diff -Naurp a/src/tool_cb_hdr.c b/src/tool_cb_hdr.c
--- a/src/tool_cb_hdr.c	2017-01-13 10:55:20.000000000 +0100
+++ b/src/tool_cb_hdr.c	2017-03-28 10:50:17.874383326 +0200
@@ -40,7 +40,7 @@ static char *parse_filename(const char *
 ** callback for CURLOPT_HEADERFUNCTION
 */
 
-size_t tool_header_cb(void *ptr, size_t size, size_t nmemb, void *userdata)
+size_t tool_header_cb(char *ptr, size_t size, size_t nmemb, void *userdata)
 {
   struct HdrCbData *hdrcbdata = userdata;
   struct OutStruct *outs = hdrcbdata->outs;
diff -Naurp a/src/tool_cb_hdr.h b/src/tool_cb_hdr.h
--- a/src/tool_cb_hdr.h	2016-11-07 10:18:18.000000000 +0100
+++ b/src/tool_cb_hdr.h	2017-03-28 10:50:45.528385413 +0200
@@ -48,7 +48,7 @@ struct HdrCbData {
 ** callback for CURLOPT_HEADERFUNCTION
 */
 
-size_t tool_header_cb(void *ptr, size_t size, size_t nmemb, void *userdata);
+size_t tool_header_cb(char *ptr, size_t size, size_t nmemb, void *userdata);
 
 #endif /* HEADER_CURL_TOOL_CB_HDR_H */
 
diff -Naurp a/src/tool_cb_wrt.c b/src/tool_cb_wrt.c
--- a/src/tool_cb_wrt.c	2016-11-07 10:18:18.000000000 +0100
+++ b/src/tool_cb_wrt.c	2017-03-28 10:54:42.996403336 +0200
@@ -72,7 +72,7 @@ bool tool_create_output_file(struct OutS
 ** callback for CURLOPT_WRITEFUNCTION
 */
 
-size_t tool_write_cb(void *buffer, size_t sz, size_t nmemb, void *userdata)
+size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
 {
   size_t rc;
   struct OutStruct *outs = userdata;
diff -Naurp a/src/tool_cb_wrt.h b/src/tool_cb_wrt.h
--- a/src/tool_cb_wrt.h	2016-11-07 10:18:18.000000000 +0100
+++ b/src/tool_cb_wrt.h	2017-03-28 10:54:54.957404238 +0200
@@ -27,7 +27,7 @@
 ** callback for CURLOPT_WRITEFUNCTION
 */
 
-size_t tool_write_cb(void *buffer, size_t sz, size_t nmemb, void *userdata);
+size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata);
 
 /* create a local file for writing, return TRUE on success */
 bool tool_create_output_file(struct OutStruct *outs);