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

NAME
       intro, _syscall - Introduction to system calls

DESCRIPTION
       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.  In this	case,  the  programmer	must  manually
       invoke  the  system  call  using	 either one of the _syscall macros, or
       syscall().  The latter technique is described in syscall(2).  This page
       describes  the  _syscall macros, and includes some notes on when to use
       one or other mechanism.

   Synopsis
       #include <linux/unistd.h>

       A _syscall macro

       desired system call

   Setup
       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:

	      _syscallX(type,name,type1,arg1,type2,arg2,...)

		     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.

EXAMPLE
       #include <stdio.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);
	    return(0);
       }

   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

NOTES
       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
       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).

       The preferred way to invoke system calls that glibc does not know about
       yet is via syscall(2).  However, this mechanism can  only  be  used  if
       using  a	 libc  (such  as  glibc)  that supports syscall(2), and if the
       <sys/syscall.h> header file contains the required  SYS_foo  definition.
       Otherwise, the use of a _syscall macro is required.

       Some  architectures,  notably ia64, do not provide the _syscall macros.
       On these architectures, syscall(2) must be used.

CONFORMING TO
       Certain codes are used to indicate Unix variants and standards to which
       calls in the section conform.  See standards(7).

FILES
       /usr/include/linux/unistd.h

SEE ALSO
       syscall(2), errno(3), feature_test_macros(7), standards(7)

Linux 1.2.13			  1996-05-22			      INTRO(2)
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