_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 specify.  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 create 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 variable 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).

EXAMPLES

       #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

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       https://www.kernel.org/doc/man-pages/.

Linux                                              2020-06-09                                        _SYSCALL(2)