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

       syscall - indirect system call

       #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)):
           Since glibc 2.19:
           Before glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE

       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  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 ker-
       nel, so you might have to refer to vdso(7)), the register used to indi-
       cate  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                -       a1   -    -        2
       arm/EABI    swi 0x0               r7      r0   r1   -
       arm64       svc #0                x8      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   -    -


       [1] On a few architectures, a register is used as a boolean (0 indicat-
           ing no error, and -1 indicating an error) to signal that the system
           call failed.  The actual error value is still contained in the  re-
           turn register.  On sparc, the carry bit (csr) in the processor sta-
           tus register (psr) is used instead of a  full  register.   On  pow-
           erpc64,  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  di-
           rectly 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:

           o  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

           o  Since  x32  has different sizes for long and pointer types, lay-
              outs of some (but not all; struct timeval or struct  rlimit  are
              64-bit, for example) structures are different.  In order to han-
              dle 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 addi-
              tional system calls are actually identical to the  system  calls
              used  for  providing i386 compat.  There are some notable excep-
              tions, however, such as preadv2(2), which uses struct iovec  en-
              tities  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  getx-
           pid(2), getxuid(2), and getxgid(2) system calls as well.  Other ar-
           chitectures do not use the second return value register in the sys-
           tem call interface, even if it is defined in the System V ABI.

       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
       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    -


       [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)

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Linux                             2020-02-09                        SYSCALL(2)
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