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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 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 condi-
       tions.  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 information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2007-10-23                          INTRO(2)

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