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

NAME         top

       syscall - indirect system call

SYNOPSIS         top

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

       long syscall(long number, ...);

DESCRIPTION         top

       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 <sys/syscall.h>.

RETURN VALUE         top

       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         top

       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 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 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 sys‐
       tem 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      -


           [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-argu‐
               ment 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

       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    -


           [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

EXAMPLE         top

       #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);

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.16 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

Linux                            2018-04-30                       SYSCALL(2)

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