    /* pngrio.c - functions for data input
     *
     * 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 input.  Users who need
     * special handling are expected to write a function that has the same
     * arguments as this and performs a similar function, but that possibly
     * has a different input method.  Note that you shouldn't change this
     * function, but rather write a replacement function and then make
     * libpng use it at run time with png_set_read_fn(...).
     */

    #include "pngpriv.h"

    #ifdef PNG_READ_SUPPORTED

    /* Read the data from whatever input you are using.  The default routine
     * reads from 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 reads.  This should never be asked
     * to read more then 64K on a 16 bit machine.
     */
    void /* PRIVATE */
    png_read_data(png_structrp png_ptr, png_bytep data, png_size_t length)
    {
       png_debug1(4, "reading %d bytes", (int)length);

       if (png_ptr->read_data_fn != NULL)
          (*(png_ptr->read_data_fn))(png_ptr, data, length);

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

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

       if (png_ptr == NULL)
          return;

       /* fread() returns 0 on error, so it is OK to store this in a png_size_t
        * instead of an int, which is what fread() actually returns.
        */
       check = fread(data, 1, length, png_voidcast(png_FILE_p, png_ptr->io_ptr));

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

    /* This function allows the application to supply a new input function
     * for libpng if standard C streams aren't being used.
     *
     * This function takes as its arguments:
     *
     * png_ptr      - pointer to a png input data structure
     *
     * io_ptr       - pointer to user supplied structure containing info about
     *                the input functions.  May be NULL.
     *
     * read_data_fn - pointer to a new input function that takes as its
     *                arguments a pointer to a png_struct, a pointer to
     *                a location where input data can be stored, and a 32-bit
     *                unsigned int that is the number of bytes to be read.
     *                To exit and output any fatal error messages the new write
     *                function should call png_error(png_ptr, "Error msg").
     *                May be NULL, in which case libpng's default function will
     *                be used.
     */
    void PNGAPI
    png_set_read_fn(png_structrp png_ptr, png_voidp io_ptr,
       png_rw_ptr read_data_fn)
    {
       if (png_ptr == NULL)
          return;

       png_ptr->io_ptr = io_ptr;

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

       else
          png_ptr->read_data_fn = png_default_read_data;
    #else
       png_ptr->read_data_fn = read_data_fn;
    #endif

       /* It is an error to write to a read device */
       if (png_ptr->write_data_fn != NULL)
       {
          png_ptr->write_data_fn = NULL;
          png_warning(png_ptr,
              "Can't set both read_data_fn and write_data_fn in the"
              " same structure");
       }

    #ifdef PNG_WRITE_FLUSH_SUPPORTED
       png_ptr->output_flush_fn = NULL;
    #endif
    }
    #endif /* PNG_READ_SUPPORTED */