mongoose/docs/c-api/util.h/mg_avprintf.md

title	decl_name	symbol_kind	signature
mg_avprintf()	mg_avprintf	func	int mg_avprintf(char **buf, size_t size, const char *fmt, va_list ap);

Prints message to the buffer. If the buffer is large enough to hold the message, it returns buffer. If buffer is to small, it allocates a large enough buffer on heap and returns allocated buffer. This is a supposed use case:

char buf[5], *p = buf; p = mg_avprintf(&p, sizeof(buf), "%s", "hi there"); use_p_somehow(p); if (p != buf) { free(p); }

The purpose of this is to avoid malloc-ing if generated strings are small.