dstring: Try to avoid calling vsnprintf() twice in dstrprintf()

1 files changed, 26 insertions, 10 deletions

diff --git a/dstring.c b/dstring.c
index 8b226bc..d370598 100644
--- a/dstring.c
+++ b/dstring.c
@@ -129,28 +129,44 @@ int dstrapp(char **str, char c) {
 }
 
 char *dstrprintf(const char *format, ...) {
+	// Guess a length to try to avoid calling vsnprintf() twice
+	int len, cap = 2*strlen(format);
+	char *str = dstralloc(cap);
+	if (!str) {
+		return NULL;
+	}
+
 	va_list args;
 
 	va_start(args, format);
-	int len = vsnprintf(NULL, 0, format, args);
+	len = vsnprintf(str, cap + 1, format, args);
 	va_end(args);
 
-	assert(len > 0);
+	if (len > cap) {
+		if (dstreserve(&str, len) != 0) {
+			goto fail;
+		}
 
-	char *str = dstralloc(len);
-	if (!str) {
-		return NULL;
+		cap = len;
+
+		va_start(args, format);
+		len = vsnprintf(str, cap + 1, format, args);
+		va_end(args);
+
+		assert(len == cap);
 	}
 
-	va_start(args, format);
-	len = vsnprintf(str, len + 1, format, args);
-	va_end(args);
+	if (len < 0) {
+		goto fail;
+	}
 
 	struct dstring *header = dstrheader(str);
-	assert(len == header->capacity);
 	header->length = len;
-
 	return str;
+
+fail:
+	dstrfree(str);
+	return NULL;
 }
 
 void dstrfree(char *dstr) {