syscall manpage

Search topic Section

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

       syscall - indirect system call

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

       long syscall(long number, ...);

       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>.

       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.

       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 in little endian mode:

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

       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).  Care	also  must  be
       taken  so that the split follows endian conventions (according to the C
       ABI for the platform).

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

       Note  that  while the parisc C ABI also uses aligned register pairs, it
       uses a shim layer to hide the issue from userspace.

       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).

       This does not affect syscalls that manually split and  assemble	64-bit
       values  such  as	 _llseek(2),  preadv(2),  preadv2(2), pwritev(2).  and
       pwritev2(2).  Welcome to the wonderful world of historical baggage.

   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 ker-
       nel, so you might have to refer to vdso(7)), the register used to indi-
       cate  the  system  call	number, the register used to return the system
       call result, and the register used to signal an error.

       arch/ABI	   instruction		 syscall #  retval  error    Notes
       alpha	   callsys		 v0	    a0	    a3	     [1]
       arc	   trap0		 r8	    r0	    -
       arm/OABI	   swi NR		 -	    a1	    -	     [2]
       arm/EABI	   swi 0x0		 r7	    r0	    -
       arm64	   svc #0		 x8	    x0	    -
       blackfin	   excpt 0x0		 P0	    R0	    -
       i386	   int $0x80		 eax	    eax	    -
       ia64	   break 0x100000	 r15	    r8	    r10	     [1]
       m68k	   trap #0		 d0	    d0	    -
       microblaze  brki r14,8		 r12	    r3	    -
       mips	   syscall		 v0	    v0	    a3	     [1]
       nios2	   trap			 r2	    r2	    r7
       parisc	   ble 0x100(%sr2, %r0)	 r20	    r28	    -
       powerpc	   sc			 r0	    r3	    r0	     [1]
       s390	   svc 0		 r1	    r2	    -	     [3]
       s390x	   svc 0		 r1	    r2	    -	     [3]
       superh	   trap #0x17		 r3	    r0	    -	     [4]
       sparc/32	   t 0x10		 g1	    o0	    psr/csr  [1]
       sparc/64	   t 0x6d		 g1	    o0	    psr/csr  [1]
       tile	   swint1		 R10	    R00	    R01	     [1]
       x86_64	   syscall		 rax	    rax	    -	     [5]
       x32	   syscall		 rax	    rax	    -	     [5]
       xtensa	   syscall		 a2	    a2	    -


	   [1] On a few architectures, a register is  used  as	a  boolean  (0
	       indicating no error, and -1 indicating an error) to signal that
	       the system call failed.	The actual error value is  still  con-
	       tained  in  the return register.	 On sparc, the carry bit (csr)
	       in the processor status register (psr) is  used	instead	 of  a
	       full register.

	   [2] NR is the system call number.

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

	   [4] On SuperH, the trap number controls the maximum number of argu-
	       ments  passed.	A  trap #0x10 can be used with only 0-argument
	       system calls, a trap #0x11 can be used with  0-	or  1-argument
	       system  calls, and so on up to trap #0x17 for 7-argument system

	   [5] The x32 ABI uses the same instruction as the x86_64 ABI and  is
	       used  on	 the  same processors.	To differentiate between them,
	       the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the	system
	       call  number  for  system calls under the x32 ABI.  Both system
	       call tables are available though, so setting the bit is	not  a
	       hard requirement.

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

       arch/ABI	     arg1  arg2	 arg3  arg4  arg5  arg6	 arg7  Notes
       alpha	     a0	   a1	 a2    a3    a4	   a5	 -
       arc	     r0	   r1	 r2    r3    r4	   r5	 -
       arm/OABI	     a1	   a2	 a3    a4    v1	   v2	 v3
       arm/EABI	     r0	   r1	 r2    r3    r4	   r5	 r6
       arm64	     x0	   x1	 x2    x3    x4	   x5	 -
       blackfin	     R0	   R1	 R2    R3    R4	   R5	 -
       i386	     ebx   ecx	 edx   esi   edi   ebp	 -
       ia64	     out0  out1	 out2  out3  out4  out5	 -
       m68k	     d1	   d2	 d3    d4    d5	   a0	 -
       microblaze    r5	   r6	 r7    r8    r9	   r10	 -
       mips/o32	     a0	   a1	 a2    a3    -	   -	 -     [1]
       mips/n32,64   a0	   a1	 a2    a3    a4	   a5	 -
       nios2	     r4	   r5	 r6    r7    r8	   r9	 -
       parisc	     r26   r25	 r24   r23   r22   r21	 -
       powerpc	     r3	   r4	 r5    r6    r7	   r8	 r9
       s390	     r2	   r3	 r4    r5    r6	   r7	 -
       s390x	     r2	   r3	 r4    r5    r6	   r7	 -
       superh	     r4	   r5	 r6    r7    r0	   r1	 r2
       sparc/32	     o0	   o1	 o2    o3    o4	   o5	 -
       sparc/64	     o0	   o1	 o2    o3    o4	   o5	 -
       tile	     R00   R01	 R02   R03   R04   R05	 -
       x86_64	     rdi   rsi	 rdx   r10   r8	   r9	 -
       x32	     rdi   rsi	 rdx   r10   r8	   r9	 -
       xtensa	     a6	   a3	 a4    a5    a8	   a9	 -


	   [1] The mips/o32 system call convention passes arguments 5  through
	       8 on the user stack.

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

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

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

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

       _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)

       This  page  is  part of release 4.10 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

Linux				  2017-03-13			    SYSCALL(2)