOWTO/" rel="nofollow noreferrer">Linux Assembly Howto</a> explains how a syscall can be done at the assembly (i.e. machine instruction) level.</p> <p>The <a href="http://www.gnu.org/software/libc/" rel="nofollow noreferrer">GNU libc</a> is providing C functions corresponding to the syscalls. So for example the <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/open.2.html" rel="nofollow noreferrer">open</a> function is a tiny glue (i.e. a wrapper) above the syscall of number <code>NR__open</code> (it is making the syscall then updating <code>errno</code>). Application usually call such C functions in libc instead of doing the syscall.</p> <p><sup>You could use some other <code>libc</code>. For instance the <a href="http://musl-libc.org/" rel="nofollow noreferrer">MUSL libc</a> is somhow "simpler" and its code is perhaps easier to read. It also is wrapping the raw syscalls into corresponding C functions.</sup></p> <p>If you add your own syscall, you better also implement a similar C function (in your own library). So you should have also a header file for your library.</p> <p>See also <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/intro.2.html" rel="nofollow noreferrer">intro(2)</a> and <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/syscall.2.html" rel="nofollow noreferrer">syscall(2)</a> and <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/syscalls.2.html" rel="nofollow noreferrer">syscalls(2)</a> man pages, and the role of <a href="http://davisdoesdownunder.blogspot.fr/2011/02/linux-syscall-vsyscall-and-vdso-oh-my.html" rel="nofollow noreferrer">VDSO in syscalls</a>.</p> <p>Notice that <a href="http://syscalls.kernelgrok.com/" rel="nofollow noreferrer">syscalls</a> are not C functions. They don't use the call stack (they could even be invoked without any stack). A syscall is basically a number like <code>NR__open</code> from <code><asm/unistd.h></code>, a <code>SYSENTER</code> machine instruction with conventions about which registers hold before the arguments to the syscall and which ones hold after the result[s] of the syscall (including the failure result, to set <code>errno</code> in the C library wrapping the syscall). The conventions for syscalls are not the calling conventions for C functions in the ABI spec (e.g. <a href="https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI" rel="nofollow noreferrer">x86-64 psABI</a>). So you need a C wrapper.</p>

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