diff --git a/Makefile b/Makefile
index 4833f74b..0c4a3170 100644
--- a/Makefile
+++ b/Makefile
@@ -109,16 +109,16 @@ riscv: DEFS += -DMG_ENABLE_FILE=0 -DMG_ENABLE_MIP=1 -DMG_ARCH=MG_ARCH_NEWLIB
 riscv: mongoose.h $(SRCS)
 	$(DOCKER) mdashnet/riscv riscv-none-elf-gcc -march=rv32imc -mabi=ilp32 $(SRCS) $(OPTS) $(WARN) $(INCS) $(DEFS) $(TFLAGS) -o unit_test
 
-#vc98: VCFLAGS += -DMG_ENABLE_IPV6=1
 vc98: Makefile mongoose.h $(SRCS)
 	$(DOCKER) mdashnet/vc98 wine cl $(SRCS) $(VCFLAGS) ws2_32.lib /Fe$@.exe
 	$(DOCKER) mdashnet/vc98 wine $@.exe
 
-#vc2017: VCFLAGS += -DMG_ENABLE_IPV6=1
+# vc2017: DEFS += -DMG_ENABLE_IPV6=1
 vc2017: Makefile mongoose.h $(SRCS)
 	$(DOCKER) mdashnet/vc2017 wine64 cl $(SRCS) $(VCFLAGS) ws2_32.lib /Fe$@.exe
 	$(DOCKER) mdashnet/vc2017 wine64 $@.exe
 
+# vc22: DEFS += -DMG_ENABLE_IPV6=$(IPV6)
 vc22: Makefile mongoose.h $(SRCS)
 	$(DOCKER) mdashnet/vc22 wine64 cl $(SRCS) $(VCFLAGS) ws2_32.lib /Fe$@.exe
 	$(DOCKER) mdashnet/vc22 wine64 $@.exe
diff --git a/mongoose.c b/mongoose.c
index ac13edb5..dce2a13d 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -598,7 +598,6 @@ char *mg_mprintf(const char *fmt, ...) {
   return s;
 }
 
-
 size_t mg_rprintf(void (*out)(char, void *), void *ptr, const char *fmt, ...) {
   size_t len = 0;
   va_list ap;
@@ -610,7 +609,19 @@ size_t mg_rprintf(void (*out)(char, void *), void *ptr, const char *fmt, ...) {
 
 static void mg_putchar_iobuf_static(char ch, void *param) {
   struct mg_iobuf *io = (struct mg_iobuf *) param;
-  if (io->len < io->size) io->buf[io->len++] = (uint8_t) ch;
+  if (io->len + 2 <= io->size) {
+    io->buf[io->len++] = (uint8_t) ch;
+    io->buf[io->len] = 0;
+  }
+}
+
+static void mg_putchar_iobuf(char ch, void *param) {
+  struct mg_iobuf *io = (struct mg_iobuf *) param;
+  if (io->len + 2 > io->size) mg_iobuf_resize(io, io->size + 64);
+  if (io->len + 2 <= io->size) {
+    io->buf[io->len++] = (uint8_t) ch;
+    io->buf[io->len] = 0;
+  }
 }
 
 // We don't use realloc() in mongoose, so resort to inefficient calloc
@@ -3290,11 +3301,9 @@ struct mg_connection *mg_mqtt_listen(struct mg_mgr *mgr, const char *url,
 
 
 size_t mg_vprintf(struct mg_connection *c, const char *fmt, va_list ap) {
-  char mem[256], *buf = mem;
-  size_t len = mg_vasprintf(&buf, sizeof(mem), fmt, ap);
-  len = mg_send(c, buf, len);
-  if (buf != mem) free(buf);
-  return len;
+  size_t old = c->send.len;
+  mg_vrprintf(mg_putchar_iobuf, &c->send, fmt, &ap);
+  return c->send.len - old;
 }
 
 size_t mg_printf(struct mg_connection *c, const char *fmt, ...) {
diff --git a/mongoose.h b/mongoose.h
index 72b5c8e0..72bb82f6 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -728,6 +728,7 @@ size_t mg_dtoa(char *buf, size_t len, double d, int width);
 typedef void (*mg_pc_t)(char, void *);                  // Custom putchar
 typedef size_t (*mg_pm_t)(mg_pc_t, void *, va_list *);  // %M printer
 void mg_putchar_realloc(char ch, void *param);          // Print to malloced str
+void mg_putchar_iobuf(char ch, void *param);            // Print to iobuf
 
 size_t mg_vrprintf(void (*)(char, void *), void *, const char *fmt, va_list *);
 size_t mg_rprintf(void (*fn)(char, void *), void *, const char *fmt, ...);
diff --git a/src/fmt.c b/src/fmt.c
index 362391dd..71e2f68c 100644
--- a/src/fmt.c
+++ b/src/fmt.c
@@ -47,7 +47,6 @@ char *mg_mprintf(const char *fmt, ...) {
   return s;
 }
 
-
 size_t mg_rprintf(void (*out)(char, void *), void *ptr, const char *fmt, ...) {
   size_t len = 0;
   va_list ap;
@@ -59,7 +58,19 @@ size_t mg_rprintf(void (*out)(char, void *), void *ptr, const char *fmt, ...) {
 
 static void mg_putchar_iobuf_static(char ch, void *param) {
   struct mg_iobuf *io = (struct mg_iobuf *) param;
-  if (io->len < io->size) io->buf[io->len++] = (uint8_t) ch;
+  if (io->len + 2 <= io->size) {
+    io->buf[io->len++] = (uint8_t) ch;
+    io->buf[io->len] = 0;
+  }
+}
+
+static void mg_putchar_iobuf(char ch, void *param) {
+  struct mg_iobuf *io = (struct mg_iobuf *) param;
+  if (io->len + 2 > io->size) mg_iobuf_resize(io, io->size + 64);
+  if (io->len + 2 <= io->size) {
+    io->buf[io->len++] = (uint8_t) ch;
+    io->buf[io->len] = 0;
+  }
 }
 
 // We don't use realloc() in mongoose, so resort to inefficient calloc
diff --git a/src/net.c b/src/net.c
index 7cefd42e..00215504 100644
--- a/src/net.c
+++ b/src/net.c
@@ -6,11 +6,9 @@
 #include "util.h"
 
 size_t mg_vprintf(struct mg_connection *c, const char *fmt, va_list ap) {
-  char mem[256], *buf = mem;
-  size_t len = mg_vasprintf(&buf, sizeof(mem), fmt, ap);
-  len = mg_send(c, buf, len);
-  if (buf != mem) free(buf);
-  return len;
+  size_t old = c->send.len;
+  mg_vrprintf(mg_putchar_iobuf, &c->send, fmt, &ap);
+  return c->send.len - old;
 }
 
 size_t mg_printf(struct mg_connection *c, const char *fmt, ...) {
diff --git a/src/str.h b/src/str.h
index 4aac6960..71b89343 100644
--- a/src/str.h
+++ b/src/str.h
@@ -44,6 +44,7 @@ size_t mg_dtoa(char *buf, size_t len, double d, int width);
 typedef void (*mg_pc_t)(char, void *);                  // Custom putchar
 typedef size_t (*mg_pm_t)(mg_pc_t, void *, va_list *);  // %M printer
 void mg_putchar_realloc(char ch, void *param);          // Print to malloced str
+void mg_putchar_iobuf(char ch, void *param);            // Print to iobuf
 
 size_t mg_vrprintf(void (*)(char, void *), void *, const char *fmt, va_list *);
 size_t mg_rprintf(void (*fn)(char, void *), void *, const char *fmt, ...);