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       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

       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


       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 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
       architecture 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 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                -       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 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

       [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      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 here.


       #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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