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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 */

       int syscall(int 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 encoun-
       tered 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:

           syscall(SYS_readahead, fd, 0,
                   (unsigned int) (offset >> 32),
                   (unsigned int) (offset & 0xFFFFFFFF),
                   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).

       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 the
       VDSO), 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          r1
       blackfin   excpt 0x0            P0          R0
       i386       int $0x80            eax         eax
       ia64       break 0x100000       r15         r10/r8
       parisc     ble 0x100(%sr2, %r0) r20         r28
       s390       svc 0                r1          r2     NR may be passed directly with
       s390x      svc 0                r1          r2     "svc NR" if NR is less than 256
       sparc/32   t 0x10               g1          o0
       sparc/64   t 0x6d               g1          o0
       x86_64     syscall              rax         rax

       The second table shows the registers used to pass the system call arguments.

       arch/ABI   arg1   arg2   arg3   arg4   arg5   arg6   arg7
       ----------------------------------------------------------
       arm/OABI   a1     a2     a3     a4     v1     v2     v3
       arm/EABI   r1     r2     r3     r4     r5     r6     r7
       blackfin   R0     R1     R2     R3     R4     R5     -
       i386       ebx    ecx    edx    esi    edi    ebp    -
       ia64       r11    r9     r10    r14    r15    r13    -
       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     -

       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>

       int
       main(int argc, char *argv[])
       {
           pid_t tid;

           tid = syscall(SYS_gettid);
           tid = syscall(SYS_tgkill, getpid(), tid);
       }

SEE ALSO
       _syscall(2), intro(2), syscalls(2)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2013-06-21                                 SYSCALL(2)

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