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/.