_syscall(2)                   System Calls 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

STANDARDS
       Linux.

HISTORY
       Starting around Linux 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.)

NOTES
       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)

Linux man-pages 6.7               2024-02-26                       _syscall(2)