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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 */

       long syscall(long 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 gen-
       eral, 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.

   Architecture-specific requirements
       Each architecture ABI has its own requirements on how system call argu-
       ments are passed	to the kernel.	For system calls  that	have  a	 glibc
       wrapper (e.g., most system calls), glibc	handles	the details of copying
       arguments to the	right registers	in a manner suitable for the architec-
       ture.   However,	when using syscall() to	make a system call, the	caller
       might need to handle architecture-dependent details;  this  requirement
       is most commonly	encountered on certain 32-bit architectures.

       For  example,  on  the  ARM  architecture Embedded ABI (EABI), a	64-bit
       value (e.g., long long) must be	aligned	 to  an	 even  register	 pair.
       Thus,  using  syscall()	instead	 of the	wrapper	provided by glibc, the
       readahead() system call would be	invoked	as follows on the  ARM	archi-
       tecture with the	EABI:

	   syscall(SYS_readahead, fd, 0,
		   (unsigned int) (offset >> 32),
		   (unsigned int) (offset & 0xFFFFFFFF),
		   count);

       Since  the  offset  argument is 64 bits,	and the	first argument (fd) is
       passed in r0, the caller	must manually split and	align the 64-bit value
       so  that	it is passed in	the r2/r3 register pair.  That means inserting
       a dummy value into r1 (the second argument of 0).

       Similar issues can occur	on MIPS	with the O32 ABI, on PowerPC with  the
       32-bit ABI, and on Xtensa.

       The   affected	system	 calls	are  fadvise64_64(2),  ftruncate64(2),
       posix_fadvise(2),      pread64(2),      pwrite64(2),	 readahead(2),
       sync_file_range(2), and truncate64(2).

   Architecture	calling	conventions
       Every architecture has its own way of invoking and passing arguments to
       the kernel.  The	details	for various architectures are  listed  in  the
       two tables below.

       The  first  table  lists	 the  instruction used to transition to	kernel
       mode, (which might not be the fastest or	best way to transition to  the
       kernel,	so  you	 might have to refer to	vdso(7)), the register used to
       indicate	the system call	number,	and the	register used  to  return  the
       system call result.

       arch/ABI	  instruction	       syscall #   retval Notes
       -------------------------------------------------------------------
       arm/OABI	  swi NR	       -	   a1	  NR is	syscall	#
       arm/EABI	  swi 0x0	       r7	   r0
       blackfin	  excpt	0x0	       P0	   R0
       i386	  int $0x80	       eax	   eax
       ia64	  break	0x100000       r15	   r10/r8 bool error/
							  errno	value
       parisc	  ble 0x100(%sr2, %r0) r20	   r28
       s390	  svc 0		       r1	   r2	  See below
       s390x	  svc 0		       r1	   r2	  See below
       sparc/32	  t 0x10	       g1	   o0
       sparc/64	  t 0x6d	       g1	   o0
       x86_64	  syscall	       rax	   rax

       For  s390 and s390x, NR (the system call	number)	may be passed directly
       with "svc NR" if	it is less than	256.

       The second table	shows the registers used to pass the system call argu-
       ments.

       arch/ABI	  arg1	 arg2	arg3   arg4   arg5   arg6   arg7
       ----------------------------------------------------------
       arm/OABI	  a1	 a2	a3     a4     v1     v2	    v3
       arm/EABI	  r0	 r1	r2     r3     r4     r5	    r6
       blackfin	  R0	 R1	R2     R3     R4     R5	    -
       i386	  ebx	 ecx	edx    esi    edi    ebp    -
       ia64	  out0	 out1	out2   out3   out4   out5   -
       parisc	  r26	 r25	r24    r23    r22    r21    -
       s390	  r2	 r3	r4     r5     r6     r7	    -
       s390x	  r2	 r3	r4     r5     r6     r7	    -
       sparc/32	  o0	 o1	o2     o3     o4     o5	    -
       sparc/64	  o0	 o1	o2     o3     o4     o5	    -
       x86_64	  rdi	 rsi	rdx    r10    r8     r9	    -

       Note  that these	tables don't cover the entire calling convention--some
       architectures may indiscriminately clobber other	registers  not	listed
       here.

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

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

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

SEE ALSO
       _syscall(2), intro(2), syscalls(2), vdso(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-10			    SYSCALL(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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