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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

       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; and

       *  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 of
       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).


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


       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at

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