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 RPCs will be used instead. See GNU Mach's system calls.

In the GNU 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

IRC, freenode, #hurd, 2013-11-02

<Hiryu> how do system calls work in the hurd?
<Hiryu> is it like in linux
<braunr> yes and no
<Hiryu> you set the number in %eax and then in0x80?
<Hiryu> int, even
<braunr> no
<braunr> but that's really a detail
<Hiryu> I'm just curious how the flow goes
<braunr> gnumach uses call gates, not interrupt gates
<braunr> but that's just another way to enter the kernel
<braunr> the mechanism itself is almost irrelevant, it matters only for
<Hiryu> ah
<braunr> what's truely interesting is that there are very few system calls
<Hiryu> so it goes straight to gnumach, which then figures out where to
  relay the call to?
<braunr> the main one being mach_msg
<braunr> yes
<braunr> one of the arguments to mach_msg is the name of a right
<braunr> (as file descriptors are names for open files)
<Hiryu> hmm okay, so we get to the kernel, go to a kernel server (one
  context switch), then get back to the calling process, and that's 2
  context switches per system call?
<braunr> a receive right or send right
<braunr> no
<braunr> if you send-recv, it's only one
<braunr> but i'm not sure we do that on the hurd
<braunr> again, that's a detail
<Hiryu> what is send-recv?
<braunr> send and receive in the same system call
<Hiryu> hmm
<braunr> then, we also use system calls to denote the unix-like RPCs of the
<braunr> i mean
<braunr> i also call them system calls
<braunr> for example, read, write, stat, etc..
<Hiryu> I see
<Hiryu> braunr: thanks