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Linux 2.6 - man page for _syscall (linux section 2)

_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).

EXAMPLE
       #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);

       /* Note: if you copy directly from the nroff source, remember to
       REMOVE the extra backslashes in the printf statement. */

       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)

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2007-12-19				      _SYSCALL(2)


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