qemu-print: New qemu_fprintf(), qemu_vfprintf()

Code that doesn't want to know about current monitor vs. stdout
vs. stderr takes an fprintf_function callback and a FILE * argument to
pass to it.  Actual arguments are either fprintf() and stdout or
stderr, or monitor_fprintf() and the current monitor cast to FILE *.
monitor_fprintf() casts it right back, and is otherwise identical to
monitor_printf().  The type-punning is ugly.

New qemu_fprintf() and qemu_vprintf() address this need without type
punning: they are like fprintf() and vfprintf(), except they print to
the current monitor when passed a null FILE *.  The next commits will
put them to use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417191805.28198-14-armbru@redhat.com>

include/qemu/qemu-print.h

@@ -16,4 +16,8 @@
 int qemu_vprintf(const char *fmt, va_list ap) GCC_FMT_ATTR(1, 0);
 int qemu_printf(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 
+int qemu_vfprintf(FILE *stream, const char *fmt, va_list ap)
+    GCC_FMT_ATTR(2, 0);
+int qemu_fprintf(FILE *stream, const char *fmt, ...) GCC_FMT_ATTR(2, 3);
+
 #endif

util/qemu-print.c

@@ -40,3 +40,30 @@ int qemu_printf(const char *fmt, ...)
     va_end(ap);
     return ret;
 }
+
+/*
+ * Print like vfprintf()
+ * Print to @stream if non-null, else to current monitor.
+ */
+int qemu_vfprintf(FILE *stream, const char *fmt, va_list ap)
+{
+    if (!stream) {
+        return monitor_vprintf(cur_mon, fmt, ap);
+    }
+    return vfprintf(stream, fmt, ap);
+}
+
+/*
+ * Print like fprintf().
+ * Print to @stream if non-null, else to current monitor.
+ */
+int qemu_fprintf(FILE *stream, const char *fmt, ...)
+{
+    va_list ap;
+    int ret;
+
+    va_start(ap, fmt);
+    ret = qemu_vfprintf(stream, fmt, ap);
+    va_end(ap);
+    return ret;
+}