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NAME

       syscall - indirect system call

SYNOPSIS

       #include <unistd.h>
       #include <sys/syscall.h>   /* For SYS_xxx definitions */

       long syscall(long number, ...);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
       syscall():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           Before glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE

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 returned by the system call in errno(3).

       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 number is
       stored in errno.

NOTES

       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 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(2) system call would be invoked as follows on the ARM
       architecture with the EABI in little endian mode:

           syscall(SYS_readahead, fd, 0,
                   (unsigned int) (offset & 0xFFFFFFFF),
                   (unsigned int) (offset >> 32),
                   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).
       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 and parisc 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 user space.

       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 kernel, so you might have to refer to
       vdso(7)), the register used to indicate the system call number, the  register(s)  used  to
       return the system call result, and the register used to signal an error.

       Arch/ABI    Instruction           System  Ret  Ret  Error    Notes
                                         call #  val  val2
       ───────────────────────────────────────────────────────────────────
       alpha       callsys               v0      v0   a4   a3       1, 6
       arc         trap0                 r8      r0   -    -
       arm/OABI    swi NR                -       r0   -    -        2
       arm/EABI    swi 0x0               r7      r0   r1   -
       arm64       svc #0                w8      x0   x1   -
       blackfin    excpt 0x0             P0      R0   -    -
       i386        int $0x80             eax     eax  edx  -
       ia64        break 0x100000        r15     r8   r9   r10      1, 6
       m68k        trap #0               d0      d0   -    -
       microblaze  brki r14,8            r12     r3   -    -
       mips        syscall               v0      v0   v1   a3       1, 6
       nios2       trap                  r2      r2   -    r7
       parisc      ble 0x100(%sr2, %r0)  r20     r28  -    -
       powerpc     sc                    r0      r3   -    r0       1
       powerpc64   sc                    r0      r3   -    cr0.SO   1
       riscv       ecall                 a7      a0   a1   -
       s390        svc 0                 r1      r2   r3   -        3
       s390x       svc 0                 r1      r2   r3   -        3
       superh      trap #0x17            r3      r0   r1   -        4, 6
       sparc/32    t 0x10                g1      o0   o1   psr/csr  1, 6
       sparc/64    t 0x6d                g1      o0   o1   psr/csr  1, 6
       tile        swint1                R10     R00  -    R01      1
       x86-64      syscall               rax     rax  rdx  -        5
       x32         syscall               rax     rax  rdx  -        5
       xtensa      syscall               a2      a2   -    -

       Notes:

       [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.  On powerpc64, the
           summary overflow bit (SO) in field 0 of the condition register (cr0) is used.

       [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 shares syscall table with x86-64 ABI, but there are some nuances:

           ·  In  order to indicate that a system call is called under the x32 ABI, an additional
              bit, __X32_SYSCALL_BIT, is bitwise-ORed with the system call number.  The ABI  used
              by  a  process  affects some process behaviors, including signal handling or system
              call restarting.

           ·  Since x32 has different sizes for long and pointer types, layouts of some (but  not
              all;  struct  timeval  or  struct  rlimit  are  64-bit, for example) structures are
              different.  In order to handle this, additional  system  calls  are  added  to  the
              system  call  table, starting from number 512 (without the __X32_SYSCALL_BIT).  For
              example, __NR_readv is defined as 19 for the x86-64 ABI and as __X32_SYSCALL_BIT  |
              515  for the x32 ABI.  Most of these additional system calls are actually identical
              to the system calls used  for  providing  i386  compat.   There  are  some  notable
              exceptions,  however,  such  as  preadv2(2),  which uses struct iovec entities with
              4-byte pointers and sizes ("compat_iovec" in kernel terms), but  passes  an  8-byte
              pos argument in a single register and not two, as is done in every other ABI.

       [6] Some architectures (namely, Alpha, IA-64, MIPS, SuperH, sparc/32, and sparc/64) use an
           additional register ("Retval2" in the above table) to pass back a second return  value
           from  the  pipe(2) system call; Alpha uses this technique in the architecture-specific
           getxpid(2), getxuid(2), and getxgid(2) system calls as well.  Other  architectures  do
           not  use  the second return value register in the system call interface, even if it is
           defined in the System V ABI.

       The second table shows the registers used to pass the system call arguments.

       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      r0    r1    r2    r3    r4    r5    r6
       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
       powerpc64     r3    r4    r5    r6    r7    r8    -
       riscv         a0    a1    a2    a3    a4    a5    -
       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    -

       Notes:

       [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 here.

EXAMPLES

       #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);
           syscall(SYS_tgkill, getpid(), tid, SIGHUP);
       }

SEE ALSO

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

COLOPHON

       This  page  is  part of release 5.10 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 https://www.kernel.org/doc/man-pages/.