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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
       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 3.21 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/.