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

       intro - Introduction to system calls

       This  chapter  describes	the Linux system calls.	 For a list of the 164
       syscalls	present	in Linux 2.0, see syscalls(2).

   Calling Directly
       In most cases, it is unnecessary	to invoke a system call	directly,  but
       there  are  times when the Standard C library does not implement	a nice
       function	call for you.

       #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	six macros that	make the actual	call into  the	system
       easier.	They have the form:


		     where  X  is 0-5, 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 spec-
       ify.  Once you include the _syscall() in	your source file, you call the
       system call by name.

       #include	<stdio.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 =	%ds\nLoad: 1 min %d / 5	min %d / 15 min	%d\n"
		       "RAM: total %d /	free %d	/ shared %d\n"
		       "Memory in buffers = %d\nSwap: total %d / free %d\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

       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, e.g.,
       -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 er-
       rno to -r when r	is negative.  For the error codes, see errno(3).

       Some system calls, such as mmap,	 require  more	than  five  arguments.
       These  are  handled by pushing the arguments on the stack and passing a
       pointer to the block of arguments.

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

       Certain codes are used to indicate Unix variants	and standards to which
       calls in	the section conform.  These are:

       SVr4   System V Release 4 Unix, as described in the "Programmer's  Ref-
	      erence  Manual:  Operating System	API (Intel processors)"	(Pren-
	      tice-Hall	1992, ISBN 0-13-951294-2)

       SVID   System V Interface Definition, as	described in "The System V In-
	      terface Definition, Fourth Edition".

	      IEEE  1003.1-1990	 part  1,  aka ISO/IEC 9945-1:1990s, aka "IEEE
	      Portable Operating System	Interface for Computing	Environments",
	      as  elucidated  in  Donald  Lewine's  "POSIX Programmer's	Guide"
	      (O'Reilly	& Associates, Inc., 1991, ISBN 0-937175-73-0.

	      IEEE Std 1003.1b-1993 (POSIX.1b standard)	 describing  real-time
	      facilities   for	 portable   operating	systems,  aka  ISO/IEC
	      9945-1:1996, as elucidated in "Programming for the real world  -
	      POSIX.4"	by  Bill  O.  Gallmeister (O'Reilly & Associates, Inc.
	      ISBN 1-56592-074-0).

       SUS, SUSv2
	      Single Unix Specification.  (Developed by	X/Open	and  The  Open
	      Group. See also .)

	      The  4.3 and 4.4 distributions of	Berkeley Unix.	4.4BSD was up-
	      ward-compatible from 4.3.

       V7     Version 7, the ancestral Unix from Bell Labs.



Linux 1.2.13			  1996-05-22			      INTRO(2)


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