diff --git a/qrexec-lib/libqrexec-utils.h b/qrexec-lib/libqrexec-utils.h
index 9b03caf..8e7680c 100644
--- a/qrexec-lib/libqrexec-utils.h
+++ b/qrexec-lib/libqrexec-utils.h
@@ -43,7 +43,7 @@ void do_fork_exec(const char *cmdline, int *pid, int *stdin_fd, int *stdout_fd,
 		  int *stderr_fd);
 void wait_for_vchan_or_argfd(libvchan_t *vchan, int max, fd_set * rdset, fd_set * wrset);
 int read_vchan_all(libvchan_t *vchan, void *data, size_t size);
-int write_vchan_all(libvchan_t *vchan, void *data, size_t size);
+int write_vchan_all(libvchan_t *vchan, const void *data, size_t size);
 int read_all(int fd, void *buf, int size);
 int write_all(int fd, const void *buf, int size);
 void fix_fds(int fdin, int fdout, int fderr);
diff --git a/qrexec-lib/txrx-vchan.c b/qrexec-lib/txrx-vchan.c
index c552b84..fc6f774 100644
--- a/qrexec-lib/txrx-vchan.c
+++ b/qrexec-lib/txrx-vchan.c
@@ -75,8 +75,9 @@ void wait_for_vchan_or_argfd(libvchan_t *ctrl, int max, fd_set * rdset, fd_set *
 	while (wait_for_vchan_or_argfd_once(ctrl, max, rdset, wrset) == 0);
 }
 
-int write_vchan_all(libvchan_t *vchan, void *data, size_t size) {
-    int pos, ret;
+int write_vchan_all(libvchan_t *vchan, const void *data, size_t size) {
+    size_t pos;
+    int ret;
 
     pos = 0;
     while (pos < size) {
@@ -89,7 +90,8 @@ int write_vchan_all(libvchan_t *vchan, void *data, size_t size) {
 }
 
 int read_vchan_all(libvchan_t *vchan, void *data, size_t size) {
-    int pos, ret;
+    size_t pos;
+    int ret;
 
     pos = 0;
     while (pos < size) {