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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 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)

Linux                                                                          2021-03-22                                                                    _SYSCALL(2)