shared: add str_printf()

asprintf() has the problem that it leaves *strp undefined when it
fails. Here is a simple wrapper that ensures NULL if asprintf() fails,
which is much more convenient to use.

This will be useful in future patches, where one needs to return error
messages from maybe failing functions, and more.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

shared/string-helpers.h

@@ -31,6 +31,8 @@
 #include <stdint.h>
 #include <errno.h>
 #include <assert.h>
+#include <stdarg.h>
+#include <stdio.h>
 
 /* Convert string to integer
  *
@@ -68,4 +70,29 @@ safe_strtoint(const char *str, int32_t *value)
 	return true;
 }
 
+/**
+ * Exactly like asprintf(), but sets *str_out to NULL if it fails.
+ *
+ * If str_out is NULL, does nothing.
+ */
+static inline void __attribute__ ((format (printf, 2, 3)))
+str_printf(char **str_out, const char *fmt, ...)
+{
+	char *msg;
+	va_list ap;
+	int ret;
+
+	if (!str_out)
+		return;
+
+	va_start(ap, fmt);
+	ret = vasprintf(&msg, fmt, ap);
+	va_end(ap);
+
+	if (ret >= 0)
+		*str_out = msg;
+	else
+		*str_out = NULL;
+}
+
 #endif /* WESTON_STRING_HELPERS_H */