linux man page for intro

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)