X-Git-Url: https://git.saurik.com/wxWidgets.git/blobdiff_plain/a626cc03053700ca5bdbf0243831bec528e6fc4d..cc4d5638c66a409e421420ed7110917755a66788:/src/png/pngmem.c?ds=sidebyside

diff --git a/src/png/pngmem.c b/src/png/pngmem.c
index 06c4c1b7ea..b9b3efb442 100644
--- a/src/png/pngmem.c
+++ b/src/png/pngmem.c
@@ -1,11 +1,14 @@
 
 /* pngmem.c - stub functions for memory allocation
  *
- * libpng 1.0.3 - January 14, 1999
- * For conditions of distribution and use, see copyright notice in png.h
- * Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.
- * Copyright (c) 1996, 1997 Andreas Dilger
- * Copyright (c) 1998, 1999 Glenn Randers-Pehrson
+ * 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 memory allocation.  Users who
  * need special memory handling are expected to supply replacement
@@ -14,498 +17,261 @@
  * identify the replacement functions.
  */
 
-#define PNG_INTERNAL
-#include "png.h"
-
-/* Borland DOS special memory handler */
-#if defined(__TURBOC__) && !defined(_Windows) && !defined(__FLAT__)
-/* if you change this, be sure to change the one in png.h also */
-
-/* Allocate memory for a png_struct.  The malloc and memset can be replaced
-   by a single call to calloc() if this is thought to improve performance. */
-png_voidp
-png_create_struct(int type)
-{
-#ifdef PNG_USER_MEM_SUPPORTED
-   return (png_create_struct_2(type, NULL));
-}
-
-/* Alternate version of png_create_struct, for use with user-defined malloc. */
-png_voidp
-png_create_struct_2(int type, png_malloc_ptr malloc_fn)
-{
-#endif /* PNG_USER_MEM_SUPPORTED */
-   png_size_t size;
-   png_voidp struct_ptr;
-
-   if (type == PNG_STRUCT_INFO)
-     size = sizeof(png_info);
-   else if (type == PNG_STRUCT_PNG)
-     size = sizeof(png_struct);
-   else
-     return ((png_voidp)NULL);
-
-#ifdef PNG_USER_MEM_SUPPORTED
-   if(malloc_fn != NULL)
-   {
-      if ((struct_ptr = (*(malloc_fn))(NULL, size)) != NULL)
-         png_memset(struct_ptr, 0, size);
-         return (struct_ptr);
-   }
-#endif /* PNG_USER_MEM_SUPPORTED */
-   if ((struct_ptr = (png_voidp)farmalloc(size)) != NULL)
-   {
-      png_memset(struct_ptr, 0, size);
-   }
-   return (struct_ptr);
-}
-
+#include "pngpriv.h"
 
-/* Free memory allocated by a png_create_struct() call */
-void
-png_destroy_struct(png_voidp struct_ptr)
+#if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
+/* Free a png_struct */
+void /* PRIVATE */
+png_destroy_png_struct(png_structrp png_ptr)
 {
-#ifdef PNG_USER_MEM_SUPPORTED
-   png_destroy_struct_2(struct_ptr, (png_free_ptr)NULL);
-}
-
-/* Free memory allocated by a png_create_struct() call */
-void
-png_destroy_struct_2(png_voidp struct_ptr, png_free_ptr free_fn)
-{
-#endif
-   if (struct_ptr != NULL)
+   if (png_ptr != NULL)
    {
-#ifdef PNG_USER_MEM_SUPPORTED
-      if(free_fn != NULL)
-      {
-         png_struct dummy_struct;
-         png_structp png_ptr = &dummy_struct;
-         (*(free_fn))(png_ptr, struct_ptr);
-         struct_ptr = NULL;
-         return;
-      }
-#endif /* PNG_USER_MEM_SUPPORTED */
-      farfree (struct_ptr);
-      struct_ptr = NULL;
+      /* png_free might call png_error and may certainly call
+       * png_get_mem_ptr, so fake a temporary png_struct to support this.
+       */
+      png_struct dummy_struct = *png_ptr;
+      memset(png_ptr, 0, (sizeof *png_ptr));
+      png_free(&dummy_struct, png_ptr);
+
+#     ifdef PNG_SETJMP_SUPPORTED
+         /* We may have a jmp_buf left to deallocate. */
+         png_free_jmpbuf(&dummy_struct);
+#     endif
    }
 }
 
 /* Allocate memory.  For reasonable files, size should never exceed
  * 64K.  However, zlib may allocate more then 64K if you don't tell
- * it not to.  See zconf.h and png.h for more information. zlib does
+ * it not to.  See zconf.h and png.h for more information.  zlib does
  * need to allocate exactly 64K, so whatever you call here must
  * have the ability to do that.
- *
- * Borland seems to have a problem in DOS mode for exactly 64K.
- * It gives you a segment with an offset of 8 (perhaps to store its
- * memory stuff).  zlib doesn't like this at all, so we have to
- * detect and deal with it.  This code should not be needed in
- * Windows or OS/2 modes, and only in 16 bit mode.  This code has
- * been updated by Alexander Lehmann for version 0.89 to waste less
- * memory.
- *
- * Note that we can't use png_size_t for the "size" declaration,
- * since on some systems a png_size_t is a 16-bit quantity, and as a
- * result, we would be truncating potentially larger memory requests
- * (which should cause a fatal error) and introducing major problems.
  */
-png_voidp
-png_malloc(png_structp png_ptr, png_uint_32 size)
+PNG_FUNCTION(png_voidp,PNGAPI
+png_calloc,(png_const_structrp png_ptr, png_alloc_size_t size),PNG_ALLOCATED)
 {
-#ifndef PNG_USER_MEM_SUPPORTED
    png_voidp ret;
-#endif
-   if (png_ptr == NULL || size == 0)
-      return ((png_voidp)NULL);
 
-#ifdef PNG_USER_MEM_SUPPORTED
-   if(png_ptr->malloc_fn != NULL)
-       return ((png_voidp)(*(png_ptr->malloc_fn))(png_ptr, size));
-   else
-       return png_malloc_default(png_ptr, size);
+   ret = png_malloc(png_ptr, size);
+
+   if (ret != NULL)
+      memset(ret, 0, size);
+
+   return ret;
 }
 
-png_voidp
-png_malloc_default(png_structp png_ptr, png_uint_32 size)
+/* png_malloc_base, an internal function added at libpng 1.6.0, does the work of
+ * allocating memory, taking into account limits and PNG_USER_MEM_SUPPORTED.
+ * Checking and error handling must happen outside this routine; it returns NULL
+ * if the allocation cannot be done (for any reason.)
+ */
+PNG_FUNCTION(png_voidp /* PRIVATE */,
+png_malloc_base,(png_const_structrp png_ptr, png_alloc_size_t size),
+   PNG_ALLOCATED)
 {
-   png_voidp ret;
-#endif /* PNG_USER_MEM_SUPPORTED */
-
-#ifdef PNG_MAX_MALLOC_64K
-   if (size > (png_uint_32)65536L)
-      png_error(png_ptr, "Cannot Allocate > 64K");
+   /* Moved to png_malloc_base from png_malloc_default in 1.6.0; the DOS
+    * allocators have also been removed in 1.6.0, so any 16-bit system now has
+    * to implement a user memory handler.  This checks to be sure it isn't
+    * called with big numbers.
+    */
+#ifdef PNG_USER_MEM_SUPPORTED
+   PNG_UNUSED(png_ptr)
 #endif
-
-   if (size == (png_uint_32)65536L)
+   if (size > 0 && size <= PNG_SIZE_MAX
+#     ifdef PNG_MAX_MALLOC_64K
+         && size <= 65536U
+#     endif
+      )
    {
-      if (png_ptr->offset_table == NULL)
-      {
-         /* try to see if we need to do any of this fancy stuff */
-         ret = farmalloc(size);
-         if (ret == NULL || ((png_size_t)ret & 0xffff))
-         {
-            int num_blocks;
-            png_uint_32 total_size;
-            png_bytep table;
-            int i;
-            png_byte huge * hptr;
-
-            if (ret != NULL)
-            {
-               farfree(ret);
-               ret = NULL;
-            }
-
-            num_blocks = (int)(1 << (png_ptr->zlib_window_bits - 14));
-            if (num_blocks < 1)
-               num_blocks = 1;
-            if (png_ptr->zlib_mem_level >= 7)
-               num_blocks += (int)(1 << (png_ptr->zlib_mem_level - 7));
-            else
-               num_blocks++;
-
-            total_size = ((png_uint_32)65536L) * (png_uint_32)num_blocks+16;
-
-            table = farmalloc(total_size);
-
-            if (table == NULL)
-            {
-               png_error(png_ptr, "Out Of Memory."); /* Note "O" and "M" */
-            }
-
-            if ((png_size_t)table & 0xfff0)
-            {
-               png_error(png_ptr, "Farmalloc didn't return normalized pointer");
-            }
-
-            png_ptr->offset_table = table;
-            png_ptr->offset_table_ptr = farmalloc(num_blocks *
-               sizeof (png_bytep));
-
-            if (png_ptr->offset_table_ptr == NULL)
-            {
-               png_error(png_ptr, "Out Of memory.");
-            }
-
-            hptr = (png_byte huge *)table;
-            if ((png_size_t)hptr & 0xf)
-            {
-               hptr = (png_byte huge *)((long)(hptr) & 0xfffffff0L);
-               hptr += 16L;
-            }
-            for (i = 0; i < num_blocks; i++)
-            {
-               png_ptr->offset_table_ptr[i] = (png_bytep)hptr;
-               hptr += (png_uint_32)65536L;
-            }
-
-            png_ptr->offset_table_number = num_blocks;
-            png_ptr->offset_table_count = 0;
-            png_ptr->offset_table_count_free = 0;
-         }
-      }
-
-      if (png_ptr->offset_table_count >= png_ptr->offset_table_number)
-         png_error(png_ptr, "Out of Memory.");
+#ifdef PNG_USER_MEM_SUPPORTED
+      if (png_ptr != NULL && png_ptr->malloc_fn != NULL)
+         return png_ptr->malloc_fn(png_constcast(png_structrp,png_ptr), size);
 
-      ret = png_ptr->offset_table_ptr[png_ptr->offset_table_count++];
+      else
+#endif
+         return malloc((size_t)size); /* checked for truncation above */
    }
+
    else
-      ret = farmalloc(size);
+      return NULL;
+}
 
-   if (ret == NULL)
-   {
-      png_error(png_ptr, "Out of memory."); /* Note "o" and "m" */
-   }
+/* This is really here only to work round a spurious warning in GCC 4.6 and 4.7
+ * that arises because of the checks in png_realloc_array that are repeated in
+ * png_malloc_array.
+ */
+static png_voidp
+png_malloc_array_checked(png_const_structrp png_ptr, int nelements,
+   size_t element_size)
+{
+   png_alloc_size_t req = nelements; /* known to be > 0 */
 
-   return (ret);
+   if (req <= PNG_SIZE_MAX/element_size)
+      return png_malloc_base(png_ptr, req * element_size);
+
+   /* The failure case when the request is too large */
+   return NULL;
 }
 
-/* free a pointer allocated by png_malloc().  In the default
-   configuration, png_ptr is not used, but is passed in case it
-   is needed.  If ptr is NULL, return without taking any action. */
-void
-png_free(png_structp png_ptr, png_voidp ptr)
+PNG_FUNCTION(png_voidp /* PRIVATE */,
+png_malloc_array,(png_const_structrp png_ptr, int nelements,
+   size_t element_size),PNG_ALLOCATED)
 {
-   if (png_ptr == NULL || ptr == NULL)
-      return;
+   if (nelements <= 0 || element_size == 0)
+      png_error(png_ptr, "internal error: array alloc");
 
-#ifdef PNG_USER_MEM_SUPPORTED
-   if (png_ptr->free_fn != NULL)
-   {
-      (*(png_ptr->free_fn))(png_ptr, ptr);
-      ptr = NULL;
-      return;
-   }
-   else png_free_default(png_ptr, ptr);
+   return png_malloc_array_checked(png_ptr, nelements, element_size);
 }
 
-void
-png_free_default(png_structp png_ptr, png_voidp ptr)
+PNG_FUNCTION(png_voidp /* PRIVATE */,
+png_realloc_array,(png_const_structrp png_ptr, png_const_voidp old_array,
+   int old_elements, int add_elements, size_t element_size),PNG_ALLOCATED)
 {
-#endif /* PNG_USER_MEM_SUPPORTED */
-
-   if (png_ptr->offset_table != NULL)
+   /* These are internal errors: */
+   if (add_elements <= 0 || element_size == 0 || old_elements < 0 ||
+      (old_array == NULL && old_elements > 0))
+      png_error(png_ptr, "internal error: array realloc");
+
+   /* Check for overflow on the elements count (so the caller does not have to
+    * check.)
+    */
+   if (add_elements <= INT_MAX - old_elements)
    {
-      int i;
+      png_voidp new_array = png_malloc_array_checked(png_ptr,
+         old_elements+add_elements, element_size);
 
-      for (i = 0; i < png_ptr->offset_table_count; i++)
-      {
-         if (ptr == png_ptr->offset_table_ptr[i])
-         {
-            ptr = NULL;
-            png_ptr->offset_table_count_free++;
-            break;
-         }
-      }
-      if (png_ptr->offset_table_count_free == png_ptr->offset_table_count)
+      if (new_array != NULL)
       {
-         farfree(png_ptr->offset_table);
-         farfree(png_ptr->offset_table_ptr);
-         png_ptr->offset_table = NULL;
-         png_ptr->offset_table_ptr = NULL;
-      }
-   }
+         /* Because png_malloc_array worked the size calculations below cannot
+          * overflow.
+          */
+         if (old_elements > 0)
+            memcpy(new_array, old_array, element_size*(unsigned)old_elements);
 
-   if (ptr != NULL)
-   {
-      farfree(ptr);
-      ptr = NULL;
-   }
-}
+         memset((char*)new_array + element_size*(unsigned)old_elements, 0,
+            element_size*(unsigned)add_elements);
 
-#else /* Not the Borland DOS special memory handler */
+         return new_array;
+      }
+   }
 
-/* Allocate memory for a png_struct or a png_info.  The malloc and
-   memset can be replaced by a single call to calloc() if this is thought
-   to improve performance noticably.*/
-png_voidp
-png_create_struct(int type)
-{
-#ifdef PNG_USER_MEM_SUPPORTED
-   return (png_create_struct_2(type, NULL));
+   return NULL; /* error */
 }
 
-/* Allocate memory for a png_struct or a png_info.  The malloc and
-   memset can be replaced by a single call to calloc() if this is thought
-   to improve performance noticably.*/
-png_voidp
-png_create_struct_2(int type, png_malloc_ptr malloc_fn)
+/* Various functions that have different error handling are derived from this.
+ * png_malloc always exists, but if PNG_USER_MEM_SUPPORTED is defined a separate
+ * function png_malloc_default is also provided.
+ */
+PNG_FUNCTION(png_voidp,PNGAPI
+png_malloc,(png_const_structrp png_ptr, png_alloc_size_t size),PNG_ALLOCATED)
 {
-#endif /* PNG_USER_MEM_SUPPORTED */
-   png_size_t size;
-   png_voidp struct_ptr;
+   png_voidp ret;
 
-   if (type == PNG_STRUCT_INFO)
-      size = sizeof(png_info);
-   else if (type == PNG_STRUCT_PNG)
-      size = sizeof(png_struct);
-   else
-      return ((png_voidp)NULL);
+   if (png_ptr == NULL)
+      return NULL;
 
-#ifdef PNG_USER_MEM_SUPPORTED
-   if(malloc_fn != NULL)
-   {
-      if ((struct_ptr = (*(malloc_fn))(NULL, size)) != NULL)
-         png_memset(struct_ptr, 0, size);
-      return (struct_ptr);
-   }
-#endif /* PNG_USER_MEM_SUPPORTED */
+   ret = png_malloc_base(png_ptr, size);
 
-#if defined(__TURBOC__) && !defined(__FLAT__)
-   if ((struct_ptr = (png_voidp)farmalloc(size)) != NULL)
-#else
-# if defined(_MSC_VER) && defined(MAXSEG_64K)
-   if ((struct_ptr = (png_voidp)halloc(size,1)) != NULL)
-# else
-   if ((struct_ptr = (png_voidp)malloc(size)) != NULL)
-# endif
-#endif
-   {
-      png_memset(struct_ptr, 0, size);
-   }
+   if (ret == NULL)
+       png_error(png_ptr, "Out of memory"); /* 'm' means png_malloc */
 
-   return (struct_ptr);
+   return ret;
 }
 
-
-/* Free memory allocated by a png_create_struct() call */
-void
-png_destroy_struct(png_voidp struct_ptr)
-{
 #ifdef PNG_USER_MEM_SUPPORTED
-   png_destroy_struct_2(struct_ptr, (png_free_ptr)NULL);
-}
-
-/* Free memory allocated by a png_create_struct() call */
-void
-png_destroy_struct_2(png_voidp struct_ptr, png_free_ptr free_fn)
+PNG_FUNCTION(png_voidp,PNGAPI
+png_malloc_default,(png_const_structrp png_ptr, png_alloc_size_t size),
+   PNG_ALLOCATED PNG_DEPRECATED)
 {
-#endif /* PNG_USER_MEM_SUPPORTED */
-   if (struct_ptr != NULL)
-   {
-#ifdef PNG_USER_MEM_SUPPORTED
-      if(free_fn != NULL)
-      {
-         png_struct dummy_struct;
-         png_structp png_ptr = &dummy_struct;
-         (*(free_fn))(png_ptr, struct_ptr);
-         struct_ptr = NULL;
-         return;
-      }
-#endif /* PNG_USER_MEM_SUPPORTED */
-#if defined(__TURBOC__) && !defined(__FLAT__)
-      farfree(struct_ptr);
-      struct_ptr = NULL;
-#else
-# if defined(_MSC_VER) && defined(MAXSEG_64K)
-      hfree(struct_ptr);
-      struct_ptr = NULL;
-# else
-      free(struct_ptr);
-      struct_ptr = NULL;
-# endif
-#endif
-   }
-}
+   png_voidp ret;
 
+   if (png_ptr == NULL)
+      return NULL;
 
-/* Allocate memory.  For reasonable files, size should never exceed
-   64K.  However, zlib may allocate more then 64K if you don't tell
-   it not to.  See zconf.h and png.h for more information.  zlib does
-   need to allocate exactly 64K, so whatever you call here must
-   have the ability to do that. */
+   /* Passing 'NULL' here bypasses the application provided memory handler. */
+   ret = png_malloc_base(NULL/*use malloc*/, size);
 
-png_voidp
-png_malloc(png_structp png_ptr, png_uint_32 size)
-{
-#ifndef PNG_USER_MEM_SUPPORTED
-   png_voidp ret;
-#endif
-   if (png_ptr == NULL || size == 0)
-      return ((png_voidp)NULL);
+   if (ret == NULL)
+      png_error(png_ptr, "Out of Memory"); /* 'M' means png_malloc_default */
 
-#ifdef PNG_USER_MEM_SUPPORTED
-   if(png_ptr->malloc_fn != NULL)
-       return ((png_voidp)(*(png_ptr->malloc_fn))(png_ptr, size));
-   else
-       return (png_malloc_default(png_ptr, size));
+   return ret;
 }
-png_voidp
-png_malloc_default(png_structp png_ptr, png_uint_32 size)
-{
-   png_voidp ret;
 #endif /* PNG_USER_MEM_SUPPORTED */
 
-#ifdef PNG_MAX_MALLOC_64K
-   if (size > (png_uint_32)65536L)
-      png_error(png_ptr, "Cannot Allocate > 64K");
-#endif
+/* This function was added at libpng version 1.2.3.  The png_malloc_warn()
+ * function will issue a png_warning and return NULL instead of issuing a
+ * png_error, if it fails to allocate the requested memory.
+ */
+PNG_FUNCTION(png_voidp,PNGAPI
+png_malloc_warn,(png_const_structrp png_ptr, png_alloc_size_t size),
+   PNG_ALLOCATED)
+{
+   if (png_ptr != NULL)
+   {
+      png_voidp ret = png_malloc_base(png_ptr, size);
 
-#if defined(__TURBOC__) && !defined(__FLAT__)
-   ret = farmalloc(size);
-#else
-# if defined(_MSC_VER) && defined(MAXSEG_64K)
-   ret = halloc(size, 1);
-# else
-   ret = malloc((size_t)size);
-# endif
-#endif
+      if (ret != NULL)
+         return ret;
 
-   if (ret == NULL)
-   {
-      png_error(png_ptr, "Out of Memory");
+      png_warning(png_ptr, "Out of memory");
    }
 
-   return (ret);
+   return NULL;
 }
 
 /* Free a pointer allocated by png_malloc().  If ptr is NULL, return
-   without taking any action. */
-void
-png_free(png_structp png_ptr, png_voidp ptr)
+ * without taking any action.
+ */
+void PNGAPI
+png_free(png_const_structrp png_ptr, png_voidp ptr)
 {
    if (png_ptr == NULL || ptr == NULL)
       return;
 
 #ifdef PNG_USER_MEM_SUPPORTED
    if (png_ptr->free_fn != NULL)
-   {
-      (*(png_ptr->free_fn))(png_ptr, ptr);
-      ptr = NULL;
-      return;
-   }
-   else png_free_default(png_ptr, ptr);
-}
-void
-png_free_default(png_structp png_ptr, png_voidp ptr)
-{
-#endif /* PNG_USER_MEM_SUPPORTED */
+      png_ptr->free_fn(png_constcast(png_structrp,png_ptr), ptr);
 
-#if defined(__TURBOC__) && !defined(__FLAT__)
-   farfree(ptr);
-   ptr = NULL;
-#else
-# if defined(_MSC_VER) && defined(MAXSEG_64K)
-   hfree(ptr);
-   ptr = NULL;
-# else
-   free(ptr);
-   ptr = NULL;
-# endif
-#endif
-}
-
-#endif /* Not Borland DOS special memory handler */
-
-png_voidp
-png_memcpy_check (png_structp png_ptr, png_voidp s1, png_voidp s2,
-   png_uint_32 length)
-{
-   png_size_t size;
-
-   size = (png_size_t)length;
-   if ((png_uint_32)size != length)
-      png_error(png_ptr,"Overflow in png_memcpy_check.");
-
-   return(png_memcpy (s1, s2, size));
+   else
+      png_free_default(png_ptr, ptr);
 }
 
-png_voidp
-png_memset_check (png_structp png_ptr, png_voidp s1, int value,
-   png_uint_32 length)
+PNG_FUNCTION(void,PNGAPI
+png_free_default,(png_const_structrp png_ptr, png_voidp ptr),PNG_DEPRECATED)
 {
-   png_size_t size;
-
-   size = (png_size_t)length;
-   if ((png_uint_32)size != length)
-      png_error(png_ptr,"Overflow in png_memset_check.");
-
-   return (png_memset (s1, value, size));
+   if (png_ptr == NULL || ptr == NULL)
+      return;
+#endif /* PNG_USER_MEM_SUPPORTED */
 
+   free(ptr);
 }
 
 #ifdef PNG_USER_MEM_SUPPORTED
 /* This function is called when the application wants to use another method
  * of allocating and freeing memory.
  */
-void
-png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr, png_malloc_ptr
+void PNGAPI
+png_set_mem_fn(png_structrp png_ptr, png_voidp mem_ptr, png_malloc_ptr
   malloc_fn, png_free_ptr free_fn)
 {
-   png_ptr->mem_ptr = mem_ptr;
-   png_ptr->malloc_fn = malloc_fn;
-   png_ptr->free_fn = free_fn;
+   if (png_ptr != NULL)
+   {
+      png_ptr->mem_ptr = mem_ptr;
+      png_ptr->malloc_fn = malloc_fn;
+      png_ptr->free_fn = free_fn;
+   }
 }
 
 /* This function returns a pointer to the mem_ptr associated with the user
  * functions.  The application should free any memory associated with this
  * pointer before png_write_destroy and png_read_destroy are called.
  */
-png_voidp
-png_get_mem_ptr(png_structp png_ptr)
+png_voidp PNGAPI
+png_get_mem_ptr(png_const_structrp png_ptr)
 {
-   return ((png_voidp)png_ptr->mem_ptr);
+   if (png_ptr == NULL)
+      return NULL;
+
+   return png_ptr->mem_ptr;
 }
 #endif /* PNG_USER_MEM_SUPPORTED */
+#endif /* PNG_READ_SUPPORTED || PNG_WRITE_SUPPORTED */