_syscall(2) [centos man page]

_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
", error);
	   printf("Uptime = %lds
Load: 1 min %lu / 5 min %lu / 15 min %lu
"
		  "RAM: total %lu / free %lu / shared %lu
"
		  "Memory in buffers = %lu
Swap: total %lu / free %lu
"
		  "Number of processes = %d
",
		  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 3.53 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)

Check Out this Related Man Page

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

NAME

       intro - Introduction to system calls

DESCRIPTION

       Section	2  of  the manual describes the Linux system calls.  A system call is an entry point into the Linux kernel.  Usually, system calls
       are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required (e.g.,
       trapping  to kernel mode) in order to invoke the system call.  Thus, making a system call looks the same as invoking a normal library func-
       tion.

       For a list of the Linux system calls, see syscalls(2).

RETURN VALUE

       On error, most system calls return a negative error number (i.e., the negated value of one of the constants described in errno(3)).  The  C
       library	wrapper hides this detail from the caller: when a system call returns a negative value, the wrapper copies the absolute value into
       the errno variable, and returns -1 as the return value of the wrapper.

       The value returned by a successful system call depends on the call.  Many system calls return 0 on success, but	some  can  return  nonzero
       values from a successful call.  The details are described in the individual manual pages.

       In  some  cases,  the  programmer must define a feature test macro in order to obtain the declaration of a system call from the header file
       specified in the man page SYNOPSIS section.  (Where required, these feature test macros must be defined before including any header files.)
       In  such  cases,  the  required	macro  is  described  in  the  man  page.   For  further  information  on  feature  test  macros, see fea-
       ture_test_macros(7).

CONFORMING TO

       Certain terms and abbreviations are used to indicate Unix variants and standards to which calls in this section conform.  See standards(7).

NOTES

   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 wrapper function for you.  In this case, the programmer must manually invoke the system call using syscall(2).  Historically, this was
       also possible using one of the _syscall macros described in _syscall(2).

   Authors and Copyright Conditions
       Look at the header of the manual page source for the author(s) and copyright conditions.  Note that these can be  different  from  page	to
       page!

SEE ALSO

       _syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)

COLOPHON

       This  page is part of release 3.27 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								    2010-09-10								  INTRO(2)

Linux and UNIX Man Pages

_syscall(2) [centos man page]

Check Out this Related Man Page