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In an [[UNIX]]-like system, a *system call* (*syscall*) is used to request all
kinds of functionality from the operating system kernel.
A [[microkernel]]-based system typically won't offer a lot of system calls --
apart from one central one, and that is *send message* -- but instead [[RPC]]s
will be used instead.
See [[GNU Mach's system calls|microkernel/mach/gnumach/interface/syscall]].
In the [[GNU Hurd|hurd]], a lot of what is traditionlly considered to be a UNIX
system call is implemented (primarily by means of [[RPC]]) inside [[glibc]].
# IRC, freenode, #hurd, 2013-06-15
<braunr> true system calls are always implemented the same way, by the
kernel, using traps or specialized instructions that enable crossing from
user to kernel space
<braunr> glibc simply translates function calls to system calls by packing
arguments appropriately and using that trap or syscall instruction
<braunr> on microkernel based systems however, true system calls are
normally used only for IPC
<braunr> so we also use the term syscall to refer to those RPCs that
provide system services
<braunr> e.G. open() is a call to a file system server (and maybe several