diff --git a/tools/fpack.c b/tools/fpack.c
index 84dd837..717b5c3 100644
--- a/tools/fpack.c
+++ b/tools/fpack.c
@@ -1,3 +1,8 @@
+#if !defined(_WIN32)
+#define _FILE_OFFSET_BITS 64
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include "fpk.h"
 #include <dirent.h>
 #include <getopt.h>
@@ -8,8 +13,12 @@
 
 #ifdef _WIN32
 #define PROGRAM_NAME "fpack.exe"
+#define fpack_fseek64(f, off, whence) _fseeki64((f), (off), (whence))
+#define fpack_ftell64(f) _ftelli64((f))
 #else
 #define PROGRAM_NAME "fpack"
+#define fpack_fseek64(f, off, whence) fseeko((f), (off_t)(off), (whence))
+#define fpack_ftell64(f) ftello((f))
 #endif
 
 #define FPACK_QUIET    (1u << 0)
@@ -92,16 +101,22 @@ static void normalize_path(char *s)
 static void *read_file(const char *path, size_t *out_size)
 {
 	FILE *fp;
-	long len;
+	int64_t len;
 	void *buf;
 
 	fp = fopen(path, "rb");
 	if (!fp)
 		return NULL;
 
-	fseek(fp, 0, SEEK_END);
-	len = ftell(fp);
-	fseek(fp, 0, SEEK_SET);
+	if (fpack_fseek64(fp, 0, SEEK_END) != 0) {
+		fclose(fp);
+		return NULL;
+	}
+	len = fpack_ftell64(fp);
+	if (len < 0 || fpack_fseek64(fp, 0, SEEK_SET) != 0) {
+		fclose(fp);
+		return NULL;
+	}
 
 	buf = malloc((size_t)len);
 	if (!buf) {