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.\"     @(#)syscall.2	8.1 (Berkeley) 6/16/93
.\"
.Dd June 16, 1993
.Dt SYSCALL 2
.Os
.Sh NAME
.Nm syscall ,
.Nm __syscall
.Nd indirect system call
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/syscall.h
.In unistd.h
.Ft int
.Fn syscall "int number" "..."
.Ft quad_t
.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
.Ao Pa sys/syscall.h Ac .
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, 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
.Fn syscall
function call appeared in
.Bx 4.0 .
.Sh BUGS
There is no way to simulate system calls that have multiple return values
such as
.Xr pipe 2 .
.Pp
Due to ABI implementation differences in passing struct or union
type arguments to system calls between different processors, all
system calls pass instead pointers to such structs or unions, even
when the documentation of the system call mentions otherwise.
The conversion between passing structs and unions is handled normally
via userland stubs.
The correct arguments for the kernel entry
points for each system call can be found in the header file
.Ao Pa sys/syscallargs.h Ac