INTRO(2)		   Linux Programmers 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 non-zero
       values from a successful call.  The details are described in the  indi
       vidual 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	speci
       fied  in  the  man  page SYNOPSIS section.  In such cases, the required
       macro is described in the man page.  For further information on feature
       test macros, see feature_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 Terms
       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 3.05 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				  2007-10-23			      INTRO(2)