diff --git a/contrib/libtests/pngstest.c b/contrib/libtests/pngstest.c
index 0f9b16cb53..56c5b4a196 100644
--- a/contrib/libtests/pngstest.c
+++ b/contrib/libtests/pngstest.c
@@ -1,7 +1,7 @@
 /* pngstest.c
  *
  * Copyright (c) 2021-2024 Cosmin Truta
- * Copyright (c) 2013-2017 John Cunningham Bowler
+ * Copyright (c) 2013-2017,2024 John Cunningham Bowler
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
@@ -46,6 +46,18 @@
 #ifdef PNG_SIMPLIFIED_READ_SUPPORTED /* Else nothing can be done */
 #include "../tools/sRGB.h"
 
+/* This is a sanity check on PNG_HAS_ATTRIBUTE.
+ */
+#if PNG_HAS_ATTRIBUTE(__unknown__::attribute)
+#  error PNG_HAS_ATTRIBUTE is not working correctly
+#endif
+
+#if PNG_HAS_ATTRIBUTE(fallthrough)
+#  define FALLTHROUGH PNG_ATTRIBUTE(fallthrough)
+#else
+#  define FALLTRHOUGH /* FALLTHROUGH */
+#endif
+
 /* KNOWN ISSUES
  *
  * These defines switch on alternate algorithms for format conversions to match
@@ -2885,13 +2897,13 @@ compare_two_images(Image *a, Image *b, int via_linear,
                {
                   case 4:
                      if (pua[btoa[3]] != pub[3]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 3:
                      if (pua[btoa[2]] != pub[2]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 2:
                      if (pua[btoa[1]] != pub[1]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 1:
                      if (pua[btoa[0]] != pub[0]) break;
                      if (alpha_added != 4 && pub[alpha_added] != 65535) break;
@@ -2907,13 +2919,13 @@ compare_two_images(Image *a, Image *b, int via_linear,
                {
                   case 4:
                      if (psa[btoa[3]] != psb[3]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 3:
                      if (psa[btoa[2]] != psb[2]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 2:
                      if (psa[btoa[1]] != psb[1]) break;
-                     /* FALLTHROUGH */
+                     FALLTHROUGH; /* FALLTHROUGH */
                   case 1:
                      if (psa[btoa[0]] != psb[0]) break;
                      if (alpha_added != 4 && psb[alpha_added] != 255) break;
diff --git a/contrib/libtests/pngunknown.c b/contrib/libtests/pngunknown.c
index 9528ab9f09..0f5105033a 100644
--- a/contrib/libtests/pngunknown.c
+++ b/contrib/libtests/pngunknown.c
@@ -2,6 +2,7 @@
  *
  * Copyright (c) 2021-2024 Cosmin Truta
  * Copyright (c) 2015,2017 Glenn Randers-Pehrson
+ * Copyright (c) 2015-2016,2024 John Bowler
  * Written by John Cunningham Bowler
  *
  * This code is released under the libpng license.
@@ -38,6 +39,12 @@
 #  define SKIP 0
 #endif
 
+#if PNG_HAS_ATTRIBUTE(fallthrough)
+#  define FALLTHROUGH PNG_ATTRIBUTE(fallthrough)
+#else
+#  define FALLTRHOUGH /* FALLTHROUGH */
+#endif
+
 
 /* Since this program tests the ability to change the unknown chunk handling
  * these must be defined:
@@ -433,7 +440,7 @@ clean_display(display *d)
    }
 }
 
-PNG_FUNCTION(void, display_exit, (display *d), static PNG_NORETURN)
+PNG_FUNCTION(void, display_exit, (display *d), PNG_NORETURN static)
 {
    ++(d->error_count);
 
@@ -457,7 +464,7 @@ display_rc(const display *d, int strict)
 
 /* libpng error and warning callbacks */
 PNG_FUNCTION(void, (PNGCBAPI error), (png_structp png_ptr, const char *message),
-   static PNG_NORETURN)
+   PNG_NORETURN static)
 {
    display *d = (display*)png_get_error_ptr(png_ptr);
 
@@ -623,7 +630,7 @@ get_unknown(display *d, png_infop info_ptr, int after_IDAT)
                   ++(d->error_count);
                   break;
                }
-               /* FALLTHROUGH */ /* (safe) */
+               FALLTHROUGH; /* (safe) */ /* FALLTHROUGH */
             case PNG_HANDLE_CHUNK_ALWAYS:
                break;
          }
@@ -1104,7 +1111,7 @@ static const char *standard_tests[] =
    NULL /*end*/
 };
 
-static PNG_NORETURN void
+PNG_NORETURN static void
 usage(const char *program, const char *reason)
 {
    fprintf(stderr, "pngunknown: %s: usage:\n %s [--strict] "
diff --git a/contrib/libtests/pngvalid.c b/contrib/libtests/pngvalid.c
index d7d7a2ef77..2d607c4fc9 100644
--- a/contrib/libtests/pngvalid.c
+++ b/contrib/libtests/pngvalid.c
@@ -1,7 +1,7 @@
 /* pngvalid.c - validate libpng by constructing then reading png files.
  *
  * Copyright (c) 2021-2024 Cosmin Truta
- * Copyright (c) 2014-2017 John Cunningham Bowler
+ * Copyright (c) 2014-2017,2024 John Cunningham Bowler
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
@@ -156,6 +156,13 @@ typedef png_byte *png_const_bytep;
 #  endif
 #endif
 
+#if PNG_HAS_ATTRIBUTE(fallthrough)
+#  define FALLTHROUGH PNG_ATTRIBUTE(fallthrough)
+#else
+#  define FALLTRHOUGH /* FALLTHROUGH */
+#endif
+
+
 /***************************** EXCEPTION HANDLING *****************************/
 #ifdef PNG_FREESTANDING_TESTS
 #  include <cexcept.h>
@@ -6589,12 +6596,12 @@ transform_info_imp(transform_display *dp, png_structp pp, png_infop pi)
    {
    case PNG_COLOR_TYPE_PALETTE:
       if (dp->output_bit_depth > 8) goto error;
-      /* FALLTHROUGH */
+      FALLTHROUGH; /* FALLTHROUGH */
    case PNG_COLOR_TYPE_GRAY:
       if (dp->output_bit_depth == 1 || dp->output_bit_depth == 2 ||
          dp->output_bit_depth == 4)
          break;
-      /* FALLTHROUGH */
+      FALLTHROUGH; /* FALLTHROUGH */
    default:
       if (dp->output_bit_depth == 8 || dp->output_bit_depth == 16)
          break;
@@ -10001,7 +10008,7 @@ gamma_component_validate(const char *name, const validate_info *vi,
                   use_background = (alpha >= 0 && alpha < 1);
 #           endif
 #           ifdef PNG_READ_ALPHA_MODE_SUPPORTED
-               /* FALLTHROUGH */
+                  FALLTHROUGH; /* FALLTHROUGH */
                case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
                case ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN:
                case ALPHA_MODE_OFFSET + PNG_ALPHA_OPTIMIZED:
diff --git a/contrib/tools/pngfix.c b/contrib/tools/pngfix.c
index e4a5d5cf39..411010dbd5 100644
--- a/contrib/tools/pngfix.c
+++ b/contrib/tools/pngfix.c
@@ -31,6 +31,12 @@
 #  include "../../png.h"
 #endif
 
+#if PNG_HAS_ATTRIBUTE(fallthrough)
+#  define FALLTHROUGH PNG_ATTRIBUTE(fallthrough)
+#else
+#  define FALLTRHOUGH /* FALLTHROUGH */
+#endif
+
 #ifdef PNG_SETJMP_SUPPORTED
 #include <setjmp.h>
 
@@ -2399,7 +2405,7 @@ zlib_advance(struct zlib *zlib, png_uint_32 nbytes)
                   endrc = ZLIB_TOO_FAR_BACK;
                   break;
                }
-               /* FALLTHROUGH */
+               FALLTHROUGH; /* FALLTHROUGH */
 
             default:
                zlib_message(zlib, 0/*stream error*/);
@@ -2553,7 +2559,7 @@ zlib_run(struct zlib *zlib)
                   list->lengths[i] -= zlib->extra_bytes;
                   list->count = i+1;
                   zlib->idat->idat_list_tail = list;
-                  /* FALLTHROUGH */
+                  FALLTHROUGH; /* FALLTHROUGH */
 
                default:
                   return rc;
@@ -2656,7 +2662,7 @@ zlib_check(struct file *file, png_uint_32 offset)
             /* Truncated stream; unrecoverable, gets converted to ZLIB_FATAL */
             zlib.z.msg = PNGZ_MSG_CAST("[truncated]");
             zlib_message(&zlib, 0/*expected*/);
-            /* FALLTHROUGH */
+            FALLTHROUGH; /* FALLTHROUGH */
 
          default:
             /* Unrecoverable error; skip the chunk; a zlib_message has already
@@ -3324,7 +3330,7 @@ read_callback(png_structp png_ptr, png_bytep buffer, size_t count)
                if (file->state != STATE_IDAT && length > 0)
                   setpos(chunk);
             }
-            /* FALLTHROUGH */
+            FALLTHROUGH; /* FALLTHROUGH */
 
          default:
             assert(chunk != NULL);
diff --git a/pngconf.h b/pngconf.h
index df9e099563..6e27dabd72 100644
--- a/pngconf.h
+++ b/pngconf.h
@@ -298,7 +298,7 @@
 #ifndef PNG_EXPORTA
 #  define PNG_EXPORTA(ordinal, type, name, args, attributes) \
       PNG_FUNCTION(PNG_EXPORT_TYPE(type), (PNGAPI name), args, \
-      PNG_LINKAGE_API attributes)
+      attributes PNG_LINKAGE_API)
 #endif
 
 /* ANSI-C (C90) does not permit a macro to be invoked with an empty argument,
@@ -318,130 +318,166 @@
 #  define PNG_CALLBACK(type, name, args) type (PNGCBAPI name) args
 #endif
 
-/* Support for compiler specific function attributes.  These are used
- * so that where compiler support is available incorrect use of API
- * functions in png.h will generate compiler warnings.
+/* "Pedantic" warnings.  These are warnings which are about code which does not
+ * conform to style guidelines issued by one or other authority but which is
+ * still completely compliant with the C standards.
  *
- * Added at libpng-1.2.41.
+ * Some (but not all) of these can be disabled by definining
+ * PNG_NO_PEDANTIC_WARNINGS when application code is compiled against pngconf.h
+ * All can be removed by PNG_NO_ATTRIBUTES
+ *
+ * "Informational" notations.  These are notations which make statements about
+ * how the the libpng code.  They cannot be disabled if the compiler supports
+ * them.
+ *
+ * Both these classes of notation are implemented using the "attribute" syntax
+ * defined along with the ISO-C23 standard.  The test, however, is for the
+ * standard defined "attribute testing" mechanism.  This is now supported in the
+ * common compilers, gcc clang and Microsoft VisualC in a consistent way.
+ *
+ * If necessary users of older compiles may define the individual macros to
+ * preempt the standard definition however it is much better to upgrade to a
+ * compiler with the ISO standard support!
+ *
+ * [[changed in libpng 1.8 to use the standard notation]]
  */
 
+#ifndef PNG_NO_ATTRIBUTES
+#  ifndef PNG_ATTRIBUTES_SUPPORTED
+#    define PNG_ATTRIBUTES_SUPPORTED
+#  endif
+#endif
+
 #ifndef PNG_NO_PEDANTIC_WARNINGS
 #  ifndef PNG_PEDANTIC_WARNINGS_SUPPORTED
 #    define PNG_PEDANTIC_WARNINGS_SUPPORTED
 #  endif
 #endif
 
-#ifdef PNG_PEDANTIC_WARNINGS_SUPPORTED
-  /* Support for compiler specific function attributes.  These are used
-   * so that where compiler support is available, incorrect use of API
-   * functions in png.h will generate compiler warnings.  Added at libpng
-   * version 1.2.41.  Disabling these removes the warnings but may also produce
-   * less efficient code.
-   */
-#  if defined(__clang__) && defined(__has_attribute)
-   /* Clang defines both __clang__ and __GNUC__. Check __clang__ first. */
-#    if !defined(PNG_USE_RESULT) && __has_attribute(__warn_unused_result__)
-#      define PNG_USE_RESULT __attribute__((__warn_unused_result__))
-#    endif
-#    if !defined(PNG_NORETURN) && __has_attribute(__noreturn__)
-#      define PNG_NORETURN __attribute__((__noreturn__))
-#    endif
-#    if !defined(PNG_ALLOCATED) && __has_attribute(__malloc__)
-#      define PNG_ALLOCATED __attribute__((__malloc__))
-#    endif
-#    if !defined(PNG_DEPRECATED) && __has_attribute(__deprecated__)
-#      define PNG_DEPRECATED __attribute__((__deprecated__))
-#    endif
-#    if !defined(PNG_PRIVATE)
-#      ifdef __has_extension
-#        if __has_extension(attribute_unavailable_with_message)
-#          define PNG_PRIVATE __attribute__((__unavailable__(\
-             "This function is not exported by libpng.")))
-#        endif
-#      endif
-#    endif
-#    ifndef PNG_RESTRICT
-#      define PNG_RESTRICT __restrict
-#    endif
+/* IMPORTANT: the following produces a warning with GCC if used with an
+ * attribute which GCC does not support even though it is part of the standard
+ * therefore a test on __has_c_attribute(token) is used below to prevent such
+ * warnings.
+ *
+ * If the build system provides a definition of PNG_ATTRIBUTE it must also
+ * define PNG_HAS_ATTRIBUTE somehow (e.g. to the value 1) to get the
+ * attributes to be used.
+ */
+#ifndef PNG_ATTRIBUTE
+#  if defined(PNG_ATTRIBUTES_SUPPORTED) && defined(__has_c_attribute)
+#     define PNG_ATTRIBUTE(token) [/**/[token]/**/]
+#     ifndef PNG_HAS_ATTRIBUTE
+#        define PNG_HAS_ATTRIBUTE(token) __has_c_attribute(token)
+#     endif
+#  else
+#     define PNG_ATTRIBUTE(token) /*token*/
+#  endif
+#endif
 
-#  elif defined(__GNUC__)
-#    ifndef PNG_USE_RESULT
-#      define PNG_USE_RESULT __attribute__((__warn_unused_result__))
-#    endif
-#    ifndef PNG_NORETURN
-#      define PNG_NORETURN   __attribute__((__noreturn__))
-#    endif
-#    if __GNUC__ >= 3
-#      ifndef PNG_ALLOCATED
-#        define PNG_ALLOCATED  __attribute__((__malloc__))
-#      endif
-#      ifndef PNG_DEPRECATED
-#        define PNG_DEPRECATED __attribute__((__deprecated__))
-#      endif
-#      ifndef PNG_PRIVATE
-#        if 0 /* Doesn't work so we use deprecated instead*/
-#          define PNG_PRIVATE \
-            __attribute__((warning("This function is not exported by libpng.")))
-#        else
-#          define PNG_PRIVATE \
-            __attribute__((__deprecated__))
-#        endif
-#      endif
-#      if ((__GNUC__ > 3) || !defined(__GNUC_MINOR__) || (__GNUC_MINOR__ >= 1))
-#        ifndef PNG_RESTRICT
-#          define PNG_RESTRICT __restrict
-#        endif
-#      endif /* __GNUC__.__GNUC_MINOR__ > 3.0 */
-#    endif /* __GNUC__ >= 3 */
-
-#  elif defined(_MSC_VER)  && (_MSC_VER >= 1300)
-#    ifndef PNG_USE_RESULT
-#      define PNG_USE_RESULT /* not supported */
-#    endif
-#    ifndef PNG_NORETURN
-#      define PNG_NORETURN   __declspec(noreturn)
-#    endif
-#    ifndef PNG_ALLOCATED
-#      if (_MSC_VER >= 1400)
-#        define PNG_ALLOCATED __declspec(restrict)
-#      endif
-#    endif
-#    ifndef PNG_DEPRECATED
-#      define PNG_DEPRECATED __declspec(deprecated)
-#    endif
-#    ifndef PNG_PRIVATE
-#      define PNG_PRIVATE __declspec(deprecated)
-#    endif
-#    ifndef PNG_RESTRICT
-#      if (_MSC_VER >= 1400)
-#        define PNG_RESTRICT __restrict
-#      endif
-#    endif
+#ifndef PNG_HAS_ATTRIBUTE
+#  define PNG_HAS_ATTRIBUTE(token) 0
+   /* Do this to be safe: */
+#  undef PNG_ATTRIBUTE
+#  define PNG_ATTRIBUTE(token) /*token*/
+#endif
 
-#  elif defined(__WATCOMC__)
-#    ifndef PNG_RESTRICT
-#      define PNG_RESTRICT __restrict
-#    endif
+#ifndef PNG_WARN
+#  ifdef PNG_PEDANTIC_WARNINGS_SUPPORTED
+#     define PNG_WARN(token) PNG_ATTRIBUTE(token)
+#  else
+#     define PNG_WARN(token) /*token*/
 #  endif
-#endif /* PNG_PEDANTIC_WARNINGS */
+#endif
 
-#ifndef PNG_DEPRECATED
-#  define PNG_DEPRECATED  /* Use of this function is deprecated */
+/* Hence these notations (or attributes as that is how they are implemented).
+ *
+ * NOTE: use __token__ forms here because these identifiers might otherwise be
+ * #defined in the application code.  This is an issue because the token is
+ * passed in a macro argument, normally __has_c_attribute and [[token]] would
+ * not expand the token (apparently).
+ *
+ * The [[deprecated]] warning is now forced on unless disabled by the user
+ * of libpng (-DPNG_WARN=).  This is deliberate to encourage users to remove
+ * deprecated functionality before it is removed.
+ *
+ * __malloc__ is a GNU extension (not standardized, or not yet standardized.)
+ */
+#ifndef PNG_DEPRECATE
+#  if PNG_HAS_ATTRIBUTE(__deprecated__)
+#     define PNG_DEPRECATE(reason) PNG_ATTRIBUTE(__deprecated__(reason))
+#  else
+#     define PNG_DEPRECATE(reason) /*deprecated*/
+#  endif
 #endif
+
 #ifndef PNG_USE_RESULT
-#  define PNG_USE_RESULT  /* The result of this function must be checked */
+#  if PNG_HAS_ATTRIBUTE(__nodiscard__)
+#     define PNG_USE_RESULT PNG_WARN(__nodiscard__)
+#     define PNG_NODISCARD(reason) PNG_WARN(__nodiscard__(reason))
+#  else
+#     define PNG_USE_RESULT /*nodiscard*/
+#     define PNG_NODISCARD(reason) /*nodiscard*/
+#  endif
 #endif
+
 #ifndef PNG_NORETURN
-#  define PNG_NORETURN    /* This function does not return */
+#  if PNG_HAS_ATTRIBUTE(__noreturn__)
+#     define PNG_NORETURN PNG_ATTRIBUTE(__noreturn__)
+#  else
+#     define PNG_NORETURN /*noreturn*/
+#  endif
 #endif
+
+#ifndef PNG_MALLOCED /* added to PNG_ALLOCATED below */
+#  if PNG_HAS_ATTRIBUTE(__gnu__::__malloc__)
+      /* This is informational: it tells the caller that the result is a freshly
+       * allocated pointer.  See the GNU documentation.
+       */
+#     define PNG_MALLOCED PNG_ATTRIBUTE(__gnu__::__malloc__)
+#  else
+#     define PNG_MALLOCED /*allocated*/
+#  endif
+#endif
+
 #ifndef PNG_ALLOCATED
-#  define PNG_ALLOCATED   /* The result of the function is new memory */
+#  define PNG_ALLOCATED PNG_NODISCARD(\
+      "The pointer to allocated memory returned by this function is ignored.")\
+      PNG_MALLOCED
+#endif
+
+#ifndef PNG_DEPRECATED
+#  define PNG_DEPRECATED PNG_DEPRECATE(\
+      "This function will be removed in the next major release of libpng.")
 #endif
+
 #ifndef PNG_PRIVATE
-#  define PNG_PRIVATE     /* This is a private libpng function */
+   /* This warns that the build of the application code will fail to link:
+    */
+#  define PNG_PRIVATE PNG_DEPRECATE(\
+      "This function is not exported by this build of libpng.")
 #endif
+
+/* C99 'restrict' is not an attribute, it's just a type qualified like const
+ * or volatile.  PNG_RESTRICT is defined to either 'restrict' (if available)
+ * or empty.  Note that some environments, like 'configure' ones, work out
+ * a #define for 'restrict' so the following might evaluated to something other
+ * than the keyword 'restrict' on C99.
+ */
+#if !defined(PNG_RESTRICT) && defined(__STDC_VERSION__)
+#  if __STDC_VERSION__ >= 199901L/*C99*/
+#     define PNG_RESTRICT restrict
+#  endif
+#endif
+
+/* This is the case if the app has included something like the configure
+ * config.h file:
+ */
+#if !defined(PNG_RESTRICT) && defined(restrict)
+#  define PNG_RESTRICT restrict
+#endif
+
 #ifndef PNG_RESTRICT
-#  define PNG_RESTRICT    /* The C99 "restrict" feature */
+#  define PNG_RESTRICT /*restrict*/
 #endif
 
 #ifndef PNG_FP_EXPORT     /* A floating point API. */
diff --git a/pngerror.c b/pngerror.c
index aa0ae58e15..757a757315 100644
--- a/pngerror.c
+++ b/pngerror.c
@@ -19,9 +19,8 @@
 
 #if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
 
-static PNG_FUNCTION(void /* PRIVATE */,
-png_default_error,(png_const_structrp png_ptr, png_const_charp error_message),
-    PNG_NORETURN);
+PNG_FUNCTION(void, png_default_error,(png_const_structrp png_ptr,
+    png_const_charp error_message),PNG_NORETURN static);
 
 #ifdef PNG_WARNINGS_SUPPORTED
 static void /* PRIVATE */
@@ -163,7 +162,7 @@ png_format_number(png_const_charp start, png_charp end, int format,
          case PNG_NUMBER_FORMAT_02u:
             /* Expects at least 2 digits. */
             mincount = 2;
-            /* FALLTHROUGH */
+            PNG_FALLTHROUGH; /* FALLTHROUGH */
 
          case PNG_NUMBER_FORMAT_u:
             *--end = digits[number % 10];
@@ -173,7 +172,7 @@ png_format_number(png_const_charp start, png_charp end, int format,
          case PNG_NUMBER_FORMAT_02x:
             /* This format expects at least two digits */
             mincount = 2;
-            /* FALLTHROUGH */
+            PNG_FALLTHROUGH; /* FALLTHROUGH */
 
          case PNG_NUMBER_FORMAT_x:
             *--end = digits[number & 0xf];
@@ -707,9 +706,9 @@ png_free_jmpbuf(png_structrp png_ptr)
  * function is used by default, or if the program supplies NULL for the
  * error function pointer in png_set_error_fn().
  */
-static PNG_FUNCTION(void /* PRIVATE */,
+PNG_FUNCTION(void /* PRIVATE */,
 png_default_error,(png_const_structrp png_ptr, png_const_charp error_message),
-    PNG_NORETURN)
+    PNG_NORETURN static)
 {
 #ifdef PNG_CONSOLE_IO_SUPPORTED
 #ifdef PNG_ERROR_NUMBERS_SUPPORTED
diff --git a/pngpriv.h b/pngpriv.h
index 102ccbae2c..3b4ba36417 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -46,22 +46,6 @@
 
 #define PNGLIB_BUILD /*libpng is being built, not used*/
 
-/* If HAVE_CONFIG_H is defined during the build then the build system must
- * provide an appropriate "config.h" file on the include path.  The header file
- * must provide definitions as required below (search for "HAVE_CONFIG_H");
- * see configure.ac for more details of the requirements.  The macro
- * "PNG_NO_CONFIG_H" is provided for maintainers to test for dependencies on
- * 'configure'; define this macro to prevent the configure build including the
- * configure generated config.h.  Libpng is expected to compile without *any*
- * special build system support on a reasonably ANSI-C compliant system.
- */
-#if defined(HAVE_CONFIG_H) && !defined(PNG_NO_CONFIG_H)
-#  include <config.h>
-
-   /* Pick up the definition of 'restrict' from config.h if it was read: */
-#  define PNG_RESTRICT restrict
-#endif
-
 /* To support symbol prefixing it is necessary to know *before* including png.h
  * whether the fixed point (and maybe other) APIs are exported, because if they
  * are not internal definitions may be required.  This is handled below just
@@ -165,13 +149,12 @@
 
 #ifndef PNG_INTERNAL_FUNCTION
 #  define PNG_INTERNAL_FUNCTION(type, name, args, attributes)\
-      PNG_LINKAGE_FUNCTION PNG_FUNCTION(type, name, args, PNG_EMPTY attributes)
+      PNG_FUNCTION(type, name, args, attributes PNG_LINKAGE_FUNCTION)
 #endif
 
 #ifndef PNG_INTERNAL_CALLBACK
 #  define PNG_INTERNAL_CALLBACK(type, name, args, attributes)\
-      PNG_LINKAGE_CALLBACK PNG_FUNCTION(type, (PNGCBAPI name), args,\
-         PNG_EMPTY attributes)
+      PNG_FUNCTION(type, (PNGCBAPI name), args, attributes PNG_LINKAGE_CALLBACK)
 #endif
 
 /* If floating or fixed point APIs are disabled they may still be compiled
@@ -247,16 +230,43 @@
 #  define PNG_MAX_MALLOC_64K
 #endif
 
+#ifndef PNG_MAYBE_UNUSED
+   /* Unused formal parameter warnings can be silenced by using this macro
+    * to qualify the declaration, but see PNG_UNUSED below.
+    * macro.  Note this is used in the #if code path where the parameter
+    * is not used.
+    */
+#  if PNG_HAS_ATTRIBUTE(maybe_unused)
+#     define PNG_MAYBE_UNUSED PNG_ATTRIBUTE(maybe_unused)
+#  else
+#     define PNG_MAYBE_UNUSED /*maybe unused*/
+#  endif
+#endif
+
 #ifndef PNG_UNUSED
-/* Unused formal parameter warnings are silenced using the following macro
- * which is expected to have no bad effects on performance (optimizing
- * compilers will probably remove it entirely).  Note that if you replace
- * it with something other than whitespace, you must include the terminating
- * semicolon.
- */
+   /* As a better alternative to the above use this macro in (at the end of)
+    * the #if code tree where the parameter is not used.  Note that this macro
+    * includes a ";" if it is defined and the parameter must be a single token.
+    */
 #  define PNG_UNUSED(param) (void)param;
 #endif
 
+#ifndef PNG_FALLTHROUGH
+   /* This notation is used immediately above a switch case label to indicate
+    * that the preceding code "falls through" to the next label.  Some
+    * authorities regard this as a bad thing to do but libpng does it a lot.
+    *
+    * NOTE: this must be used in the form:
+    *
+    *    PNG_FALLTHROUGH;
+    */
+#  if PNG_HAS_ATTRIBUTE(fallthrough)
+#     define PNG_FALLTHROUGH PNG_ATTRIBUTE(fallthrough)
+#  else
+#     define PNG_FALLTHROUGH /* FALLTHROUGH */
+#  endif
+#endif
+
 /* Just a little check that someone hasn't tried to define something
  * contradictory.
  */
diff --git a/pngread.c b/pngread.c
index 3ec5623a00..fab303ca13 100644
--- a/pngread.c
+++ b/pngread.c
@@ -1918,7 +1918,7 @@ png_create_colormap_entry(png_image_read_control *display,
          {
             case 4:
                entry[afirst ? 0 : 3] = (png_uint_16)alpha;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
 
             case 3:
                if (alpha < 65535)
@@ -1940,7 +1940,7 @@ png_create_colormap_entry(png_image_read_control *display,
 
             case 2:
                entry[1 ^ afirst] = (png_uint_16)alpha;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
 
             case 1:
                if (alpha < 65535)
@@ -1969,7 +1969,7 @@ png_create_colormap_entry(png_image_read_control *display,
          {
             case 4:
                entry[afirst ? 0 : 3] = (png_byte)alpha;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
             case 3:
                entry[afirst + (2 ^ bgr)] = (png_byte)blue;
                entry[afirst + 1] = (png_byte)green;
@@ -1978,7 +1978,7 @@ png_create_colormap_entry(png_image_read_control *display,
 
             case 2:
                entry[1 ^ afirst] = (png_byte)alpha;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
             case 1:
                entry[afirst] = (png_byte)green;
                break;
@@ -2898,7 +2898,7 @@ png_image_read_colormap(png_voidp argument)
       case P_sRGB:
          /* Change to 8-bit sRGB */
          png_set_alpha_mode_fixed(png_ptr, PNG_ALPHA_PNG, PNG_GAMMA_sRGB);
-         /* FALLTHROUGH */
+         PNG_FALLTHROUGH; /* FALLTHROUGH */
 
       case P_FILE:
          if (png_ptr->bit_depth > 8)
diff --git a/pngrtran.c b/pngrtran.c
index e82e467bf5..b171bc047b 100644
--- a/pngrtran.c
+++ b/pngrtran.c
@@ -48,7 +48,7 @@ png_set_crc_action(png_structrp png_ptr, int crit_action, int ancil_action)
       case PNG_CRC_WARN_DISCARD:    /* Not a valid action for critical data */
          png_warning(png_ptr,
              "Can't discard critical data on CRC error");
-         /* FALLTHROUGH */
+         PNG_FALLTHROUGH; /* FALLTHROUGH */
       case PNG_CRC_ERROR_QUIT:                                /* Error/quit */
 
       case PNG_CRC_DEFAULT:
@@ -1251,7 +1251,7 @@ png_init_rgb_transformations(png_structrp png_ptr)
             default:
 
             case 8:
-               /* FALLTHROUGH */ /*  (Already 8 bits) */
+               PNG_FALLTHROUGH; /*  (Already 8 bits) */ /* FALLTHROUGH */
 
             case 16:
                /* Already a full 16 bits */
diff --git a/pngrutil.c b/pngrutil.c
index eacc88bd0b..f017ddef17 100644
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -3104,7 +3104,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
          case 2:
             png_ptr->user_chunk_cache_max = 1;
             png_chunk_benign_error(png_ptr, "no space in chunk cache");
-            /* FALLTHROUGH */
+            PNG_FALLTHROUGH; /* FALLTHROUGH */
          case 1:
             /* NOTE: prior to 1.6.0 this case resulted in an unknown critical
              * chunk being skipped, now there will be a hard error below.
@@ -3113,7 +3113,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
          default: /* not at limit */
             --(png_ptr->user_chunk_cache_max);
-            /* FALLTHROUGH */
+            PNG_FALLTHROUGH; /* FALLTHROUGH */
          case 0: /* no limit */
 #  endif /* USER_LIMITS */
             /* Here when the limit isn't reached or when limits are compiled
diff --git a/pngwrite.c b/pngwrite.c
index 18b29e38a4..3488a236d7 100644
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -1043,7 +1043,7 @@ png_set_filter(png_structrp png_ptr, int method, int filters)
          case 6:
          case 7: png_app_error(png_ptr, "Unknown row filter for method 0");
 #endif /* WRITE_FILTER */
-            /* FALLTHROUGH */
+            PNG_FALLTHROUGH; /* FALLTHROUGH */
          case PNG_FILTER_VALUE_NONE:
             png_ptr->do_filter = PNG_FILTER_NONE; break;
 
@@ -1887,7 +1887,7 @@ png_image_set_PLTE(png_image_write_control *display)
                tRNS[i] = entry[afirst ? 0 : 3];
                if (tRNS[i] < 255)
                   num_trans = i+1;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
             case 3:
                palette[i].blue = entry[afirst + (2 ^ bgr)];
                palette[i].green = entry[afirst + 1];
@@ -1898,7 +1898,7 @@ png_image_set_PLTE(png_image_write_control *display)
                tRNS[i] = entry[1 ^ afirst];
                if (tRNS[i] < 255)
                   num_trans = i+1;
-               /* FALLTHROUGH */
+               PNG_FALLTHROUGH; /* FALLTHROUGH */
             case 1:
                palette[i].blue = palette[i].red = palette[i].green =
                   entry[afirst];