INTRO(2)                   Linux Programmer's Manual                  INTRO(2)

       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  func-

       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 be-
       fore 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 DE-
       SCRIPTION 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 val-
       ues  from a successful call.  The details are described in the individ-
       ual 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 speci-
       fied 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  fur-
       ther 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), symlink(7),
       sysvipc(7), time(7)

       This page is part of release 5.05 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

Linux                             2019-08-02                          INTRO(2)
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