debian man page for syscall

SYSCALL(2)						     Linux Programmer's Manual							SYSCALL(2)

NAME
       syscall - indirect system call

SYNOPSIS
       #define _GNU_SOURCE	   /* See feature_test_macros(7) */
       #include <unistd.h>
       #include <sys/syscall.h>   /* For SYS_xxx definitions */

       int syscall(int number, ...);

DESCRIPTION
       syscall()  is  a  small	library  function that invokes the system call whose assembly language interface has the specified number with the
       specified arguments.  Employing syscall() is useful, for example, when invoking a system call  that  has  no  wrapper  function	in  the  C
       library.

       syscall()  saves CPU registers before making the system call, restores the registers upon return from the system call, and stores any error
       code returned by the system call in errno(3) if an error occurs.

       Symbolic constants for system call numbers can be found in the header file <sys/syscall.h>.

RETURN VALUE
       The return value is 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.

NOTES
       syscall() first appeared in 4BSD.

EXAMPLE
       #define _GNU_SOURCE
       #include <unistd.h>
       #include <sys/syscall.h>
       #include <sys/types.h>

       int
       main(int argc, char *argv[])
       {
	   pid_t tid;

	   tid = syscall(SYS_gettid);
	   tid = syscall(SYS_tgkill, getpid(), tid);
       }

SEE ALSO
       _syscall(2), intro(2), syscalls(2)

COLOPHON
       This  page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-08-14								SYSCALL(2)