[glib: 1/2] Add and use G_SIZEOF_MEMBER() macro

[Philip Withnall <pwithnall src gnome org>]

commit fd61a21ecdbbef5c1cf65d989ea42f4fc70b9b24
Author: Marc-André Lureau <marcandre lureau redhat com>
Date:   Mon Jan 20 16:41:34 2020 +0400

    Add and use G_SIZEOF_MEMBER() macro
    
    A convenient macro present in the Linux kernel, named FIELD_SIZE() there.
    
    Signed-off-by: Marc-André Lureau <marcandre lureau redhat com>

 docs/reference/glib/glib-sections.txt |  1 +
 gio/glocalfileoutputstream.c          |  4 ++--
 gio/gunixoutputstream.c               |  4 ++--
 gio/gunixsocketaddress.c              |  2 +-
 glib/gmacros.h                        | 14 ++++++++++++++
 glib/tests/macros.c                   |  8 ++++++++
 6 files changed, 28 insertions(+), 5 deletions(-)
---
diff --git a/docs/reference/glib/glib-sections.txt b/docs/reference/glib/glib-sections.txt
index dd5f923f3..63f40baf5 100644
--- a/docs/reference/glib/glib-sections.txt
+++ b/docs/reference/glib/glib-sections.txt
@@ -424,6 +424,7 @@ CLAMP
 G_APPROX_VALUE
 
 <SUBSECTION>
+G_SIZEOF_MEMBER
 G_STRUCT_MEMBER
 G_STRUCT_MEMBER_P
 G_STRUCT_OFFSET
diff --git a/gio/glocalfileoutputstream.c b/gio/glocalfileoutputstream.c
index a7bcb04a6..a8161bd52 100644
--- a/gio/glocalfileoutputstream.c
+++ b/gio/glocalfileoutputstream.c
@@ -224,9 +224,9 @@ g_local_file_output_stream_write (GOutputStream  *stream,
 #ifdef G_OS_UNIX
 /* Macro to check if struct iovec and GOutputVector have the same ABI */
 #define G_OUTPUT_VECTOR_IS_IOVEC (sizeof (struct iovec) == sizeof (GOutputVector) && \
-      sizeof ((struct iovec *) 0)->iov_base == sizeof ((GOutputVector *) 0)->buffer && \
+      G_SIZEOF_MEMBER (struct iovec, iov_base) == G_SIZEOF_MEMBER (GOutputVector, buffer) && \
       G_STRUCT_OFFSET (struct iovec, iov_base) == G_STRUCT_OFFSET (GOutputVector, buffer) && \
-      sizeof ((struct iovec *) 0)->iov_len == sizeof((GOutputVector *) 0)->size && \
+      G_SIZEOF_MEMBER (struct iovec, iov_len) == G_SIZEOF_MEMBER (GOutputVector, size) && \
       G_STRUCT_OFFSET (struct iovec, iov_len) == G_STRUCT_OFFSET (GOutputVector, size))
 
 static gboolean
diff --git a/gio/gunixoutputstream.c b/gio/gunixoutputstream.c
index 506e09a8c..1af0b44d2 100644
--- a/gio/gunixoutputstream.c
+++ b/gio/gunixoutputstream.c
@@ -403,9 +403,9 @@ g_unix_output_stream_write (GOutputStream  *stream,
 
 /* Macro to check if struct iovec and GOutputVector have the same ABI */
 #define G_OUTPUT_VECTOR_IS_IOVEC (sizeof (struct iovec) == sizeof (GOutputVector) && \
-      sizeof ((struct iovec *) 0)->iov_base == sizeof ((GOutputVector *) 0)->buffer && \
+      G_SIZEOF_MEMBER (struct iovec, iov_base) == G_SIZEOF_MEMBER (GOutputVector, buffer) && \
       G_STRUCT_OFFSET (struct iovec, iov_base) == G_STRUCT_OFFSET (GOutputVector, buffer) && \
-      sizeof ((struct iovec *) 0)->iov_len == sizeof((GOutputVector *) 0)->size && \
+      G_SIZEOF_MEMBER (struct iovec, iov_len) == G_SIZEOF_MEMBER (GOutputVector, size) && \
       G_STRUCT_OFFSET (struct iovec, iov_len) == G_STRUCT_OFFSET (GOutputVector, size))
 
 static gboolean
diff --git a/gio/gunixsocketaddress.c b/gio/gunixsocketaddress.c
index 27e195e47..0ab1a62e9 100644
--- a/gio/gunixsocketaddress.c
+++ b/gio/gunixsocketaddress.c
@@ -67,7 +67,7 @@ enum
 };
 
 #ifndef UNIX_PATH_MAX
-#define UNIX_PATH_MAX sizeof (((struct sockaddr_un *) 0)->sun_path)
+#define UNIX_PATH_MAX G_SIZEOF_MEMBER (struct sockaddr_un, sun_path)
 #endif
 
 struct _GUnixSocketAddressPrivate
diff --git a/glib/gmacros.h b/glib/gmacros.h
index e829f207b..d9c7906cd 100644
--- a/glib/gmacros.h
+++ b/glib/gmacros.h
@@ -1096,4 +1096,18 @@
 
 #endif /* __GI_SCANNER__ */
 
+/**
+ * G_SIZEOF_MEMBER:
+ * @struct_type: a structure type, e.g. #GOutputVector
+ * @member: a field in the structure, e.g. `size`
+ *
+ * Returns: the size of @member in the struct definition without having a
+ * declared instance of @struct_type.
+ *
+ * Since: 2.64
+ */
+#define G_SIZEOF_MEMBER(struct_type, member) \
+    GLIB_AVAILABLE_MACRO_IN_2_64 \
+    sizeof (((struct_type *) 0)->member)
+
 #endif /* __G_MACROS_H__ */
diff --git a/glib/tests/macros.c b/glib/tests/macros.c
index 89a1322ee..317832a41 100644
--- a/glib/tests/macros.c
+++ b/glib/tests/macros.c
@@ -50,6 +50,13 @@ test_alignof_fallback (void)
   check_alignof (struct { char a; int b; });
 }
 
+static void
+test_struct_sizeof_member (void)
+{
+    G_STATIC_ASSERT (G_SIZEOF_MEMBER (struct { char a; int b; }, a) == sizeof (char));
+    g_assert_cmpint (G_SIZEOF_MEMBER (struct { char a; int b; }, b), ==, sizeof (int));
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -58,6 +65,7 @@ main (int   argc,
 
   g_test_add_func ("/alignof/fallback", test_alignof_fallback);
   g_test_add_func ("/assert/static", test_assert_static);
+  g_test_add_func ("/struct/sizeof_member", test_struct_sizeof_member);
 
   return g_test_run ();
 }