NAME
syscall,
__syscall —
indirect system call
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/syscall.h>
#include <unistd.h>
int
syscall(
int
number,
...);
quad_t
__syscall(
quad_t
number,
...);
DESCRIPTION
syscall() performs the system call whose assembly language
interface has the specified
number with the specified
arguments. Symbolic constants for system calls can be found in the header file
⟨
sys/syscall.h⟩. The
__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. It should not be used in normal applications.
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
errno.
HISTORY
The
syscall() function call appeared in
4.0BSD.
BUGS
There is no way to simulate system calls that have multiple return values such
as
pipe(2).
Since architectures return 32 bit and 64 bit results in different registers, it
may be impossible to portably convert the result of
__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.
Many architectures mask off the unwanted high bits of the syscall number, rather
than returning an error.
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
⟨
sys/syscallargs.h⟩