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 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
       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 different from page to page!

SEE ALSO

       _syscall(2), syscall(2), syscalls(2), errno(3), intro(3), capabilities(7), credentials(7),
       feature_test_macros(7), mq_overview(7), path_resolution(7), pipe(7), pty(7), sem_overview(7),
       shm_overview(7), signal(7), socket(7), standards(7), svipc(7), symlink(7), time(7)

COLOPHON

       This page is part of release 3.54 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                                              2010-11-11                                           INTRO(2)