readline: Add missing GCC_FMT_ATTR

This fixes a compiler warning with -Werror=missing-format-attribute
and allows improved compiler checks for variable argument lists.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

include/qemu/readline.h

diff --git a/include/qemu/readline.h b/include/qemu/readline.h
index a89fe4a..49efe4e 100644
--- a/include/qemu/readline.h
+++ b/include/qemu/readline.h
@@ -5,7 +5,8 @@
 #define READLINE_MAX_CMDS 64
 #define READLINE_MAX_COMPLETIONS 256
 
-typedef void ReadLinePrintfFunc(void *opaque, const char *fmt, ...);
+typedef void GCC_FMT_ATTR(2, 3) ReadLinePrintfFunc(void *opaque,
+                                                   const char *fmt, ...);
 typedef void ReadLineFlushFunc(void *opaque);
 typedef void ReadLineFunc(void *opaque, const char *str,
                           void *readline_opaque);

monitor.c

diff --git a/monitor.c b/monitor.c
index 80456fb..5ba541d 100644
--- a/monitor.c
+++ b/monitor.c
@@ -4888,7 +4888,8 @@
 /* These functions just adapt the readline interface in a typesafe way.  We
  * could cast function pointers but that discards compiler checks.
  */
-static void monitor_readline_printf(void *opaque, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) monitor_readline_printf(void *opaque,
+                                                       const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);

qemu-io.c

diff --git a/qemu-io.c b/qemu-io.c
index d669028..7f459d8 100644
--- a/qemu-io.c
+++ b/qemu-io.c
@@ -219,7 +219,8 @@
     return prompt;
 }
 
-static void readline_printf_func(void *opaque, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) readline_printf_func(void *opaque,
+                                                    const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);