Add seprintf

Based on seprint(2) from Plan 9. I'm not sure if my return value
exactly matches Plan 9's in the case of truncation. seprint(2) is
described only as returning a pointer to the terminating '\0', but
if it does so even in the case of truncation, it is awkward for the
caller to detect. This implementation returns end in the truncation
case, so that (ptr == end) indicates truncation.

1 files changed, 13 insertions, 0 deletions

diff --git a/bounce.h b/bounce.h
index a2265c8..2010d45 100644
--- a/bounce.h
+++ b/bounce.h
@@ -26,6 +26,7 @@
  */
 
 #include <limits.h>
+#include <stdarg.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -49,6 +50,18 @@
 
 typedef unsigned char byte;
 
+static inline char *seprintf(char *ptr, char *end, const char *fmt, ...)
+	__attribute__((format(printf, 3, 4)));
+static inline char *seprintf(char *ptr, char *end, const char *fmt, ...) {
+	va_list ap;
+	va_start(ap, fmt);
+	int n = vsnprintf(ptr, end - ptr, fmt, ap);
+	va_end(ap);
+	if (n < 0) return NULL;
+	if (n > end - ptr) return end;
+	return ptr + n;
+}
+
 enum { MessageCap = 8191 + 512 };
 
 enum { ParamCap = 15 };