diff --git a/CHANGES b/CHANGES
index f4dc5fd..2b35f10 100644
--- a/CHANGES
+++ b/CHANGES
@@ -16,6 +16,11 @@ corrections:
 	compiling with MUSL libc failed.
 	Problem reported by Kennedy33.
 
+	The async signal safe diagnostic system used FDs 3 and 4 internally, so
+	use of appropriate fdin or fdout led to failures.
+	Test: DIAG_FDIN
+	Problem reported by Onur Sentürk.
+
 testing:
 	test.sh: Show a warning when phase-1 (insecure phase) of a security
 	test fails
diff --git a/error.c b/error.c
index bc60b1e..7de3d39 100644
--- a/error.c
+++ b/error.c
@@ -210,6 +210,19 @@ const char *diag_get_string(char what) {
    return NULL;
 }
 
+/* make sure that the diag_sock fds do not have this num */
+int diag_reserve_fd(int fd) {
+   DIAG_INIT;
+   if (diag_sock_send == fd) {
+      diag_sock_send = Dup(fd);
+      Close(fd);
+   }
+   if (diag_sock_recv == fd) {
+      diag_sock_recv = Dup(fd);
+      Close(fd);
+   }
+   return 0;
+}
 
 /* Linux and AIX syslog format:
 Oct  4 17:10:37 hostname socat[52798]: D signal(13, 1)
diff --git a/error.h b/error.h
index 09c0727..d37982f 100644
--- a/error.h
+++ b/error.h
@@ -236,6 +236,7 @@ extern void diag_set(char what, const char *arg);
 extern void diag_set_int(char what, int arg);
 extern int diag_get_int(char what);
 extern const char *diag_get_string(char what);
+extern int diag_reserve_fd(int fd);
 extern int diag_dup(void);
 extern int diag_dup2(int newfd);
 extern void msg(int level, const char *format, ...);
diff --git a/test.sh b/test.sh
index 2a2659c..9f9787f 100755
--- a/test.sh
+++ b/test.sh
@@ -12799,6 +12799,48 @@ esac
 PORT=$((PORT+1))
 N=$((N+1))
 
+
+# The fix to "Make code async-signal-safe" used internally FD 3 and FD 4.
+# Using option fdin=3 did not pass data to executed program.
+NAME=DIAG_FDIN
+case "$TESTS" in
+*%$N%*|*%functions%*|*%bugs%*|*%exec%*|*%$NAME%*)
+TEST="$NAME: test use of fdin=3"
+# Use FD 3 explicitely with fdin and test if Socat passes data to executed
+# program
+if ! eval $NUMCOND; then :; else
+tf="$td/test$N.stdout"
+te="$td/test$N.stderr"
+tdiff="$td/test$N.diff"
+da="test$N $(date) $RANDOM"
+CMD0="$TRACE $SOCAT $opts - SYSTEM:\"cat >&3 <&4\",fdin=4,fdout=3"
+printf "test $F_n $TEST... " $N
+echo "$da" |$TRACE $SOCAT $opts - SYSTEM:"cat <&3 >&4",fdin=3,fdout=4 >${tf}0 2>"${te}0"
+rc0=$?
+if [ $rc0 -ne 0 ]; then
+    $PRINTF "$FAILED\n"
+    echo "$CMD0"
+    cat "${te}0"
+    numFAIL=$((numFAIL+1))
+    listFAIL="$listFAIL $N"
+elif echo "$da" |diff - ${tf}0 >$tdiff; then
+    $PRINTF "$OK\n"
+    numOK=$((numOK+1))
+else
+    $PRINTF "$FAILED\n"
+    echo "$CMD0"
+    cat "${te}0"
+    cat "$tdiff"
+    numFAIL=$((numFAIL+1))
+    listFAIL="$listFAIL $N"
+fi
+fi # NUMCOND
+ ;;
+esac
+PORT=$((PORT+1))
+N=$((N+1))
+
+
 ##################################################################################
 #=================================================================================
 # here come tests that might affect your systems integrity. Put normal tests
diff --git a/xio-progcall.c b/xio-progcall.c
index 1196cf4..bcb7d34 100644
--- a/xio-progcall.c
+++ b/xio-progcall.c
@@ -168,7 +168,7 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
       }
 #endif
 
-      /* remember: fdin is the fs where the sub program reads from, thus it is
+      /* remember: fdin is the fd where the sub program reads from, thus it is
 	 sock0[]'s read fd */
       /*! problem: when fdi==WRFD(sock[0]) or fdo==RDFD(sock[0]) */
       if (rw != XIO_WRONLY) {
@@ -177,6 +177,9 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 	       Warn2("fcntl(%d, F_SETFD, 0): %s", fdo, strerror(errno));
 	    }
 	 } else {
+	    /* make sure that the internal diagnostic socket pair fds do not conflict
+	       with our choices */
+	    diag_reserve_fd(fdo);
 	    if (Dup2(XIO_GETWRFD(sock[0]), fdo) < 0) {
 	       Error3("dup2(%d, %d): %s",
 		      XIO_GETWRFD(sock[0]), fdo, strerror(errno));
@@ -190,6 +193,9 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 	       Warn2("fcntl(%d, F_SETFD, 0): %s", fdi, strerror(errno));
 	    }
 	 } else {
+	    /* make sure that the internal diagnostic socket pair fds do not conflict
+	       with our choices */
+	    diag_reserve_fd(fdi);
 	    if (Dup2(XIO_GETRDFD(sock[0]), fdi) < 0) {
 	       Error3("dup2(%d, %d): %s)",
 		      XIO_GETRDFD(sock[0]), fdi, strerror(errno));
@@ -425,12 +431,18 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 	 if (usepty) {
 	    Close(ptyfd);
 	    if (rw != XIO_RDONLY && fdi != ttyfd) {
+	       /* make sure that the internal diagnostic socket pair fds do not conflict
+		  with our choices */
+	       diag_reserve_fd(fdi);
 	       if (Dup2(ttyfd, fdi) < 0) {
 		  Error3("dup2(%d, %d): %s", ttyfd, fdi, strerror(errno));
 		  return -1; }
 	       /*0 Info2("dup2(%d, %d)", ttyfd, fdi);*/
 	    }
 	    if (rw != XIO_WRONLY && fdo != ttyfd) {
+	       /* make sure that the internal diagnostic socket pair fds do not conflict
+		  with our choices */
+	       diag_reserve_fd(fdo);
 	       if (Dup2(ttyfd, fdo) < 0) {
 		  Error3("dup2(%d, %d): %s", ttyfd, fdo, strerror(errno));
 		  return -1; }
@@ -472,6 +484,9 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 	       }
 	       
 	       if (rw != XIO_WRONLY && rdpip[1] != fdo) {
+		  /* make sure that the internal diagnostic socket pair fds do not conflict
+		     with our choices */
+		  diag_reserve_fd(fdo);
 		  if (Dup2(rdpip[1], fdo) < 0) {
 		     Error3("dup2(%d, %d): %s", rdpip[1], fdo, strerror(errno));
 		     return -1;
@@ -481,6 +496,9 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 		  /*0 applyopts_cloexec(fdo, *copts);*/
 	       }
 	       if (rw != XIO_RDONLY && wrpip[0] != fdi) {
+		  /* make sure that the internal diagnostic socket pair fds do not conflict
+		     with our choices */
+		  diag_reserve_fd(fdi);
 		  if (Dup2(wrpip[0], fdi) < 0) {
 		     Error3("dup2(%d, %d): %s", wrpip[0], fdi, strerror(errno));
 		     return -1;
@@ -499,12 +517,18 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
 	    } else {	/* socketpair */
 	       Close(sv[0]);
 	       if (rw != XIO_RDONLY && fdi != sv[1]) {
+		  /* make sure that the internal diagnostic socket pair fds do not conflict
+		     with our choices */
+		  diag_reserve_fd(fdi);
 		  if (Dup2(sv[1], fdi) < 0) {
 		     Error3("dup2(%d, %d): %s", sv[1], fdi, strerror(errno));
 		     return -1; }
 		  /*0 Info2("dup2(%d, %d)", sv[1], fdi);*/
 	       }
 	       if (rw != XIO_WRONLY && fdo != sv[1]) {
+		  /* make sure that the internal diagnostic socket pair fds do not conflict
+		     with our choices */
+		  diag_reserve_fd(fdo);
 		  if (Dup2(sv[1], fdo) < 0) {
 		     Error3("dup2(%d, %d): %s", sv[1], fdo, strerror(errno));
 		     return -1; }