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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.  Usu-
       ally, 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 sys-
       tem 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).

       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 infor-
       mation 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 differ-
       ent from page to page!

       _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 4.16 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/.

Linux                                                      2017-09-15                                                   INTRO(2)

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