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       intro - Introduction to system calls


       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

       For a list of the Linux system calls, see syscalls(2).


       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).


       Certain  terms and abbreviations are used to indicate UNIX variants and
       standards to which calls in this section conform.  See standards(7).


   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


       _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), svipc(7), standards(7), socket(7),
       symlink(7), time(7)


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