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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  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)