From aa9dcbffe848d618366827d444a4d725ebf34d4a Mon Sep 17 00:00:00 2001
From: Jonathan Campbell <jonathan@castus.tv>
Date: Wed, 21 May 2025 15:41:42 -0700
Subject: [PATCH] If file://con input is using O_NONBLOCK, that is fine, but
 use FIONREAD to make sure all the data requested is there to read first to
 prevent short reads that cause zero padding in the SRT stream

---
 apps/transmitmedia.cpp | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/apps/transmitmedia.cpp b/apps/transmitmedia.cpp
index 862875fa2..5a20d9b7f 100644
--- a/apps/transmitmedia.cpp
+++ b/apps/transmitmedia.cpp
@@ -710,8 +710,10 @@ class ConsoleSource: public Source
     ConsoleSource()
 #ifdef _WIN32
     : may_block(true)
-#else
+#elif defined(FIONREAD)
     : may_block(fcntl(fileno(stdin), F_SETFL, fcntl(fileno(stdin), F_GETFL) | O_NONBLOCK) < 0)
+#else
+    : may_block(true)
 #endif
     {
 #ifdef _WIN32
@@ -723,6 +725,21 @@ class ConsoleSource: public Source
 
     int Read(size_t chunk, MediaPacket& pkt, ostream & ignored SRT_ATR_UNUSED = cout) override
     {
+#if defined(FIONREAD)
+        // The trouble with O_NONBLOCK is that read() may return less than the expected data if not enough available
+        // which then causes a full size SRT packet padded with zeros. Use FIONREAD, if possible, to make sure that
+	// read() returns either chunk bytes or nothing at all.
+        if (!may_block) {
+            int br = 0;
+            if (ioctl(GetSysSocket(), FIONREAD, &br) >= 0) {
+                if (br < 0 || size_t(br) < chunk) {
+                    pkt.payload.clear();
+                    return 0;
+                }
+            }
+        }
+#endif
+
         if (pkt.payload.size() < chunk)
             pkt.payload.resize(chunk);