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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.  Usually, system calls are not invoked directly: instead, most sys‐
       tem  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 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 dec‐
       laration 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 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 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-02-20                                   INTRO(2)

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