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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
      actually)