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.\"     @(#)syscall.2	8.1 (Berkeley) 6/16/93
.\"
.Dd August 7, 2009
.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.
.Pp
This system call is useful for testing new system calls that
do not have entries in the C library.
It should not be used in normal applications.
.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
Since architectures return 32 bit and 64 bit results in different registers,
it may be impossible to portably convert the result of
.Fn __syscall
to a 32bit value.
For instance sparc returns 32 bit values in %o0 and 64 bit values in %o0:%o1
(with %o0 containing the most significant part) so a 32 bit right shift
of the result is needed to get a correct 32 bit result.
.Pp
Many architectures mask off the unwanted high bits of the syscall number,
rather than returning an error.
.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