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NAME

       syscall - indirect system call

SYNOPSIS

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <unistd.h>
       #include <sys/syscall.h>   /* For SYS_xxx definitions */

       long syscall(long number, ...);

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 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>.

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  code  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() 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 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 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]
       riscv       scall                 a7         a0      -
       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      -

       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.

           [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 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
       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.

EXAMPLE

       #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 4.16 of the Linux man-pages project.  A description of the
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       found at https://www.kernel.org/doc/man-pages/.