Print error on useless fdout,fdin options

CHANGES

@@ -81,6 +81,11 @@ corrections:
 	Help for filan -L was bad, is now corrected to:
 	"follow symbolic links instead of showing their properties"
 
+	Address options fdin and fdout were silently ignored when not applicable
+	due to -u or -U option. Now these combinations are caught as errors.
+	Test: FDOUT_ERROR
+	Issue reported by Hendrik.
+
 porting:
 	Socat included <sys/poll.h> instead of POSIX <poll.h>
 	Thanks to John Spencer for reporting this issue.

test.sh

@@ -12149,6 +12149,39 @@ N=$((N+1))
 done
 
 
+# give a description of what is tested (a bugfix, a new feature...)
+NAME=FDOUT_ERROR
+case "$TESTS" in
+*%$N%*|*%functions%*|*%bugs%*|*%socket%*|*%$NAME%*)
+TEST="$NAME: fdout bails out in write-only context"
+# use EXEC in write-only context with option fdout. Expected behaviour: error
+if ! eval $NUMCOND; then :; else
+tf="$td/test$N.stdout"
+te="$td/test$N.stderr"
+tdiff="$td/test$N.diff"
+da="test$N $(date) $RANDOM"
+CMD="$SOCAT $opts -u /dev/null EXEC:cat,fdout=1"
+printf "test $F_n $TEST... " $N
+$CMD >/dev/null 2>"${te}"
+rc=$?
+if [ $rc -eq 1 ]; then
+    $PRINTF "$OK\n"
+    numOK=$((numOK+1))
+else
+    $PRINTF "$FAILED\n"
+    echo "$CMD"
+    cat "${te}"
+    echo "command did not terminate with error!"
+    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

xio-progcall.c

@@ -103,8 +103,17 @@ int _xioopen_foxec(int xioflags,	/* XIO_RDONLY etc. */
       usepipes = false;
    }
 #endif /* HAVE_PTY */
-   retropt_ushort(popts, OPT_FDIN,  (unsigned short *)&fdi);
+
-   retropt_ushort(popts, OPT_FDOUT, (unsigned short *)&fdo);
+   if (retropt_ushort(popts, OPT_FDIN,  (unsigned short *)&fdi) >= 0) {
+      if ((xioflags&XIO_ACCMODE) == XIO_RDONLY) {
+	 Error("_xioopen_foxec(): option fdin is useless in read-only mode");
+      }
+   }
+   if (retropt_ushort(popts, OPT_FDOUT, (unsigned short *)&fdo) >= 0) {
+      if ((xioflags&XIO_ACCMODE) == XIO_WRONLY) {
+	 Error("_xioopen_foxec(): option fdout is useless in write-only mode");
+      }
+   }
 
    if (withfork) {
       if (!(xioflags&XIO_MAYCHILD)) {