This is libc.info, produced by makeinfo version 4.0 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Portable Positioning,  Next: Stream Buffering,  Prev: File Positioning,  Up: I/O on Streams

Portable File-Position Functions
================================

   On the GNU system, the file position is truly a character count.  You
can specify any character count value as an argument to `fseek' or
`fseeko' and get reliable results for any random access file.  However,
some ISO C systems do not represent file positions in this way.

   On some systems where text streams truly differ from binary streams,
it is impossible to represent the file position of a text stream as a
count of characters from the beginning of the file.  For example, the
file position on some systems must encode both a record offset within
the file, and a character offset within the record.

   As a consequence, if you want your programs to be portable to these
systems, you must observe certain rules:

   * The value returned from `ftell' on a text stream has no predictable
     relationship to the number of characters you have read so far.
     The only thing you can rely on is that you can use it subsequently
     as the OFFSET argument to `fseek' or `fseeko' to move back to the
     same file position.

   * In a call to `fseek' or `fseeko' on a text stream, either the
     OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the
     OFFSET must be the result of an earlier call to `ftell' on the
     same stream.

   * The value of the file position indicator of a text stream is
     undefined while there are characters that have been pushed back
     with `ungetc' that haven't been read or discarded.  *Note
     Unreading::.

   But even if you observe these rules, you may still have trouble for
long files, because `ftell' and `fseek' use a `long int' value to
represent the file position.  This type may not have room to encode all
the file positions in a large file.  Using the `ftello' and `fseeko'
functions might help here since the `off_t' type is expected to be able
to hold all file position values but this still does not help to handle
additional information which must be associated with a file position.

   So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions `fgetpos' and
`fsetpos' instead.  These functions represent the file position using
the data type `fpos_t', whose internal representation varies from
system to system.

   These symbols are declared in the header file `stdio.h'.

 - Data Type: fpos_t
     This is the type of an object that can encode information about the
     file position of a stream, for use by the functions `fgetpos' and
     `fsetpos'.

     In the GNU system, `fpos_t' is an opaque data structure that
     contains internal data to represent file offset and conversion
     state information.  In other systems, it might have a different
     internal representation.

     When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine
     this type is in fact equivalent to `fpos64_t' since the LFS
     interface transparently replaces the old interface.

 - Data Type: fpos64_t
     This is the type of an object that can encode information about the
     file position of a stream, for use by the functions `fgetpos64' and
     `fsetpos64'.

     In the GNU system, `fpos64_t' is an opaque data structure that
     contains internal data to represent file offset and conversion
     state information.  In other systems, it might have a different
     internal representation.

 - Function: int fgetpos (FILE *STREAM, fpos_t *POSITION)
     This function stores the value of the file position indicator for
     the stream STREAM in the `fpos_t' object pointed to by POSITION.
     If successful, `fgetpos' returns zero; otherwise it returns a
     nonzero value and stores an implementation-defined positive value
     in `errno'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system the function is in fact `fgetpos64'.  I.e., the LFS
     interface transparently replaces the old interface.

 - Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)
     This function is similar to `fgetpos' but the file position is
     returned in a variable of type `fpos64_t' to which POSITION points.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fgetpos'
     and so transparently replaces the old interface.

 - Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION)
     This function sets the file position indicator for the stream
     STREAM to the position POSITION, which must have been set by a
     previous call to `fgetpos' on the same stream.  If successful,
     `fsetpos' clears the end-of-file indicator on the stream, discards
     any characters that were "pushed back" by the use of `ungetc', and
     returns a value of zero.  Otherwise, `fsetpos' returns a nonzero
     value and stores an implementation-defined positive value in
     `errno'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system the function is in fact `fsetpos64'.  I.e., the LFS
     interface transparently replaces the old interface.

 - Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)
     This function is similar to `fsetpos' but the file position used
     for positioning is provided in a variable of type `fpos64_t' to
     which POSITION points.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fsetpos'
     and so transparently replaces the old interface.


File: libc.info,  Node: Stream Buffering,  Next: Other Kinds of Streams,  Prev: Portable Positioning,  Up: I/O on Streams

Stream Buffering
================

   Characters that are written to a stream are normally accumulated and
transmitted asynchronously to the file in a block, instead of appearing
as soon as they are output by the application program.  Similarly,
streams often retrieve input from the host environment in blocks rather
than on a character-by-character basis.  This is called "buffering".

   If you are writing programs that do interactive input and output
using streams, you need to understand how buffering works when you
design the user interface to your program.  Otherwise, you might find
that output (such as progress or prompt messages) doesn't appear when
you intended it to, or displays some other unexpected behavior.

   This section deals only with controlling when characters are
transmitted between the stream and the file or device, and _not_ with
how things like echoing, flow control, and the like are handled on
specific classes of devices.  For information on common control
operations on terminal devices, see *Note Low-Level Terminal
Interface::.

   You can bypass the stream buffering facilities altogether by using
the low-level input and output functions that operate on file
descriptors instead.  *Note Low-Level I/O::.

* Menu:

* Buffering Concepts::          Terminology is defined here.
* Flushing Buffers::            How to ensure that output buffers are flushed.
* Controlling Buffering::       How to specify what kind of buffering to use.


File: libc.info,  Node: Buffering Concepts,  Next: Flushing Buffers,  Up: Stream Buffering

Buffering Concepts
------------------

   There are three different kinds of buffering strategies:

   * Characters written to or read from an "unbuffered" stream are
     transmitted individually to or from the file as soon as possible.

   * Characters written to a "line buffered" stream are transmitted to
     the file in blocks when a newline character is encountered.

   * Characters written to or read from a "fully buffered" stream are
     transmitted to or from the file in blocks of arbitrary size.

   Newly opened streams are normally fully buffered, with one
exception: a stream connected to an interactive device such as a
terminal is initially line buffered.  *Note Controlling Buffering::,
for information on how to select a different kind of buffering.
Usually the automatic selection gives you the most convenient kind of
buffering for the file or device you open.

   The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want.  Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with `fflush', as described in
*Note Flushing Buffers::.


File: libc.info,  Node: Flushing Buffers,  Next: Controlling Buffering,  Prev: Buffering Concepts,  Up: Stream Buffering

Flushing Buffers
----------------

   "Flushing" output on a buffered stream means transmitting all
accumulated characters to the file.  There are many circumstances when
buffered output on a stream is flushed automatically:

   * When you try to do output and the output buffer is full.

   * When the stream is closed.  *Note Closing Streams::.

   * When the program terminates by calling `exit'.  *Note Normal
     Termination::.

   * When a newline is written, if the stream is line buffered.

   * Whenever an input operation on _any_ stream actually reads data
     from its file.

   If you want to flush the buffered output at another time, call
`fflush', which is declared in the header file `stdio.h'.

 - Function: int fflush (FILE *STREAM)
     This function causes any buffered output on STREAM to be delivered
     to the file.  If STREAM is a null pointer, then `fflush' causes
     buffered output on _all_ open output streams to be flushed.

     This function returns `EOF' if a write error occurs, or zero
     otherwise.

 - Function: int fflush_unlocked (FILE *STREAM)
     The `fflush_unlocked' function is equivalent to the `fflush'
     function except that it does not implicitly lock the stream.

   The `fflush' function can be used to flush all streams currently
opened.  While this is useful in some situations it does often more than
necessary since it might be done in situations when terminal input is
required and the program wants to be sure that all output is visible on
the terminal.  But this means that only line buffered streams have to be
flushed.  Solaris introduced a function especially for this.  It was
always available in the GNU C library in some form but never officially
exported.

 - Function: void _flushlbf (void)
     The `_flushlbf' function flushes all line buffered streams
     currently opened.

     This function is declared in the `stdio_ext.h' header.

   *Compatibility Note:* Some brain-damaged operating systems have been
known to be so thoroughly fixated on line-oriented input and output
that flushing a line buffered stream causes a newline to be written!
Fortunately, this "feature" seems to be becoming less common.  You do
not need to worry about this in the GNU system.

   In some situations it might be useful to not flush the output pending
for a stream but instead simply forget it.  If transmission is costly
and the output is not needed anymore this is valid reasoning.  In this
situation a non-standard function introduced in Solaris and available in
the GNU C library can be used.

 - Function: void __fpurge (FILE *STREAM)
     The `__fpurge' function causes the buffer of the stream STREAM to
     be emptied.  If the stream is currently in read mode all input in
     the buffer is lost.  If the stream is in output mode the buffered
     output is not written to the device (or whatever other underlying
     storage) and the buffer the cleared.

     This function is declared in `stdio_ext.h'.


File: libc.info,  Node: Controlling Buffering,  Prev: Flushing Buffers,  Up: Stream Buffering

Controlling Which Kind of Buffering
-----------------------------------

   After opening a stream (but before any other operations have been
performed on it), you can explicitly specify what kind of buffering you
want it to have using the `setvbuf' function.

   The facilities listed in this section are declared in the header
file `stdio.h'.

 - Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t
          SIZE)
     This function is used to specify that the stream STREAM should
     have the buffering mode MODE, which can be either `_IOFBF' (for
     full buffering), `_IOLBF' (for line buffering), or `_IONBF' (for
     unbuffered input/output).

     If you specify a null pointer as the BUF argument, then `setvbuf'
     allocates a buffer itself using `malloc'.  This buffer will be
     freed when you close the stream.

     Otherwise, BUF should be a character array that can hold at least
     SIZE characters.  You should not free the space for this array as
     long as the stream remains open and this array remains its buffer.
     You should usually either allocate it statically, or `malloc'
     (*note Unconstrained Allocation::) the buffer.  Using an automatic
     array is not a good idea unless you close the file before exiting
     the block that declares the array.

     While the array remains a stream buffer, the stream I/O functions
     will use the buffer for their internal purposes.  You shouldn't
     try to access the values in the array directly while the stream is
     using it for buffering.

     The `setvbuf' function returns zero on success, or a nonzero value
     if the value of MODE is not valid or if the request could not be
     honored.

 - Macro: int _IOFBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be fully buffered.

 - Macro: int _IOLBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be line buffered.

 - Macro: int _IONBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be unbuffered.

 - Macro: int BUFSIZ
     The value of this macro is an integer constant expression that is
     good to use for the SIZE argument to `setvbuf'.  This value is
     guaranteed to be at least `256'.

     The value of `BUFSIZ' is chosen on each system so as to make stream
     I/O efficient.  So it is a good idea to use `BUFSIZ' as the size
     for the buffer when you call `setvbuf'.

     Actually, you can get an even better value to use for the buffer
     size by means of the `fstat' system call: it is found in the
     `st_blksize' field of the file attributes.  *Note Attribute
     Meanings::.

     Sometimes people also use `BUFSIZ' as the allocation size of
     buffers used for related purposes, such as strings used to receive
     a line of input with `fgets' (*note Character Input::).  There is
     no particular reason to use `BUFSIZ' for this instead of any other
     integer, except that it might lead to doing I/O in chunks of an
     efficient size.

 - Function: void setbuf (FILE *STREAM, char *BUF)
     If BUF is a null pointer, the effect of this function is
     equivalent to calling `setvbuf' with a MODE argument of `_IONBF'.
     Otherwise, it is equivalent to calling `setvbuf' with BUF, and a
     MODE of `_IOFBF' and a SIZE argument of `BUFSIZ'.

     The `setbuf' function is provided for compatibility with old code;
     use `setvbuf' in all new programs.

 - Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE)
     If BUF is a null pointer, this function makes STREAM unbuffered.
     Otherwise, it makes STREAM fully buffered using BUF as the buffer.
     The SIZE argument specifies the length of BUF.

     This function is provided for compatibility with old BSD code.  Use
     `setvbuf' instead.

 - Function: void setlinebuf (FILE *STREAM)
     This function makes STREAM be line buffered, and allocates the
     buffer for you.

     This function is provided for compatibility with old BSD code.  Use
     `setvbuf' instead.

   It is possible to query whether a given stream is line buffered or
not using a non-standard function introduced in Solaris and available
in the GNU C library.

 - Function: int __flbf (FILE *STREAM)
     The `__flbf' function will return a nonzero value in case the
     stream STREAM is line buffered.  Otherwise the return value is
     zero.

     This function is declared in the `stdio_ext.h' header.

   Two more extensions allow to determine the size of the buffer and how
much of it is used.  These functions were also introduced in Solaris.

 - Function: size_t __fbufsize (FILE *STREAM)
     The `__fbufsize' function return the size of the buffer in the
     stream STREAM.  This value can be used to optimize the use of the
     stream.

     This function is declared in the `stdio_ext.h' header.

 - Function: size_t __fpending (FILE *STREAM) The `__fpending'
     function returns the number of bytes currently in the output
     buffer.  For wide-oriented stream the measuring unit is wide
     characters.  This function should not be used on buffers in read
     mode or opened read-only.

     This function is declared in the `stdio_ext.h' header.


File: libc.info,  Node: Other Kinds of Streams,  Next: Formatted Messages,  Prev: Stream Buffering,  Up: I/O on Streams

Other Kinds of Streams
======================

   The GNU library provides ways for you to define additional kinds of
streams that do not necessarily correspond to an open file.

   One such type of stream takes input from or writes output to a
string.  These kinds of streams are used internally to implement the
`sprintf' and `sscanf' functions.  You can also create such a stream
explicitly, using the functions described in *Note String Streams::.

   More generally, you can define streams that do input/output to
arbitrary objects using functions supplied by your program.  This
protocol is discussed in *Note Custom Streams::.

   *Portability Note:* The facilities described in this section are
specific to GNU.  Other systems or C implementations might or might not
provide equivalent functionality.

* Menu:

* String Streams::              Streams that get data from or put data in
                                 a string or memory buffer.
* Obstack Streams::		Streams that store data in an obstack.
* Custom Streams::              Defining your own streams with an arbitrary
                                 input data source and/or output data sink.


File: libc.info,  Node: String Streams,  Next: Obstack Streams,  Up: Other Kinds of Streams

String Streams
--------------

   The `fmemopen' and `open_memstream' functions allow you to do I/O to
a string or memory buffer.  These facilities are declared in `stdio.h'.

 - Function: FILE * fmemopen (void *BUF, size_t SIZE, const char
          *OPENTYPE)
     This function opens a stream that allows the access specified by
     the OPENTYPE argument, that reads from or writes to the buffer
     specified by the argument BUF.  This array must be at least SIZE
     bytes long.

     If you specify a null pointer as the BUF argument, `fmemopen'
     dynamically allocates an array SIZE bytes long (as with `malloc';
     *note Unconstrained Allocation::).  This is really only useful if
     you are going to write things to the buffer and then read them back
     in again, because you have no way of actually getting a pointer to
     the buffer (for this, try `open_memstream', below).  The buffer is
     freed when the stream is closed.

     The argument OPENTYPE is the same as in `fopen' (*note Opening
     Streams::).  If the OPENTYPE specifies append mode, then the
     initial file position is set to the first null character in the
     buffer.  Otherwise the initial file position is at the beginning
     of the buffer.

     When a stream open for writing is flushed or closed, a null
     character (zero byte) is written at the end of the buffer if it
     fits.  You should add an extra byte to the SIZE argument to
     account for this.  Attempts to write more than SIZE bytes to the
     buffer result in an error.

     For a stream open for reading, null characters (zero bytes) in the
     buffer do not count as "end of file".  Read operations indicate
     end of file only when the file position advances past SIZE bytes.
     So, if you want to read characters from a null-terminated string,
     you should supply the length of the string as the SIZE argument.

   Here is an example of using `fmemopen' to create a stream for
reading from a string:

     #include <stdio.h>
     
     static char buffer[] = "foobar";
     
     int
     main (void)
     {
       int ch;
       FILE *stream;
     
       stream = fmemopen (buffer, strlen (buffer), "r");
       while ((ch = fgetc (stream)) != EOF)
         printf ("Got %c\n", ch);
       fclose (stream);
     
       return 0;
     }

   This program produces the following output:

     Got f
     Got o
     Got o
     Got b
     Got a
     Got r

 - Function: FILE * open_memstream (char **PTR, size_t *SIZELOC)
     This function opens a stream for writing to a buffer.  The buffer
     is allocated dynamically (as with `malloc'; *note Unconstrained
     Allocation::) and grown as necessary.

     When the stream is closed with `fclose' or flushed with `fflush',
     the locations PTR and SIZELOC are updated to contain the pointer
     to the buffer and its size.  The values thus stored remain valid
     only as long as no further output on the stream takes place.  If
     you do more output, you must flush the stream again to store new
     values before you use them again.

     A null character is written at the end of the buffer.  This null
     character is _not_ included in the size value stored at SIZELOC.

     You can move the stream's file position with `fseek' or `fseeko'
     (*note File Positioning::).  Moving the file position past the end
     of the data already written fills the intervening space with
     zeroes.

   Here is an example of using `open_memstream':

     #include <stdio.h>
     
     int
     main (void)
     {
       char *bp;
       size_t size;
       FILE *stream;
     
       stream = open_memstream (&bp, &size);
       fprintf (stream, "hello");
       fflush (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
       fprintf (stream, ", world");
       fclose (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
     
       return 0;
     }

   This program produces the following output:

     buf = `hello', size = 5
     buf = `hello, world', size = 12


File: libc.info,  Node: Obstack Streams,  Next: Custom Streams,  Prev: String Streams,  Up: Other Kinds of Streams

Obstack Streams
---------------

   You can open an output stream that puts it data in an obstack.
*Note Obstacks::.

 - Function: FILE * open_obstack_stream (struct obstack *OBSTACK)
     This function opens a stream for writing data into the obstack
     OBSTACK.  This starts an object in the obstack and makes it grow
     as data is written (*note Growing Objects::).

     Calling `fflush' on this stream updates the current size of the
     object to match the amount of data that has been written.  After a
     call to `fflush', you can examine the object temporarily.

     You can move the file position of an obstack stream with `fseek' or
     `fseeko' (*note File Positioning::).  Moving the file position past
     the end of the data written fills the intervening space with zeros.

     To make the object permanent, update the obstack with `fflush', and
     then use `obstack_finish' to finalize the object and get its
     address.  The following write to the stream starts a new object in
     the obstack, and later writes add to that object until you do
     another `fflush' and `obstack_finish'.

     But how do you find out how long the object is?  You can get the
     length in bytes by calling `obstack_object_size' (*note Status of
     an Obstack::), or you can null-terminate the object like this:

          obstack_1grow (OBSTACK, 0);

     Whichever one you do, you must do it _before_ calling
     `obstack_finish'.  (You can do both if you wish.)

   Here is a sample function that uses `open_obstack_stream':

     char *
     make_message_string (const char *a, int b)
     {
       FILE *stream = open_obstack_stream (&message_obstack);
       output_task (stream);
       fprintf (stream, ": ");
       fprintf (stream, a, b);
       fprintf (stream, "\n");
       fclose (stream);
       obstack_1grow (&message_obstack, 0);
       return obstack_finish (&message_obstack);
     }


File: libc.info,  Node: Custom Streams,  Prev: Obstack Streams,  Up: Other Kinds of Streams

Programming Your Own Custom Streams
-----------------------------------

   This section describes how you can make a stream that gets input
from an arbitrary data source or writes output to an arbitrary data sink
programmed by you.  We call these "custom streams".  The functions and
types described here are all GNU extensions.

* Menu:

* Streams and Cookies::         The "cookie" records where to fetch or
                                 store data that is read or written.
* Hook Functions::              How you should define the four "hook
                                 functions" that a custom stream needs.


File: libc.info,  Node: Streams and Cookies,  Next: Hook Functions,  Up: Custom Streams

Custom Streams and Cookies
..........................

   Inside every custom stream is a special object called the "cookie".
This is an object supplied by you which records where to fetch or store
the data read or written.  It is up to you to define a data type to use
for the cookie.  The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type `void *'.

   To implement a custom stream, you must specify _how_ to fetch or
store the data in the specified place.  You do this by defining "hook
functions" to read, write, change "file position", and close the
stream.  All four of these functions will be passed the stream's cookie
so they can tell where to fetch or store the data.  The library
functions don't know what's inside the cookie, but your functions will
know.

   When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
`cookie_io_functions_t'.

   These facilities are declared in `stdio.h'.

 - Data Type: cookie_io_functions_t
     This is a structure type that holds the functions that define the
     communications protocol between the stream and its cookie.  It has
     the following members:

    `cookie_read_function_t *read'
          This is the function that reads data from the cookie.  If the
          value is a null pointer instead of a function, then read
          operations on this stream always return `EOF'.

    `cookie_write_function_t *write'
          This is the function that writes data to the cookie.  If the
          value is a null pointer instead of a function, then data
          written to the stream is discarded.

    `cookie_seek_function_t *seek'
          This is the function that performs the equivalent of file
          positioning on the cookie.  If the value is a null pointer
          instead of a function, calls to `fseek' or `fseeko' on this
          stream can only seek to locations within the buffer; any
          attempt to seek outside the buffer will return an `ESPIPE'
          error.

    `cookie_close_function_t *close'
          This function performs any appropriate cleanup on the cookie
          when closing the stream.  If the value is a null pointer
          instead of a function, nothing special is done to close the
          cookie when the stream is closed.

 - Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE,
          cookie_io_functions_t IO-FUNCTIONS)
     This function actually creates the stream for communicating with
     the COOKIE using the functions in the IO-FUNCTIONS argument.  The
     OPENTYPE argument is interpreted as for `fopen'; see *Note Opening
     Streams::.  (But note that the "truncate on open" option is
     ignored.)  The new stream is fully buffered.

     The `fopencookie' function returns the newly created stream, or a
     null pointer in case of an error.


File: libc.info,  Node: Hook Functions,  Prev: Streams and Cookies,  Up: Custom Streams

Custom Stream Hook Functions
............................

   Here are more details on how you should define the four hook
functions that a custom stream needs.

   You should define the function to read data from the cookie as:

     ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE)

   This is very similar to the `read' function; see *Note I/O
Primitives::.  Your function should transfer up to SIZE bytes into the
BUFFER, and return the number of bytes read, or zero to indicate
end-of-file.  You can return a value of `-1' to indicate an error.

   You should define the function to write data to the cookie as:

     ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE)

   This is very similar to the `write' function; see *Note I/O
Primitives::.  Your function should transfer up to SIZE bytes from the
buffer, and return the number of bytes written.  You can return a value
of `-1' to indicate an error.

   You should define the function to perform seek operations on the
cookie as:

     int SEEKER (void *COOKIE, fpos_t *POSITION, int WHENCE)

   For this function, the POSITION and WHENCE arguments are interpreted
as for `fgetpos'; see *Note Portable Positioning::.  In the GNU
library, `fpos_t' is equivalent to `off_t' or `long int', and simply
represents the number of bytes from the beginning of the file.

   After doing the seek operation, your function should store the
resulting file position relative to the beginning of the file in
POSITION.  Your function should return a value of `0' on success and
`-1' to indicate an error.

   You should define the function to do cleanup operations on the cookie
appropriate for closing the stream as:

     int CLEANER (void *COOKIE)

   Your function should return `-1' to indicate an error, and `0'
otherwise.

 - Data Type: cookie_read_function
     This is the data type that the read function for a custom stream
     should have.  If you declare the function as shown above, this is
     the type it will have.

 - Data Type: cookie_write_function
     The data type of the write function for a custom stream.

 - Data Type: cookie_seek_function
     The data type of the seek function for a custom stream.

 - Data Type: cookie_close_function
     The data type of the close function for a custom stream.


File: libc.info,  Node: Formatted Messages,  Prev: Other Kinds of Streams,  Up: I/O on Streams

Formatted Messages
==================

   On systems which are based on System V messages of programs
(especially the system tools) are printed in a strict form using the
`fmtmsg' function.  The uniformity sometimes helps the user to
interpret messages and the strictness tests of the `fmtmsg' function
ensure that the programmer follows some minimal requirements.

* Menu:

* Printing Formatted Messages::   The `fmtmsg' function.
* Adding Severity Classes::       Add more severity classes.
* Example::                       How to use `fmtmsg' and `addseverity'.


File: libc.info,  Node: Printing Formatted Messages,  Next: Adding Severity Classes,  Up: Formatted Messages

Printing Formatted Messages
---------------------------

   Messages can be printed to standard error and/or to the console.  To
select the destination the programmer can use the following two values,
bitwise OR combined if wanted, for the CLASSIFICATION parameter of
`fmtmsg':

`MM_PRINT'
     Display the message in standard error.

`MM_CONSOLE'
     Display the message on the system console.

   The erroneous piece of the system can be signalled by exactly one of
the following values which also is bitwise ORed with the CLASSIFICATION
parameter to `fmtmsg':

`MM_HARD'
     The source of the condition is some hardware.

`MM_SOFT'
     The source of the condition is some software.

`MM_FIRM'
     The source of the condition is some firmware.

   A third component of the CLASSIFICATION parameter to `fmtmsg' can
describe the part of the system which detects the problem.  This is
done by using exactly one of the following values:

`MM_APPL'
     The erroneous condition is detected by the application.

`MM_UTIL'
     The erroneous condition is detected by a utility.

`MM_OPSYS'
     The erroneous condition is detected by the operating system.

   A last component of CLASSIFICATION can signal the results of this
message.  Exactly one of the following values can be used:

`MM_RECOVER'
     It is a recoverable error.

`MM_NRECOV'
     It is a non-recoverable error.

 - Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL,
          int SEVERITY, const char *TEXT, const char *ACTION, const
          char *TAG)
     Display a message described by its parameters on the device(s)
     specified in the CLASSIFICATION parameter.  The LABEL parameter
     identifies the source of the message.  The string should consist
     of two colon separated parts where the first part has not more
     than 10 and the second part not more than 14 characters.  The TEXT
     parameter describes the condition of the error, the ACTION
     parameter possible steps to recover from the error and the TAG
     parameter is a reference to the online documentation where more
     information can be found.  It should contain the LABEL value and a
     unique identification number.

     Each of the parameters can be a special value which means this
     value is to be omitted.  The symbolic names for these values are:

    `MM_NULLLBL'
          Ignore LABEL parameter.

    `MM_NULLSEV'
          Ignore SEVERITY parameter.

    `MM_NULLMC'
          Ignore CLASSIFICATION parameter.  This implies that nothing is
          actually printed.

    `MM_NULLTXT'
          Ignore TEXT parameter.

    `MM_NULLACT'
          Ignore ACTION parameter.

    `MM_NULLTAG'
          Ignore TAG parameter.

     There is another way certain fields can be omitted from the output
     to standard error.  This is described below in the description of
     environment variables influencing the behavior.

     The SEVERITY parameter can have one of the values in the following
     table:

    `MM_NOSEV'
          Nothing is printed, this value is the same as `MM_NULLSEV'.

    `MM_HALT'
          This value is printed as `HALT'.

    `MM_ERROR'
          This value is printed as `ERROR'.

    `MM_WARNING'
          This value is printed as `WARNING'.

    `MM_INFO'
          This value is printed as `INFO'.

     The numeric value of these five macros are between `0' and `4'.
     Using the environment variable `SEV_LEVEL' or using the
     `addseverity' function one can add more severity levels with their
     corresponding string to print.  This is described below (*note
     Adding Severity Classes::).

     If no parameter is ignored the output looks like this:

          LABEL: SEVERITY-STRING: TEXT
          TO FIX: ACTION TAG

     The colons, new line characters and the `TO FIX' string are
     inserted if necessary, i.e., if the corresponding parameter is not
     ignored.

     This function is specified in the X/Open Portability Guide.  It is
     also available on all systems derived from System V.

     The function returns the value `MM_OK' if no error occurred.  If
     only the printing to standard error failed, it returns `MM_NOMSG'.
     If printing to the console fails, it returns `MM_NOCON'.  If
     nothing is printed `MM_NOTOK' is returned.  Among situations where
     all outputs fail this last value is also returned if a parameter
     value is incorrect.

   There are two environment variables which influence the behavior of
`fmtmsg'.  The first is `MSGVERB'.  It is used to control the output
actually happening on standard error (_not_ the console output).  Each
of the five fields can explicitly be enabled.  To do this the user has
to put the `MSGVERB' variable with a format like the following in the
environment before calling the `fmtmsg' function the first time:

     MSGVERB=KEYWORD[:KEYWORD[:...]]

   Valid KEYWORDs are `label', `severity', `text', `action', and `tag'.
If the environment variable is not given or is the empty string, a not
supported keyword is given or the value is somehow else invalid, no
part of the message is masked out.

   The second environment variable which influences the behavior of
`fmtmsg' is `SEV_LEVEL'.  This variable and the change in the behavior
of `fmtmsg' is not specified in the X/Open Portability Guide.  It is
available in System V systems, though.  It can be used to introduce new
severity levels.  By default, only the five severity levels described
above are available.  Any other numeric value would make `fmtmsg' print
nothing.

   If the user puts `SEV_LEVEL' with a format like

     SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]]

in the environment of the process before the first call to `fmtmsg',
where DESCRIPTION has a value of the form

     SEVERITY-KEYWORD,LEVEL,PRINTSTRING

   The SEVERITY-KEYWORD part is not used by `fmtmsg' but it has to be
present.  The LEVEL part is a string representation of a number.  The
numeric value must be a number greater than 4.  This value must be used
in the SEVERITY parameter of `fmtmsg' to select this class.  It is not
possible to overwrite any of the predefined classes.  The PRINTSTRING
is the string printed when a message of this class is processed by
`fmtmsg' (see above, `fmtsmg' does not print the numeric value but
instead the string representation).


File: libc.info,  Node: Adding Severity Classes,  Next: Example,  Prev: Printing Formatted Messages,  Up: Formatted Messages

Adding Severity Classes
-----------------------

   There is another possibility to introduce severity classes besides
using the environment variable `SEV_LEVEL'.  This simplifies the task of
introducing new classes in a running program.  One could use the
`setenv' or `putenv' function to set the environment variable, but this
is toilsome.

 - Function: int addseverity (int SEVERITY, const char *STRING)
     This function allows the introduction of new severity classes
     which can be addressed by the SEVERITY parameter of the `fmtmsg'
     function.  The SEVERITY parameter of `addseverity' must match the
     value for the parameter with the same name of `fmtmsg', and STRING
     is the string printed in the actual messages instead of the numeric
     value.

     If STRING is `NULL' the severity class with the numeric value
     according to SEVERITY is removed.

     It is not possible to overwrite or remove one of the default
     severity classes.  All calls to `addseverity' with SEVERITY set to
     one of the values for the default classes will fail.

     The return value is `MM_OK' if the task was successfully performed.
     If the return value is `MM_NOTOK' something went wrong.  This could
     mean that no more memory is available or a class is not available
     when it has to be removed.

     This function is not specified in the X/Open Portability Guide
     although the `fmtsmg' function is.  It is available on System V
     systems.


File: libc.info,  Node: Example,  Prev: Adding Severity Classes,  Up: Formatted Messages

How to use `fmtmsg' and `addseverity'
-------------------------------------

   Here is a simple example program to illustrate the use of the both
functions described in this section.

     #include <fmtmsg.h>
     
     int
     main (void)
     {
       addseverity (5, "NOTE2");
       fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
       fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
               "UX:cat:001");
       fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
       return 0;
     }

   The second call to `fmtmsg' illustrates a use of this function as it
usually occurs on System V systems, which heavily use this function.
It seems worthwhile to give a short explanation here of how this system
works on System V.  The value of the LABEL field (`UX:cat') says that
the error occurred in the Unix program `cat'.  The explanation of the
error follows and the value for the ACTION parameter is `"refer to
manual"'.  One could be more specific here, if necessary.  The TAG
field contains, as proposed above, the value of the string given for
the LABEL parameter, and additionally a unique ID (`001' in this case).
For a GNU environment this string could contain a reference to the
corresponding node in the Info page for the program.

Running this program without specifying the `MSGVERB' and `SEV_LEVEL'
function produces the following output:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001

   We see the different fields of the message and how the extra glue
(the colons and the `TO FIX' string) are printed.  But only one of the
three calls to `fmtmsg' produced output.  The first call does not print
anything because the LABEL parameter is not in the correct form.  The
string must contain two fields, separated by a colon (*note Printing
Formatted Messages::).  The third `fmtmsg' call produced no output
since the class with the numeric value `6' is not defined.  Although a
class with numeric value `5' is also not defined by default, the call
to `addseverity' introduces it and the second call to `fmtmsg' produces
the above output.

   When we change the environment of the program to contain
`SEV_LEVEL=XXX,6,NOTE' when running it we get a different result:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001
     label:foo: NOTE: text
     TO FIX: action tag

   Now the third call to `fmtmsg' produced some output and we see how
the string `NOTE' from the environment variable appears in the message.

   Now we can reduce the output by specifying which fields we are
interested in.  If we additionally set the environment variable
`MSGVERB' to the value `severity:label:action' we get the following
output:

     UX:cat: NOTE2
     TO FIX: refer to manual
     label:foo: NOTE
     TO FIX: action

I.e., the output produced by the TEXT and the TAG parameters to
`fmtmsg' vanished.  Please also note that now there is no colon after
the `NOTE' and `NOTE2' strings in the output.  This is not necessary
since there is no more output on this line because the text is missing.


File: libc.info,  Node: Low-Level I/O,  Next: File System Interface,  Prev: I/O on Streams,  Up: Top

Low-Level Input/Output
**********************

   This chapter describes functions for performing low-level
input/output operations on file descriptors.  These functions include
the primitives for the higher-level I/O functions described in *Note
I/O on Streams::, as well as functions for performing low-level control
operations for which there are no equivalents on streams.

   Stream-level I/O is more flexible and usually more convenient;
therefore, programmers generally use the descriptor-level functions only
when necessary.  These are some of the usual reasons:

   * For reading binary files in large chunks.

   * For reading an entire file into core before parsing it.

   * To perform operations other than data transfer, which can only be
     done with a descriptor.  (You can use `fileno' to get the
     descriptor corresponding to a stream.)

   * To pass descriptors to a child process.  (The child can create its
     own stream to use a descriptor that it inherits, but cannot
     inherit a stream directly.)

* Menu:

* Opening and Closing Files::           How to open and close file
                                         descriptors.
* I/O Primitives::                      Reading and writing data.
* File Position Primitive::             Setting a descriptor's file
                                         position.
* Descriptors and Streams::             Converting descriptor to stream
                                         or vice-versa.
* Stream/Descriptor Precautions::       Precautions needed if you use both
                                         descriptors and streams.
* Scatter-Gather::                      Fast I/O to discontinuous buffers.
* Memory-mapped I/O::                   Using files like memory.
* Waiting for I/O::                     How to check for input or output
					 on multiple file descriptors.
* Synchronizing I/O::                   Making sure all I/O actions completed.
* Asynchronous I/O::                    Perform I/O in parallel.
* Control Operations::                  Various other operations on file
					 descriptors.
* Duplicating Descriptors::             Fcntl commands for duplicating
                                         file descriptors.
* Descriptor Flags::                    Fcntl commands for manipulating
                                         flags associated with file
                                         descriptors.
* File Status Flags::                   Fcntl commands for manipulating
                                         flags associated with open files.
* File Locks::                          Fcntl commands for implementing
                                         file locking.
* Interrupt Input::                     Getting an asynchronous signal when
                                         input arrives.
* IOCTLs::                              Generic I/O Control operations.

