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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 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; 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

       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


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


   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

   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 3.77 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                            2014-02-20                         INTRO(2)

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