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FreeBSD 11.0 - man page for __syscall (freebsd section 2)

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