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:

           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

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);
           tid = 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 4.04 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

Linux                            2015-03-29                       SYSCALL(2)