size_t xvasprintf(char **result, const char *format, va_list va);
size_t xasprintf(char **result, const char *format, ...);
void str_appendf(struct str *buf, const char *format, ...);
size_t xvasprintf(char **result, const char *format, va_list va);
size_t xasprintf(char **result, const char *format, ...);
void str_appendf(struct str *buf, const char *format, ...);