_syscall(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | FILES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <linux/unistd.h>

       A _syscall macro

       desired system call

DESCRIPTION         top

       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         top

       /usr/include/linux/unistd.h

CONFORMING TO         top

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

NOTES         top

       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         top

       #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         top

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

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                    _SYSCALL(2)

Pages that refer to this page: intro(2)syscall(2)