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NAME    [Toc]    [Back]

       intro - Introduction to system calls

DESCRIPTION    [Toc]    [Back]

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

   Calling Directly    [Toc]    [Back]
       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.

   Synopsis    [Toc]    [Back]
       #include <linux/unistd.h>

       A _syscall macro

       desired system call

   Setup    [Toc]    [Back]
       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 specify.
  Once you include the _syscall() in your source file, you call the
       system call by name.

EXAMPLE    [Toc]    [Back]

       #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    [Toc]    [Back]

       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

NOTES    [Toc]    [Back]

       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, 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
       errno 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).

CONFORMING TO    [Toc]    [Back]

       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  Reference
  Manual:  Operating System API (Intel processors)" (Prentice-Hall
 1992, ISBN 0-13-951294-2)

       SVID   System V Interface Definition, as described  in  "The  System  V
	      Interface 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 http://www.UNIX-systems.org/version2/ .)

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

       V7     Version 7, the ancestral Unix from Bell Labs.

FILES    [Toc]    [Back]


SEE ALSO    [Toc]    [Back]


Linux 1.2.13			  1996-05-22			      INTRO(2)
[ Back ]
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