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
   general, 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
   arguments 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
   architecture.  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

   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
   architecture with the EABI:

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

   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, 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
           contained  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
           arguments   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 calls.

       [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

   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 4.09 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

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