    /* pngwio.c - functions for data output
     *
     * Last changed in libpng 1.6.0 [February 14, 2013]
     * Copyright (c) 1998-2013 Glenn Randers-Pehrson
     * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
     * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
     *
     * This code is released under the libpng license.
     * For conditions of distribution and use, see the disclaimer
     * and license in png.h
     *
     * This file provides a location for all output.  Users who need
     * special handling are expected to write functions that have the same
     * arguments as these and perform similar functions, but that possibly
     * use different output methods.  Note that you shouldn't change these
     * functions, but rather write replacement functions and then change
     * them at run time with png_set_write_fn(...).
     */

    #include "pngpriv.h"

    #ifdef PNG_WRITE_SUPPORTED

    /* Write the data to whatever output you are using.  The default routine
     * writes to a file pointer.  Note that this routine sometimes gets called
     * with very small lengths, so you should implement some kind of simple
     * buffering if you are using unbuffered writes.  This should never be asked
     * to write more than 64K on a 16 bit machine.
     */

    void /* PRIVATE */
    png_write_data(png_structrp png_ptr, png_const_bytep data, png_size_t length)
    {
       /* NOTE: write_data_fn must not change the buffer! */
       if (png_ptr->write_data_fn != NULL )
          (*(png_ptr->write_data_fn))(png_ptr, png_constcast(png_bytep,data),
             length);

       else
          png_error(png_ptr, "Call to NULL write function");
    }

    #ifdef PNG_STDIO_SUPPORTED
    /* This is the function that does the actual writing of data.  If you are
     * not writing to a standard C stream, you should create a replacement
     * write_data function and use it at run time with png_set_write_fn(), rather
     * than changing the library.
     */
    void PNGCBAPI
    png_default_write_data(png_structp png_ptr, png_bytep data, png_size_t length)
    {
       png_size_t check;

       if (png_ptr == NULL)
          return;

       check = fwrite(data, 1, length, (png_FILE_p)(png_ptr->io_ptr));

       if (check != length)
          png_error(png_ptr, "Write Error");
    }
    #endif

    /* This function is called to output any data pending writing (normally
     * to disk).  After png_flush is called, there should be no data pending
     * writing in any buffers.
     */
    #ifdef PNG_WRITE_FLUSH_SUPPORTED
    void /* PRIVATE */
    png_flush(png_structrp png_ptr)
    {
       if (png_ptr->output_flush_fn != NULL)
          (*(png_ptr->output_flush_fn))(png_ptr);
    }

    #  ifdef PNG_STDIO_SUPPORTED
    void PNGCBAPI
    png_default_flush(png_structp png_ptr)
    {
       png_FILE_p io_ptr;

       if (png_ptr == NULL)
          return;

       io_ptr = png_voidcast(png_FILE_p, (png_ptr->io_ptr));
       fflush(io_ptr);
    }
    #  endif
    #endif

    /* This function allows the application to supply new output functions for
     * libpng if standard C streams aren't being used.
     *
     * This function takes as its arguments:
     * png_ptr       - pointer to a png output data structure
     * io_ptr        - pointer to user supplied structure containing info about
     *                 the output functions.  May be NULL.
     * write_data_fn - pointer to a new output function that takes as its
     *                 arguments a pointer to a png_struct, a pointer to
     *                 data to be written, and a 32-bit unsigned int that is
     *                 the number of bytes to be written.  The new write
     *                 function should call png_error(png_ptr, "Error msg")
     *                 to exit and output any fatal error messages.  May be
     *                 NULL, in which case libpng's default function will
     *                 be used.
     * flush_data_fn - pointer to a new flush function that takes as its
     *                 arguments a pointer to a png_struct.  After a call to
     *                 the flush function, there should be no data in any buffers
     *                 or pending transmission.  If the output method doesn't do
     *                 any buffering of output, a function prototype must still be
     *                 supplied although it doesn't have to do anything.  If
     *                 PNG_WRITE_FLUSH_SUPPORTED is not defined at libpng compile
     *                 time, output_flush_fn will be ignored, although it must be
     *                 supplied for compatibility.  May be NULL, in which case
     *                 libpng's default function will be used, if
     *                 PNG_WRITE_FLUSH_SUPPORTED is defined.  This is not
     *                 a good idea if io_ptr does not point to a standard
     *                 *FILE structure.
     */
    void PNGAPI
    png_set_write_fn(png_structrp png_ptr, png_voidp io_ptr,
        png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn)
    {
       if (png_ptr == NULL)
          return;

       png_ptr->io_ptr = io_ptr;

    #ifdef PNG_STDIO_SUPPORTED
       if (write_data_fn != NULL)
          png_ptr->write_data_fn = write_data_fn;

       else
          png_ptr->write_data_fn = png_default_write_data;
    #else
       png_ptr->write_data_fn = write_data_fn;
    #endif

    #ifdef PNG_WRITE_FLUSH_SUPPORTED
    #  ifdef PNG_STDIO_SUPPORTED

       if (output_flush_fn != NULL)
          png_ptr->output_flush_fn = output_flush_fn;

       else
          png_ptr->output_flush_fn = png_default_flush;

    #  else
       png_ptr->output_flush_fn = output_flush_fn;
    #  endif
    #endif /* PNG_WRITE_FLUSH_SUPPORTED */

       /* It is an error to read while writing a png file */
       if (png_ptr->read_data_fn != NULL)
       {
          png_ptr->read_data_fn = NULL;

          png_warning(png_ptr,
              "Can't set both read_data_fn and write_data_fn in the"
              " same structure");
       }
    }
    #endif /* PNG_WRITE_SUPPORTED */