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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 ker-
       nel.  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 sys-
       tem call.  Thus, making a system call looks the same as invoking a normal library function.

       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  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.  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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-10-23                          INTRO(2)