2 files changed, 26 insertions, 15 deletions

diff --git a/buffer.c b/buffer.c
index 8794e20..ddc6dba 100644
--- a/buffer.c
+++ b/buffer.c
@@ -109,29 +109,39 @@ bool buffer_prepend0(Buffer *buf, const char *data) {
 	return buffer_prepend(buf, data, strlen(data) + (buf->len == 0));
 }
 
-bool buffer_printf(Buffer *buf, const char *fmt, ...) {
-	va_list ap;
-	va_start(ap, fmt);
-	bool ret = buffer_vprintf(buf, fmt, ap);
-	va_end(ap);
-	return ret;
-}
-
-bool buffer_vprintf(Buffer *buf, const char *fmt, va_list ap) {
+static bool buffer_vappendf(Buffer *buf, const char *fmt, va_list ap) {
 	va_list ap_save;
 	va_copy(ap_save, ap);
 	int len = vsnprintf(NULL, 0, fmt, ap);
-	if (len == -1 || !buffer_grow(buf, len+1)) {
+	if (len == -1 || !buffer_grow(buf, buf->len+len+1)) {
 		va_end(ap_save);
 		return false;
 	}
-	bool ret = vsnprintf(buf->data, len+1, fmt, ap_save) == len;
-	if (ret)
-		buf->len = len+1;
+	size_t nul = (buf->len > 0 && buf->data[buf->len-1] == '\0') ? 1 : 0;
+	buf->len -= nul;
+	bool ret = vsnprintf(buf->data+buf->len, len+1, fmt, ap_save) == len;
+	buf->len += ret ? (size_t)len+1 : nul;
 	va_end(ap_save);
 	return ret;
 }
 
+bool buffer_appendf(Buffer *buf, const char *fmt, ...) {
+	va_list ap;
+	va_start(ap, fmt);
+	bool ret = buffer_vappendf(buf, fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
+bool buffer_printf(Buffer *buf, const char *fmt, ...) {
+	buffer_clear(buf);
+	va_list ap;
+	va_start(ap, fmt);
+	bool ret = buffer_vappendf(buf, fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
 size_t buffer_length0(Buffer *buf) {
 	size_t len = buf->len;
 	if (len > 0 && buf->data[len-1] == '\0')
diff --git a/buffer.h b/buffer.h
index b095572..6c6bc82 100644
--- a/buffer.h
+++ b/buffer.h
@@ -42,9 +42,10 @@ bool buffer_append0(Buffer*, const char *data);
 bool buffer_prepend(Buffer*, const void *data, size_t len);
 /* prepend NUL-terminated data */
 bool buffer_prepend0(Buffer*, const char *data);
-
+/* set formatted buffer content, ensures NUL termination on success */
 bool buffer_printf(Buffer*, const char *fmt, ...);
-bool buffer_vprintf(Buffer*, const char *fmt, va_list);
+/* append formatted buffer content, ensures NUL termination on success */
+bool buffer_appendf(Buffer*, const char *fmt, ...);
 /* return length of a buffer without trailing NUL byte */
 size_t buffer_length0(Buffer*);
 /* return length of a buffer including possible NUL byte */