_syscall(2) — Linux manual page


_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:



              X is 0–6, which are the number of arguments taken by the sys‐
              tem 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


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);

           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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                      _SYSCALL(2)

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