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.

   In many cases, the C library wrapper function does nothing more than:

   *  copying arguments and the unique system call number to the registers
      where the kernel expects them;

   *  trapping to kernel mode, at which point the  kernel  does  the  real
      work of the system call;

   *  setting  errno  if  the system call returns an error number when the
      kernel returns the CPU to user mode.

   However, in a few cases, a wrapper function may  do  rather  more  than
   this,  for  example,  performing  some  preprocessing  of the arguments
   before trapping to kernel mode, or postprocessing of values returned by
   the system call.  Where this is the case, the manual pages in Section 2
   generally try to note the details of both the (usually GNU)  C  library
   API  interface  and  the  raw  system  call.   Most  commonly, the main
   DESCRIPTION will focus on the C library interface, and differences  for
   the system call are covered in the NOTES section.

   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 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs, and the
   latest version of this page, can be found at
   https://www.kernel.org/doc/man-pages/.

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