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       _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 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.




       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 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).


       #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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