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.\"     @(#)syscall.2	8.1 (Berkeley) 6/16/93
.\"
.Dd $Mdocdate: May 30 2016 $
.Dt SYSCALL 2
.Os
.Sh NAME
.Nm syscall ,
.Nm __syscall
.Nd indirect system call
.Sh SYNOPSIS
.In sys/syscall.h
.In unistd.h
.Ft int
.Fn syscall "int number" "..."
.Fn __syscall "quad_t number" "..."
.Sh DESCRIPTION
.Fn syscall
performs the system call whose assembly language
interface has the specified
.Fa number
with the specified arguments.
Symbolic constants for system calls can be found in the header file
.In sys/syscall.h .
.Pp
Since different system calls have different return types, a
prototype of
.Nm __syscall
specifying the correct return type should be declared locally.
This is especially important for system calls returning
larger-than-int results.
.Pp
The
.Nm __syscall
form should be used when one or more of the parameters is a
64-bit argument to ensure that argument alignment is correct.
This system call is useful for testing new system calls that
do not have entries in the C library.
.Sh RETURN VALUES
The return values are defined by the system call being invoked.
In general, for system calls returning
.Va int ,
a 0 return value indicates success.
A \-1 return value indicates an error,
and an error code is stored in
.Va errno .
.Sh HISTORY
The predecessor of these functions, the former
.Fn indir
system call, first appeared in
.At v4 .
The
.Fn syscall
function first appeared in
.Bx 3 .
.Sh BUGS
This would all be easier if the layout of structures with long long
members matched how long long arguments were aligned on the stack
for syscall arguments.
They don't match for some ILP32 archs so explicit padding is necessary
for consistent handling.