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_SYSCALL(2)                Linux Programmer's Manual               _SYSCALL(2)
NAME
       _syscall - invoking a system call without library support (OBSOLETE)
SYNOPSIS
       #include <linux/unistd.h>
       A _syscall macro
       desired system call
DESCRIPTION
       The  important thing to know about a system call is its prototype.  You
       need to know how many arguments, their types, and the  function  return
       type.  There are seven macros that make the actual call into the system
       easier.  They have the form:
           _syscallX(type,name,type1,arg1,type2,arg2,...)
       where
              X is 0-6, which are the number of arguments taken by the  system
              call
              type is the return type of the system call
              name is the name of the system call
              typeN is the Nth argument's type
              argN is the name of the Nth argument
       These macros create a function called name with the arguments you spec-
       ify.  Once you include the _syscall() in your source file, you call the
       system call by name.
FILES
       /usr/include/linux/unistd.h
CONFORMING TO
       The use of these macros is Linux-specific, and deprecated.
NOTES
       Starting  around  kernel  2.6.18, the _syscall macros were removed from
       header files supplied to user space.  Use  syscall(2)  instead.   (Some
       architectures,  notably  ia64,  never  provided the _syscall macros; on
       those architectures, syscall(2) was always required.)
       The _syscall() macros do not produce a prototype.  You may have to cre-
       ate one, especially for C++ users.
       System calls are not required to return only positive or negative error
       codes.  You need to read the source to  be  sure  how  it  will  return
       errors.   Usually,  it  is  the  negative of a standard error code, for
       example, -EPERM.  The _syscall() macros will return the result r of the
       system call when r is nonnegative, but will return -1 and set the vari-
       able errno to -r  when  r  is  negative.   For  the  error  codes,  see
       errno(3).
       When defining a system call, the argument types must be passed by-value
       or by-pointer (for aggregates like structs).
EXAMPLE
       #include <stdio.h>
       #include <stdlib.h>
       #include <errno.h>
       #include <linux/unistd.h>       /* for _syscallX macros/related stuff */
       #include <linux/kernel.h>       /* for struct sysinfo */
       _syscall1(int, sysinfo, struct sysinfo *, info);
       int
       main(void)
       {
           struct sysinfo s_info;
           int error;
           error = sysinfo(&s_info);
           printf("code error = %d\n", error);
           printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
                  "RAM: total %lu / free %lu / shared %lu\n"
                  "Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
                  "Number of processes = %d\n",
                  s_info.uptime, s_info.loads[0],
                  s_info.loads[1], s_info.loads[2],
                  s_info.totalram, s_info.freeram,
                  s_info.sharedram, s_info.bufferram,
                  s_info.totalswap, s_info.freeswap,
                  s_info.procs);
           exit(EXIT_SUCCESS);
       }
   Sample output
       code error = 0
       uptime = 502034s
       Load: 1 min 13376 / 5 min 5504 / 15 min 1152
       RAM: total 15343616 / free 827392 / shared 8237056
       Memory in buffers = 5066752
       Swap: total 27881472 / free 24698880
       Number of processes = 40
SEE ALSO
       intro(2), syscall(2), errno(3)
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/.
Linux                             2017-09-15                       _SYSCALL(2)