3 files changed, 54 insertions, 3 deletions

diff --git a/hw/qdev-properties-system.c b/hw/qdev-properties-system.c
index d9934b5b9c..a22b155b6f 100644
--- a/hw/qdev-properties-system.c
+++ b/hw/qdev-properties-system.c
@@ -123,11 +123,10 @@ static int parse_chr(DeviceState *dev, const char *str, void **ptr)
     if (chr == NULL) {
         return -ENOENT;
     }
-    if (chr->avail_connections < 1) {
+    if (qemu_chr_fe_claim(chr) != 0) {
         return -EEXIST;
     }
     *ptr = chr;
-    --chr->avail_connections;
     return 0;
 }
 
@@ -140,7 +139,7 @@ static void release_chr(Object *obj, const char *name, void *opaque)
 
     if (chr) {
         qemu_chr_add_handlers(chr, NULL, NULL, NULL, NULL);
-        ++chr->avail_connections;
+        qemu_chr_fe_release(chr);
     }
 }
 
diff --git a/include/char/char.h b/include/char/char.h
index 32c9999113..9d1ea46117 100644
--- a/include/char/char.h
+++ b/include/char/char.h
@@ -203,6 +203,35 @@ int qemu_chr_fe_ioctl(CharDriverState *s, int cmd, void *arg);
 int qemu_chr_fe_get_msgfd(CharDriverState *s);
 
 /**
+ * @qemu_chr_fe_claim:
+ *
+ * Claim a backend before using it, should be called before calling
+ * qemu_chr_add_handlers(). 
+ *
+ * Returns: -1 if the backend is already in use by another frontend, 0 on
+ *          success.
+ */
+int qemu_chr_fe_claim(CharDriverState *s);
+
+/**
+ * @qemu_chr_fe_claim_no_fail:
+ *
+ * Like qemu_chr_fe_claim, but will exit qemu with an error when the
+ * backend is already in use.
+ */
+void qemu_chr_fe_claim_no_fail(CharDriverState *s);
+
+/**
+ * @qemu_chr_fe_claim:
+ *
+ * Release a backend for use by another frontend.
+ *
+ * Returns: -1 if the backend is already in use by another frontend, 0 on
+ *          success.
+ */
+void qemu_chr_fe_release(CharDriverState *s);
+
+/**
  * @qemu_chr_be_can_write:
  *
  * Determine how much data the front end can currently accept.  This function
diff --git a/qemu-char.c b/qemu-char.c
index 505a773330..100f9a968c 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -3452,6 +3452,29 @@ int qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
     return tag;
 }
 
+int qemu_chr_fe_claim(CharDriverState *s)
+{
+    if (s->avail_connections < 1) {
+        return -1;
+    }
+    s->avail_connections--;
+    return 0;
+}
+
+void qemu_chr_fe_claim_no_fail(CharDriverState *s)
+{
+    if (qemu_chr_fe_claim(s) != 0) {
+        fprintf(stderr, "%s: error chardev \"%s\" already used\n",
+                __func__, s->label);
+        exit(1);
+    }
+}
+
+void qemu_chr_fe_release(CharDriverState *s)
+{
+    s->avail_connections++;
+}
+
 void qemu_chr_delete(CharDriverState *chr)
 {
     QTAILQ_REMOVE(&chardevs, chr, next);