syscall

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



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 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   -    -

       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.
           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 calls.

       [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    -

       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 5.04 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/.



Linux                             2019-10-10                        SYSCALL(2)