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       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, and the register used to return the
       system call result.

       arch/ABI   instruction          syscall #   retval Notes
       arm/OABI   swi NR               -           a1     NR is syscall #
       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     See below
       mips       syscall              v0          v0     See below
       parisc     ble 0x100(%sr2, %r0) r20         r28
       s390       svc 0                r1          r2     See below
       s390x      svc 0                r1          r2     See below
       sparc/32   t 0x10               g1          o0
       sparc/64   t 0x6d               g1          o0
       x86_64     syscall              rax         rax    See below
       x32        syscall              rax         rax    See below

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

       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.

       On a few architectures, a register is used to indicate  simple  boolean
       failure  of  the  system call: ia64 uses r10 for this purpose, and mips
       uses a3.

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

       arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
       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  -
       mips/o32      a0    a1    a2    a3    -     -     -     See below
       mips/n32,64   a0    a1    a2    a3    a4    a5    -
       parisc        r26   r25   r24   r23   r22   r21   -
       s390          r2    r3    r4    r5    r6    r7    -
       s390x         r2    r3    r4    r5    r6    r7    -
       sparc/32      o0    o1    o2    o3    o4    o5    -
       sparc/64      o0    o1    o2    o3    o4    o5    -
       x86_64        rdi   rsi   rdx   r10   r8    r9    -
       x32           rdi   rsi   rdx   r10   r8    r9    -

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


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


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