freebsd man page for __syscall

SYSCALL(2)						      BSD System Calls Manual							SYSCALL(2)

NAME
     syscall, __syscall -- indirect system call

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/syscall.h>
     #include <unistd.h>

     int
     syscall(int number, ...);

     off_t
     __syscall(quad_t number, ...);

DESCRIPTION
     The syscall() function 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 arguments 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.

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 appeared in 4.0BSD.

BUGS
     There is no way to simulate system calls that have multiple return values such as pipe(2).

BSD
								   June 16, 1993							       BSD