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.  Usu-
       ally, 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 sys-
       tem 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 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.  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 Conditions
       Look at the header of the manual page source for the author(s) and copyright conditions.  Note that these can be  differ-
       ent 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 3.25 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                                                      2010-02-03                                                   INTRO(2)