bionic (2) syscall.2.gz

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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 userspace.

       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]
       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
       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  4.15  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/.