_SYSCALL(2)                Linux Programmer's Manual               _SYSCALL(2)

       _syscall - invoking a system call without library support (OBSOLETE)

       #include <linux/unistd.h>

       A _syscall macro

       desired system call

       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:



              X is 0-6, which are the number of arguments taken by the  system

              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.


       The use of these macros is Linux-specific, and deprecated.

       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  er-
       rors.   Usually, it is the negative of a standard error code, for exam-
       ple, -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  er-

       When defining a system call, the argument types must be passed by-value
       or by-pointer (for aggregates like structs).

       #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);

           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,

   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

       intro(2), syscall(2), errno(3)

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Linux                             2019-03-06                       _SYSCALL(2)
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