Prevent specifying a UDP send size that's too small.

We need at least 16 bytes to hold counters and timestamps.
Avoids a problem noted in issue #390.

src/iperf.h

@@ -321,6 +321,8 @@ struct iperf_test
 #define MB (1024 * 1024)
 #define MAX_TCP_BUFFER (512 * MB)
 #define MAX_BLOCKSIZE MB
+/* Minimum size UDP send is the size of two 32-bit ints followed by a 64-bit int */
+#define MIN_UDP_BLOCKSIZE (4 + 4 + 8)
 /* Maximum size UDP send is (64K - 1) - IP and UDP header sizes */
 #define MAX_UDP_BLOCKSIZE (65535 - 8 - 20)
 #define MIN_INTERVAL 0.1

src/iperf_api.c

@@ -1016,7 +1016,7 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
 	return -1;
     }
     if (test->protocol->id == Pudp &&
-	blksize > MAX_UDP_BLOCKSIZE) {
+	(blksize < MIN_UDP_BLOCKSIZE || blksize > MAX_UDP_BLOCKSIZE)) {
 	i_errno = IEUDPBLOCKSIZE;
 	return -1;
     }

src/iperf_api.h

@@ -292,7 +292,7 @@ enum {
     IELOGFILE = 17,	    // Can't open log file
     IENOSCTP = 18,	    // No SCTP support available
     IEBIND = 19,			// Local port specified with no local bind option
-    IEUDPBLOCKSIZE = 20,    // Block size too large. Maximum value = %dMAX_UDP_BLOCKSIZE
+    IEUDPBLOCKSIZE = 20,    // Block size invalid
     IEBADTOS = 21,	    // Bad TOS value
     /* Test errors */
     IENEWTEST = 100,        // Unable to create a new test (check perror)

src/iperf_error.c

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014, 2015, 2016, The Regents of the University of
+ * iperf, Copyright (c) 2014, 2015, 2016, 2017, 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.
@@ -126,7 +126,7 @@ iperf_strerror(int i_errno)
             snprintf(errstr, len, "--bind must be specified to use --cport");
             break;
         case IEUDPBLOCKSIZE:
-            snprintf(errstr, len, "block size too large (maximum = %d bytes)", MAX_UDP_BLOCKSIZE);
+            snprintf(errstr, len, "block size invalid (minimum = %d bytes, maximum = %d bytes)", MIN_UDP_BLOCKSIZE, MAX_UDP_BLOCKSIZE);
             break;
 	case IEBADTOS:
 	    snprintf(errstr, len, "bad TOS value (must be between 0 and 255 inclusive)");