Added code from oliverw#329 to resolve oliverw#325

.gitignore

@@ -117,3 +117,5 @@ logs/
 src/Miningcore/config.json
 src/Miningcore/config2.json
 /src/Miningcore/coins2.json
+
+!**/lib/**/*

src/Native/libethhash/io.c

@@ -23,6 +23,15 @@
 #include <stdio.h>
 #include <errno.h>
 
+int ethash_fseek(FILE* f, size_t offset, int origin)
+{
+#ifdef _WIN32
+    return _fseeki64(f, offset, origin);
+#else
+    return fseeko(f, offset, origin);
+#endif
+}
+
 enum ethash_io_rc ethash_io_prepare(
 	char const* dirname,
 	ethash_h256_t const seedhash,
@@ -91,7 +100,7 @@ enum ethash_io_rc ethash_io_prepare(
 		goto free_memo;
 	}
 	// make sure it's of the proper size
-	if (fseek(f, (long int)(file_size + ETHASH_DAG_MAGIC_NUM_SIZE - 1), SEEK_SET) != 0) {
+    if (ethash_fseek(f, file_size + ETHASH_DAG_MAGIC_NUM_SIZE - 1, SEEK_SET) != 0) {
 		fclose(f);
 		ETHASH_CRITICAL("Could not seek to the end of DAG file: \"%s\". Insufficient space?", tmpfile);
 		goto free_memo;

src/Native/libethhash/io.h

@@ -71,6 +71,16 @@ enum ethash_io_rc {
 #define ETHASH_CRITICAL(...)          
 #endif
 
+ /**
+ * An fseek wrapper for crossplatform 64-bit seek.
+ *
+ * @param f            The file stream whose fd to get
+ * @param offset       Number of bytes from @a origin
+ * @param origin       Initial position
+ * @return             Current offset or -1 to indicate an error
+ */
+int ethash_fseek(FILE* f, size_t offset, int origin);
+
 /**
  * Prepares io for ethash
  *

src/Native/libethhash/io_win32.c

@@ -76,13 +76,23 @@ char* ethash_io_create_filename(
 
 bool ethash_file_size(FILE* f, size_t* ret_size)
 {
+#ifdef _WIN32
+    struct _stat64 st;
+    int fd;
+    if ((fd = _fileno(f)) == -1 || _fstat64(fd, &st) != 0) {
+        return false;
+    }
+    *ret_size = st.st_size;
+    return true;
+#else
 	struct _stat st;
 	int fd;
 	if ((fd = _fileno(f)) == -1 || _fstat(fd, &st) != 0) {
 		return false;
 	}
 	*ret_size = st.st_size;
 	return true;
+#endif
 }
 
 bool ethash_get_default_dirname(char* strbuf, size_t buffsize)