+/* Function calling conventions.
+ * =============================
+ * Normally it is not necessary to specify to the compiler how to call
+ * a function - it just does it - however on x86 systems derived from
+ * Microsoft and Borland C compilers ('IBM PC', 'DOS', 'Windows' systems
+ * and some others) there are multiple ways to call a function and the
+ * default can be changed on the compiler command line. For this reason
+ * libpng specifies the calling convention of every exported function and
+ * every function called via a user supplied function pointer. This is
+ * done in this file by defining the following macros:
+ *
+ * PNGAPI Calling convention for exported functions.
+ * PNGCBAPI Calling convention for user provided (callback) functions.
+ * PNGCAPI Calling convention used by the ANSI-C library (required
+ * for longjmp callbacks and sometimes used internally to
+ * specify the calling convention for zlib).
+ *
+ * These macros should never be overridden. If it is necessary to
+ * change calling convention in a private build this can be done
+ * by setting PNG_API_RULE (which defaults to 0) to one of the values
+ * below to select the correct 'API' variants.
+ *
+ * PNG_API_RULE=0 Use PNGCAPI - the 'C' calling convention - throughout.
+ * This is correct in every known environment.
+ * PNG_API_RULE=1 Use the operating system convention for PNGAPI and
+ * the 'C' calling convention (from PNGCAPI) for
+ * callbacks (PNGCBAPI). This is no longer required
+ * in any known environment - if it has to be used
+ * please post an explanation of the problem to the
+ * libpng mailing list.
+ *
+ * These cases only differ if the operating system does not use the C
+ * calling convention, at present this just means the above cases
+ * (x86 DOS/Windows sytems) and, even then, this does not apply to
+ * Cygwin running on those systems.
+ *
+ * Note that the value must be defined in pnglibconf.h so that what
+ * the application uses to call the library matches the conventions
+ * set when building the library.