Fix: Avoid spamming stdout (and JSON output) with a warning.

Fixes #1223.

src/iperf_api.h

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014-2020, The Regents of the University of
+ * iperf, Copyright (c) 2014-2021, The Regents of the University of
  * California, through Lawrence Berkeley National Laboratory (subject
  * to receipt of any required approvals from the U.S. Dept. of
  * Energy).  All rights reserved.
@@ -60,6 +60,8 @@ typedef uint64_t iperf_size_t;
 #define DEFAULT_NO_MSG_RCVD_TIMEOUT 120000
 #define MIN_NO_MSG_RCVD_TIMEOUT 100
 
+#define WARN_STR_LEN 128
+
 /* short option equivalents, used to support options that only have long form */
 #define OPT_SCTP 1
 #define OPT_LOGFILE 2

src/iperf_udp.c

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014-2020, The Regents of the University of
+ * iperf, Copyright (c) 2014-2021, The Regents of the University of
  * California, through Lawrence Berkeley National Laboratory (subject
  * to receipt of any required approvals from the U.S. Dept. of
  * Energy).  All rights reserved.
@@ -403,9 +403,11 @@ iperf_udp_accept(struct iperf_test *test)
      */
     if (rc > 0) {
 	if (test->settings->socket_bufsize == 0) {
+            char str[WARN_STR_LEN];
 	    int bufsize = test->settings->blksize + UDP_BUFFER_EXTRA;
-	    printf("Increasing socket buffer size to %d\n",
-		bufsize);
+	    snprintf(str, sizeof(str), "Increasing socket buffer size to %d",
+	             bufsize);
+	    warning(str);
 	    test->settings->socket_bufsize = bufsize;
 	    rc = iperf_udp_buffercheck(test, s);
 	    if (rc < 0)
@@ -515,9 +517,11 @@ iperf_udp_connect(struct iperf_test *test)
      */
     if (rc > 0) {
 	if (test->settings->socket_bufsize == 0) {
+            char str[WARN_STR_LEN];
 	    int bufsize = test->settings->blksize + UDP_BUFFER_EXTRA;
-	    printf("Increasing socket buffer size to %d\n",
-		bufsize);
+	    snprintf(str, sizeof(str), "Increasing socket buffer size to %d",
+	             bufsize);
+	    warning(str);
 	    test->settings->socket_bufsize = bufsize;
 	    rc = iperf_udp_buffercheck(test, s);
 	    if (rc < 0)