Re: [V9][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
On Tue, 30 May 2023, Qing Zhao via Gcc-patches wrote: > Joseph, > > could you please review this patch and see whether it's Okay for commit > now? This version is OK. -- Joseph S. Myers jos...@codesourcery.com
[V9][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
Joseph,
could you please review this patch and see whether it's Okay for commit
now?
thanks a lot for all your comments and suggestions for this patch.
Qing.
==
on a structure with a C99 flexible array member being nested in
another structure.
"The GCC extension accepts a structure containing an ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are two situations:
* A structure containing a C99 flexible array member, or a union
containing such a structure, is the last field of another structure,
for example:
struct flex { int length; char data[]; };
union union_flex { int others; struct flex f; };
struct out_flex_struct { int m; struct flex flex_data; };
struct out_flex_union { int n; union union_flex flex_data; };
In the above, both 'out_flex_struct.flex_data.data[]' and
'out_flex_union.flex_data.f.data[]' are considered as flexible
arrays too.
* A structure containing a C99 flexible array member, or a union
containing such a structure, is not the last field of another structure,
for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, accessing a member of the array 'mid_flex.flex_data.data[]'
might have undefined behavior. Compilers do not handle such a case
consistently, Any code relying on this case should be modified to ensure
that flexible array members only end up at the ends of structures.
Please use the warning option '-Wflex-array-member-not-at-end' to
identify all such cases in the source code and modify them. This extension
is now deprecated.
"
gcc/c-family/ChangeLog:
* c.opt: New option -Wflex-array-member-not-at-end.
gcc/c/ChangeLog:
* c-decl.cc (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 +++
gcc/c/c-decl.cc | 9
gcc/doc/extend.texi | 44 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 +
4 files changed, 88 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index cddeece..c26d9801b63 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wflex-array-member-not-at-end
+C C++ Var(warn_flex_array_member_not_at_end) Warning
+Warn when a structure containing a C99 flexible array member as the last
+field is not at the end of another structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 0c718151f6d..a1a8e9bd341 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9293,6 +9293,15 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
TYPE_INCLUDES_FLEXARRAY (t)
= is_last_field && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x));
+ if (warn_flex_array_member_not_at_end
+ && !is_last_field
+ && RECORD_OR_UNION_TYPE_P (TREE_TYPE (x))
+ && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+ OPT_Wflex_array_member_not_at_end,
+ "structure containing a flexible array member"
+ " is not at the end of another structure");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index ed8b9c8a87b..aa1ecb6968e 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1751,7 +1751,49 @@ Flexible array members may only appear as the last
member of a
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
-permitted by GCC as extensions.)
+permitted by GCC as extensions, see details below.)
+@end itemize
+
+The GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
+member}, or a union containing such a structure (possibly recursively)
+to be a member of a structure.
+
+There are two situation
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
> On May 26, 2023, at 4:12 PM, Joseph Myers wrote: > > On Fri, 26 May 2023, Qing Zhao via Gcc-patches wrote: > >> Another question: is it better for me to rearrange the Patch 1/2 and Patch >> 2/2 a little bit, >> to put the FE , doc change and corresponding testing case together into one >> patch, (you have approved the FE part of change in Patch 1/2). >> and then the mid-end change to tree-ojbect-size.cc and the corresponding >> testing cases to another patch? > > I don't really see this patch as needing to be split up into multiple > parts at all. Okay. Will update the documentation without mentioning the future version number and send out the webrev one more time. Qing > > -- > Joseph S. Myers > jos...@codesourcery.com
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
On Fri, 26 May 2023, Qing Zhao via Gcc-patches wrote: > Another question: is it better for me to rearrange the Patch 1/2 and Patch > 2/2 a little bit, > to put the FE , doc change and corresponding testing case together into one > patch, (you have approved the FE part of change in Patch 1/2). > and then the mid-end change to tree-ojbect-size.cc and the corresponding > testing cases to another patch? I don't really see this patch as needing to be split up into multiple parts at all. -- Joseph S. Myers jos...@codesourcery.com
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
Thank you for the suggestion.
> On May 26, 2023, at 1:59 PM, Joseph Myers wrote:
>
> On Thu, 25 May 2023, Qing Zhao via Gcc-patches wrote:
>
>>> On May 25, 2023, at 4:51 PM, Joseph Myers wrote:
>>>
>>> The documentation in this case is OK, though claims about how a future
>>> version will behave have a poor track record (we tend to end up with such
>>> claims persisting in the documentation even though the change in question
>>> didn't get made and might sometimes no longer be considered desirable).
>> Then, do you have any suggestions on this claim? Shall we delete it from
>> the doc? Or keep it?
>
> My suggestion would be just to say the feature is deprecated without
> saying what a future version will do -
I will modify the doc as:
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 17ef80e75cc..e8a8be93ff1 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1792,8 +1792,8 @@ consistently. Any code relying on this case should be
modified to ensure
that flexible array members only end up at the ends of structures.
Please use the warning option @option{-Wflex-array-member-not-at-end} to
-identify all such cases in the source code and modify them. This warning
-will be on by default starting from GCC 15.
+identify all such cases in the source code and modify them. This extension
+is now deprecated.
@end itemize
Let me know if you have see any issue with the above.
> also make sure to say it's
> deprecated in the GCC 14 release notes, and then if GCC 15 starts to warn,
> put something in the GCC 15 release notes as well.
Okay, will add this to my todo list.
Another question: is it better for me to rearrange the Patch 1/2 and Patch
2/2 a little bit,
to put the FE , doc change and corresponding testing case together into one
patch, (you have approved the FE part of change in Patch 1/2).
and then the mid-end change to tree-ojbect-size.cc and the corresponding
testing cases to another patch?
Thank you!
Qing
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
On Thu, 25 May 2023, Qing Zhao via Gcc-patches wrote: > > On May 25, 2023, at 4:51 PM, Joseph Myers wrote: > > > > The documentation in this case is OK, though claims about how a future > > version will behave have a poor track record (we tend to end up with such > > claims persisting in the documentation even though the change in question > > didn't get made and might sometimes no longer be considered desirable). > Then, do you have any suggestions on this claim? Shall we delete it from > the doc? Or keep it? My suggestion would be just to say the feature is deprecated without saying what a future version will do - also make sure to say it's deprecated in the GCC 14 release notes, and then if GCC 15 starts to warn, put something in the GCC 15 release notes as well. -- Joseph S. Myers jos...@codesourcery.com
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
> On May 25, 2023, at 4:51 PM, Joseph Myers wrote: > > The documentation in this case is OK, though claims about how a future > version will behave have a poor track record (we tend to end up with such > claims persisting in the documentation even though the change in question > didn't get made and might sometimes no longer be considered desirable). Then, do you have any suggestions on this claim? Shall we delete it from the doc? Or keep it? Qing > > -- > Joseph S. Myers > jos...@codesourcery.com
Re: [V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
The documentation in this case is OK, though claims about how a future version will behave have a poor track record (we tend to end up with such claims persisting in the documentation even though the change in question didn't get made and might sometimes no longer be considered desirable). -- Joseph S. Myers jos...@codesourcery.com
[V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
on a structure with a C99 flexible array member being nested in
another structure.
"The GCC extension accepts a structure containing an ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are two situations:
* A structure containing a C99 flexible array member, or a union
containing such a structure, is the last field of another structure,
for example:
struct flex { int length; char data[]; };
union union_flex { int others; struct flex f; };
struct out_flex_struct { int m; struct flex flex_data; };
struct out_flex_union { int n; union union_flex flex_data; };
In the above, both 'out_flex_struct.flex_data.data[]' and
'out_flex_union.flex_data.f.data[]' are considered as flexible
arrays too.
* A structure containing a C99 flexible array member, or a union
containing such a structure, is not the last field of another structure,
for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, accessing a member of the array 'mid_flex.flex_data.data[]'
might have undefined behavior. Compilers do not handle such a case
consistently, Any code relying on this case should be modified to ensure
that flexible array members only end up at the ends of structures.
Please use the warning option '-Wflex-array-member-not-at-end' to
identify all such cases in the source code and modify them. This
warning will be on by default starting from GCC 15.
"
gcc/c-family/ChangeLog:
* c.opt: New option -Wflex-array-member-not-at-end.
gcc/c/ChangeLog:
* c-decl.cc (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 +++
gcc/c/c-decl.cc | 9
gcc/doc/extend.texi | 44 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 +
4 files changed, 88 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index cddeece..c26d9801b63 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wflex-array-member-not-at-end
+C C++ Var(warn_flex_array_member_not_at_end) Warning
+Warn when a structure containing a C99 flexible array member as the last
+field is not at the end of another structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index e14f514cb6e..ecd10ebb69c 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9278,6 +9278,15 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
TYPE_INCLUDES_FLEXARRAY (t)
= is_last_field && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x));
+ if (warn_flex_array_member_not_at_end
+ && !is_last_field
+ && RECORD_OR_UNION_TYPE_P (TREE_TYPE (x))
+ && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+ OPT_Wflex_array_member_not_at_end,
+ "structure containing a flexible array member"
+ " is not at the end of another structure");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f9d13b495ad..17ef80e75cc 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1751,7 +1751,49 @@ Flexible array members may only appear as the last
member of a
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
-permitted by GCC as extensions.)
+permitted by GCC as extensions, see details below.)
+@end itemize
+
+The GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
+member}, or a union containing such a structure (possibly recursively)
+to be a member of a structure.
+
+There are two situations:
+
+@itemize @bullet
+@item
+A structure containing a C99 flexible array member, or a union containing
+such a structure, is the last field of another structure, for example:
+
[PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
on a structure with a C99 flexible array member being nested in
another structure.
"The GCC extension accepts a structure containing an ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are two situations:
* A structure containing a C99 flexible array member, or a union
containing such a structure, is the last field of another structure,
for example:
struct flex { int length; char data[]; };
union union_flex { int others; struct flex f; };
struct out_flex_struct { int m; struct flex flex_data; };
struct out_flex_union { int n; union union_flex flex_data; };
In the above, both 'out_flex_struct.flex_data.data[]' and
'out_flex_union.flex_data.f.data[]' are considered as flexible
arrays too.
* A structure containing a C99 flexible array member, or a union
containing such a structure, is not the last field of another structure,
for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, accessing a member of the array 'mid_flex.flex_data.data[]'
might have undefined behavior. Compilers do not handle such a case
consistently, Any code relying on this case should be modified to ensure
that flexible array members only end up at the ends of structures.
Please use the warning option '-Wflex-array-member-not-at-end' to
identify all such cases in the source code and modify them. This
warning will be on by default starting from GCC 15.
"
gcc/c-family/ChangeLog:
* c.opt: New option -Wflex-array-member-not-at-end.
gcc/c/ChangeLog:
* c-decl.cc (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 +++
gcc/c/c-decl.cc | 9
gcc/doc/extend.texi | 44 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 +
4 files changed, 88 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index cddeece..c26d9801b63 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wflex-array-member-not-at-end
+C C++ Var(warn_flex_array_member_not_at_end) Warning
+Warn when a structure containing a C99 flexible array member as the last
+field is not at the end of another structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index e14f514cb6e..ecd10ebb69c 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9278,6 +9278,15 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
TYPE_INCLUDES_FLEXARRAY (t)
= is_last_field && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x));
+ if (warn_flex_array_member_not_at_end
+ && !is_last_field
+ && RECORD_OR_UNION_TYPE_P (TREE_TYPE (x))
+ && TYPE_INCLUDES_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+ OPT_Wflex_array_member_not_at_end,
+ "structure containing a flexible array member"
+ " is not at the end of another structure");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f9d13b495ad..17ef80e75cc 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1751,7 +1751,49 @@ Flexible array members may only appear as the last
member of a
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
-permitted by GCC as extensions.)
+permitted by GCC as extensions, see details below.)
+@end itemize
+
+The GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
+member}, or a union containing such a structure (possibly recursively)
+to be a member of a structure.
+
+There are two situations:
+
+@itemize @bullet
+@item
+A structure containing a C99 flexible array member, or a union containing
+such a structure, is the last field of another structure, for example:
+
Re: [V7][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
Hi, Joseph,
I modified the gcc/doc/extend.texi per your suggestion as following:
Let me know if you have further comment and suggestion on this patch.
I will send out the V8 of the patch after some testing.
Thanks.
Qing.
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 6425ba57e88..9aedaa802e0 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1754,7 +1754,7 @@ structure or an element of an array. (However, these
uses are
permitted by GCC as extensions, see details below.)
@end itemize
-GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
+The GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
member}, or a union containing such a structure (possibly recursively)
to be a member of a structure.
@@ -1776,10 +1776,9 @@ struct out_flex_union @{ int n; union union_flex
flex_data; @};
In the above, both @code{out_flex_struct.flex_data.data[]} and
@code{out_flex_union.flex_data.f.data[]} are considered as flexible arrays too.
-
@item
A structure containing a C99 flexible array member, or a union containing
-such a structure, is the middle field of another structure, for example:
+such a structure, is not the last field of another structure, for example:
@smallexample
struct flex @{ int length; char data[]; @};
@@ -1787,12 +1786,12 @@ struct flex @{ int length; char data[]; @};
struct mid_flex @{ int m; struct flex flex_data; int n; @};
@end smallexample
-In the above, @code{mid_flex.flex_data.data[]} has undefined behavior.
-Compilers do not handle such case consistently, Any code relying on
-such case should be modified to ensure that flexible array members
-only end up at the ends of structures.
+In the above, accessing a member of the array @code{mid_flex.flex_data.data[]}
+might have undefined behavior. Compilers do not handle such a case
+consistently. Any code relying on this case should be modified to ensure
+that flexible array members only end up at the ends of structures.
-Please use warning option @option{-Wflex-array-member-not-at-end} to
+Please use the warning option @option{-Wflex-array-member-not-at-end} to
identify all such cases in the source code and modify them. This warning
will be on by default starting from GCC 15.
@end itemize
> On May 19, 2023, at 5:12 PM, Joseph Myers wrote:
>
> On Fri, 19 May 2023, Qing Zhao via Gcc-patches wrote:
>
>> +GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
>
> "The GCC extension" or "A GCC extension".
>
>> +@item
>> +A structure containing a C99 flexible array member, or a union containing
>> +such a structure, is the middle field of another structure, for example:
>
> There might be more than one middle field, and I think this case also
> includes where it's the *first* field - any field other than the last.
>
>> +@smallexample
>> +struct flex @{ int length; char data[]; @};
>> +
>> +struct mid_flex @{ int m; struct flex flex_data; int n; @};
>> +@end smallexample
>> +
>> +In the above, @code{mid_flex.flex_data.data[]} has undefined behavior.
>
> And it's not literally mid_flex.flex_data.data[] that has undefined
> behavior, but trying to access a member of that array.
>
>> +Compilers do not handle such case consistently, Any code relying on
>
> "such a case", and "," should be "." at the end of a sentence.
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
Re: [V7][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
Joseph,
Thanks a lot for the review. And sorry for my late reply (just came back from a
short vacation).
> On May 19, 2023, at 5:12 PM, Joseph Myers wrote:
>
> On Fri, 19 May 2023, Qing Zhao via Gcc-patches wrote:
>
>> +GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
>
> "The GCC extension" or "A GCC extension".
Okay.
>
>> +@item
>> +A structure containing a C99 flexible array member, or a union containing
>> +such a structure, is the middle field of another structure, for example:
>
> There might be more than one middle field, and I think this case also
> includes where it's the *first* field - any field other than the last.
Good point. Will fix this.
>
>> +@smallexample
>> +struct flex @{ int length; char data[]; @};
>> +
>> +struct mid_flex @{ int m; struct flex flex_data; int n; @};
>> +@end smallexample
>> +
>> +In the above, @code{mid_flex.flex_data.data[]} has undefined behavior.
>
> And it's not literally mid_flex.flex_data.data[] that has undefined
> behavior, but trying to access a member of that array.
Yes, you are right. Will fix this.
>
>> +Compilers do not handle such case consistently, Any code relying on
>
> "such a case", and "," should be "." at the end of a sentence.
Okay, will fix this.
Thanks
Qing
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
Re: [V7][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
On Fri, 19 May 2023, Qing Zhao via Gcc-patches wrote:
> +GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
"The GCC extension" or "A GCC extension".
> +@item
> +A structure containing a C99 flexible array member, or a union containing
> +such a structure, is the middle field of another structure, for example:
There might be more than one middle field, and I think this case also
includes where it's the *first* field - any field other than the last.
> +@smallexample
> +struct flex @{ int length; char data[]; @};
> +
> +struct mid_flex @{ int m; struct flex flex_data; int n; @};
> +@end smallexample
> +
> +In the above, @code{mid_flex.flex_data.data[]} has undefined behavior.
And it's not literally mid_flex.flex_data.data[] that has undefined
behavior, but trying to access a member of that array.
> +Compilers do not handle such case consistently, Any code relying on
"such a case", and "," should be "." at the end of a sentence.
--
Joseph S. Myers
jos...@codesourcery.com
[V7][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]
on a structure with a C99 flexible array member being nested in
another structure.
"GCC extension accepts a structure containing an ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are two situations:
* A structure containing a C99 flexible array member, or a union
containing such a structure, is the last field of another structure,
for example:
struct flex { int length; char data[]; };
union union_flex { int others; struct flex f; };
struct out_flex_struct { int m; struct flex flex_data; };
struct out_flex_union { int n; union union_flex flex_data; };
In the above, both 'out_flex_struct.flex_data.data[]' and
'out_flex_union.flex_data.f.data[]' are considered as flexible
arrays too.
* A structure containing a C99 flexible array member, or a union
containing such a structure, is the middle field of another structure,
for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, 'mid_flex.flex_data.data[]' has undefined behavior.
Compilers do not handle such case consistently, Any code relying on
such case should be modified to ensure that flexible array members
only end up at the ends of structures.
Please use warning option '-Wflex-array-member-not-at-end' to
identify all such cases in the source code and modify them. This
warning will be on by default starting from GCC 15.
"
gcc/c-family/ChangeLog:
* c.opt: New option -Wflex-array-member-not-at-end.
gcc/c/ChangeLog:
* c-decl.cc (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 +++
gcc/c/c-decl.cc | 9
gcc/doc/extend.texi | 45 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 +
4 files changed, 89 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index cddeece..c26d9801b63 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wflex-array-member-not-at-end
+C C++ Var(warn_flex_array_member_not_at_end) Warning
+Warn when a structure containing a C99 flexible array member as the last
+field is not at the end of another structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 2c620b681d9..9a48f28788d 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9293,6 +9293,15 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
TYPE_INCLUDE_FLEXARRAY (t)
= is_last_field && TYPE_INCLUDE_FLEXARRAY (TREE_TYPE (x));
+ if (warn_flex_array_member_not_at_end
+ && !is_last_field
+ && RECORD_OR_UNION_TYPE_P (TREE_TYPE (x))
+ && TYPE_INCLUDE_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+ OPT_Wflex_array_member_not_at_end,
+ "structure containing a flexible array member"
+ " is not at the end of another structure");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index ed8b9c8a87b..6425ba57e88 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1751,7 +1751,50 @@ Flexible array members may only appear as the last
member of a
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
-permitted by GCC as extensions.)
+permitted by GCC as extensions, see details below.)
+@end itemize
+
+GCC extension accepts a structure containing an ISO C99 @dfn{flexible array
+member}, or a union containing such a structure (possibly recursively)
+to be a member of a structure.
+
+There are two situations:
+
+@itemize @bullet
+@item
+A structure containing a C99 flexible array member, or a union containing
+such a structure, is the last field of another structure, for example:
+
+@smallexample
+struct flex @{ int length; char data[]; @}
Re: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
> On Feb 23, 2023, at 7:56 PM, Joseph Myers wrote:
>
> On Thu, 23 Feb 2023, Qing Zhao via Gcc-patches wrote:
>
>> But the following:
>>
>> struct flex1 { int length1; char data1[]; };
>> struct flex2 { int length2; char data2[]; };
>> union union_flex { struct flex1 f1; struct flex2 f2; }; /* this is C
>> standard. */
>>
>> struct out_flex { int n; union union_flex flex_data1;}; /* this is GNU
>> extension. */
>>
>> Should add this item into the documentation?
>
> "union that contains a structure with a flexible array member" is just
> like "structure with a flexible array member". I suppose the
> documentation should try to make that clear, without repeating it too much
> for every separate case.
Okay, thanks.
Qing
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
Re: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
On Thu, 23 Feb 2023, Qing Zhao via Gcc-patches wrote:
> But the following:
>
> struct flex1 { int length1; char data1[]; };
> struct flex2 { int length2; char data2[]; };
> union union_flex { struct flex1 f1; struct flex2 f2; }; /* this is C
> standard. */
>
> struct out_flex { int n; union union_flex flex_data1;}; /* this is GNU
> extension. */
>
> Should add this item into the documentation?
"union that contains a structure with a flexible array member" is just
like "structure with a flexible array member". I suppose the
documentation should try to make that clear, without repeating it too much
for every separate case.
--
Joseph S. Myers
jos...@codesourcery.com
Re: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
> On Feb 23, 2023, at 5:04 PM, Qing Zhao via Gcc-patches
> wrote:
>
>
>
>> On Feb 23, 2023, at 4:24 PM, Joseph Myers wrote:
>>
>> On Thu, 23 Feb 2023, Qing Zhao via Gcc-patches wrote:
>>
>>> +@item
>>> +The structure with a C99 flexible array member is the field of
>>> +another union, for example:
>>> +
>>> +@smallexample
>>> +struct flex1 @{ int length1; char data1[]; @}
>>> +struct flex2 @{ int length2; char data2[]; @}
>>> +
>>> +union out_flex @{ struct flex1 flex_data1; struct flex2 flex_data2; @}
>>
>> I don't think this is an extension; structures with flexible array members
>> are OK in unions in standard C.
>
> Thanks for the info.
>
> Just checked the small testing case with -Wpedantic, Yes, it’s accepted
> without any warning.
> Will delete this item from the doc. And send the new patch soon.
But the following:
struct flex1 { int length1; char data1[]; };
struct flex2 { int length2; char data2[]; };
union union_flex { struct flex1 f1; struct flex2 f2; }; /* this is C standard.
*/
struct out_flex { int n; union union_flex flex_data1;}; /* this is GNU
extension. */
Should add this item into the documentation?
Qing
>
> Thanks.
>
> Qing
>>
>> --
>> Joseph S. Myers
>> jos...@codesourcery.com
Re: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
> On Feb 23, 2023, at 4:24 PM, Joseph Myers wrote:
>
> On Thu, 23 Feb 2023, Qing Zhao via Gcc-patches wrote:
>
>> +@item
>> +The structure with a C99 flexible array member is the field of
>> +another union, for example:
>> +
>> +@smallexample
>> +struct flex1 @{ int length1; char data1[]; @}
>> +struct flex2 @{ int length2; char data2[]; @}
>> +
>> +union out_flex @{ struct flex1 flex_data1; struct flex2 flex_data2; @}
>
> I don't think this is an extension; structures with flexible array members
> are OK in unions in standard C.
Thanks for the info.
Just checked the small testing case with -Wpedantic, Yes, it’s accepted without
any warning.
Will delete this item from the doc. And send the new patch soon.
Thanks.
Qing
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
Re: Fwd: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
On Thu, 23 Feb 2023, Qing Zhao via Gcc-patches wrote:
> +@item
> +The structure with a C99 flexible array member is the field of
> +another union, for example:
> +
> +@smallexample
> +struct flex1 @{ int length1; char data1[]; @}
> +struct flex2 @{ int length2; char data2[]; @}
> +
> +union out_flex @{ struct flex1 flex_data1; struct flex2 flex_data2; @}
I don't think this is an extension; structures with flexible array members
are OK in unions in standard C.
--
Joseph S. Myers
jos...@codesourcery.com
Fwd: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
Ping * 2.
Hi, Joseph and Richard,
Could you please review this patch and let me know whether it’s ready for
committing into GCC13?
thanks.
Qing
Begin forwarded message:
From: Qing Zhao mailto:qing.z...@oracle.com>>
Subject: [v3][PATCH 2/2] Update documentation to clarify a GCC extension
(PR77650)
Date: February 10, 2023 at 7:50:13 PM EST
To: jos...@codesourcery.com<mailto:jos...@codesourcery.com>,
rguent...@suse.de<mailto:rguent...@suse.de>
Cc: siddh...@gotplt.org<mailto:siddh...@gotplt.org>,
keesc...@chromium.org<mailto:keesc...@chromium.org>,
gcc-patches@gcc.gnu.org<mailto:gcc-patches@gcc.gnu.org>, Qing Zhao
mailto:qing.z...@oracle.com>>
on structure with C99 flexible array member being nested in another structure.
This is also fixed PR77650.
" GCC extension accepts a structure containing a ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are three situations:
* The structure with a C99 flexible array member is the last field of
another structure, for example:
struct flex { int length; char data[]; };
struct out_flex { int m; struct flex flex_data; };
In the above, 'flex_data.data[]' is considered as a flexible array
too.
* The structure with a C99 flexible array member is the field of
another union, for example:
struct flex1 { int length1; char data1[]; }
struct flex2 { int length2; char data2[]; }
union out_flex { struct flex1 flex_data1; struct flex2 flex_data2; }
In the above, 'flex_data1.data1[]' or 'flex_data2.data2[]' is
considered as flexible arrays too.
* The structure with a C99 flexible array member is the middle field
of another structure, for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, 'flex_data.data[]' is allowed to be extended flexibly
to the padding. E.g, up to 4 elements.
However, relying on space in struct padding is a bad programming
practice, compilers do not handle such extension consistently, Any
code relying on this behavior should be modified to ensure that
flexible array members only end up at the ends of structures.
Please use warning option '-Wgnu-variable-sized-type-not-at-end' to
identify all such cases in the source code and modify them. This
extension will be deprecated from gcc in the next release. "
gcc/c-family/ChangeLog:
PR c/77650
* c.opt: New option -Wgnu-variable-sized-type-not-at-end.
gcc/c/ChangeLog:
PR c/77650
* c-decl.cc<http://c-decl.cc> (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
PR c/77650
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
PR c/77650
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 ++
gcc/c/c-decl.cc<http://c-decl.cc> | 7 +++
gcc/doc/extend.texi | 58 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 ++
4 files changed, 100 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index c0fea56a8f5..fd720538800 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wgnu-variable-sized-type-not-at-end
+C C++ Var(warn_variable_sized_type_not_at_end) Warning
+Warn about structures or unions with C99 flexible array members are not
+at the end of a structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc<http://c-decl.cc>
b/gcc/c/c-decl.cc<http://c-decl.cc>
index 741a37560b0..041df4355da 100644
--- a/gcc/c/c-decl.cc<http://c-decl.cc>
+++ b/gcc/c/c-decl.cc<http://c-decl.cc>
@@ -9289,6 +9289,13 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
&& is_last_field)
TYPE_INCLUDE_FLEXARRAY (t) = true;
+ if (warn_variable_sized_type_not_at_end
+ && !is_last_field
+ && TYPE_INCLUDE_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+OPT_Wgnu_variable_sized_type_not_at_end,
+"variable sized type not at the end of a struct");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/g
Re: [v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
Ping…
Qing
> On Feb 10, 2023, at 7:50 PM, Qing Zhao wrote:
>
> on structure with C99 flexible array member being nested in another structure.
>
> This is also fixed PR77650.
>
> " GCC extension accepts a structure containing a ISO C99 "flexible array
> member", or a union containing such a structure (possibly recursively)
> to be a member of a structure.
>
> There are three situations:
>
> * The structure with a C99 flexible array member is the last field of
> another structure, for example:
>
> struct flex { int length; char data[]; };
>
> struct out_flex { int m; struct flex flex_data; };
>
> In the above, 'flex_data.data[]' is considered as a flexible array
> too.
>
> * The structure with a C99 flexible array member is the field of
> another union, for example:
>
> struct flex1 { int length1; char data1[]; }
> struct flex2 { int length2; char data2[]; }
>
> union out_flex { struct flex1 flex_data1; struct flex2 flex_data2; }
>
> In the above, 'flex_data1.data1[]' or 'flex_data2.data2[]' is
> considered as flexible arrays too.
>
> * The structure with a C99 flexible array member is the middle field
> of another structure, for example:
>
> struct flex { int length; char data[]; };
>
> struct mid_flex { int m; struct flex flex_data; int n; };
>
> In the above, 'flex_data.data[]' is allowed to be extended flexibly
> to the padding. E.g, up to 4 elements.
>
> However, relying on space in struct padding is a bad programming
> practice, compilers do not handle such extension consistently, Any
> code relying on this behavior should be modified to ensure that
> flexible array members only end up at the ends of structures.
>
> Please use warning option '-Wgnu-variable-sized-type-not-at-end' to
> identify all such cases in the source code and modify them. This
> extension will be deprecated from gcc in the next release. "
>
> gcc/c-family/ChangeLog:
>
> PR c/77650
> * c.opt: New option -Wgnu-variable-sized-type-not-at-end.
>
> gcc/c/ChangeLog:
>
> PR c/77650
> * c-decl.cc (finish_struct): Issue warnings for new option.
>
> gcc/ChangeLog:
>
> PR c/77650
> * doc/extend.texi: Document GCC extension on a structure containing
> a flexible array member to be a member of another structure.
>
> gcc/testsuite/ChangeLog:
>
> PR c/77650
> * gcc.dg/variable-sized-type-flex-array.c: New test.
> ---
> gcc/c-family/c.opt| 5 ++
> gcc/c/c-decl.cc | 7 +++
> gcc/doc/extend.texi | 58 ++-
> .../gcc.dg/variable-sized-type-flex-array.c | 31 ++
> 4 files changed, 100 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
>
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index c0fea56a8f5..fd720538800 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -737,6 +737,11 @@ Wformat-truncation=
> C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
> Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
> IntegerRange(0, 2)
> Warn about calls to snprintf and similar functions that truncate output.
>
> +Wgnu-variable-sized-type-not-at-end
> +C C++ Var(warn_variable_sized_type_not_at_end) Warning
> +Warn about structures or unions with C99 flexible array members are not
> +at the end of a structure.
> +
> Wif-not-aligned
> C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
> Warn when the field in a struct is not aligned.
> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
> index 741a37560b0..041df4355da 100644
> --- a/gcc/c/c-decl.cc
> +++ b/gcc/c/c-decl.cc
> @@ -9289,6 +9289,13 @@ finish_struct (location_t loc, tree t, tree fieldlist,
> tree attributes,
> && is_last_field)
> TYPE_INCLUDE_FLEXARRAY (t) = true;
>
> + if (warn_variable_sized_type_not_at_end
> + && !is_last_field
> + && TYPE_INCLUDE_FLEXARRAY (TREE_TYPE (x)))
> + warning_at (DECL_SOURCE_LOCATION (x),
> + OPT_Wgnu_variable_sized_type_not_at_end,
> + "variable sized type not at the end of a struct");
> +
> if (DECL_NAME (x)
> || RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
> saw_named_field = true;
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 5a026c4b48c..737228b35ac 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -1748,7 +1748,63 @@ Flexible array members may only appear as the last
> member of a
> A structure containing a flexible array member, or a union containing
> such a structure (possibly recursively), may not be a member of a
> structure or an element of an array. (However, these uses are
> -permitted by GCC as extensions.)
> +permitted by GCC as extensions, see details below.)
> +@end itemize
[v3][PATCH 2/2] Update documentation to clarify a GCC extension (PR77650)
on structure with C99 flexible array member being nested in another structure.
This is also fixed PR77650.
" GCC extension accepts a structure containing a ISO C99 "flexible array
member", or a union containing such a structure (possibly recursively)
to be a member of a structure.
There are three situations:
* The structure with a C99 flexible array member is the last field of
another structure, for example:
struct flex { int length; char data[]; };
struct out_flex { int m; struct flex flex_data; };
In the above, 'flex_data.data[]' is considered as a flexible array
too.
* The structure with a C99 flexible array member is the field of
another union, for example:
struct flex1 { int length1; char data1[]; }
struct flex2 { int length2; char data2[]; }
union out_flex { struct flex1 flex_data1; struct flex2 flex_data2; }
In the above, 'flex_data1.data1[]' or 'flex_data2.data2[]' is
considered as flexible arrays too.
* The structure with a C99 flexible array member is the middle field
of another structure, for example:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
In the above, 'flex_data.data[]' is allowed to be extended flexibly
to the padding. E.g, up to 4 elements.
However, relying on space in struct padding is a bad programming
practice, compilers do not handle such extension consistently, Any
code relying on this behavior should be modified to ensure that
flexible array members only end up at the ends of structures.
Please use warning option '-Wgnu-variable-sized-type-not-at-end' to
identify all such cases in the source code and modify them. This
extension will be deprecated from gcc in the next release. "
gcc/c-family/ChangeLog:
PR c/77650
* c.opt: New option -Wgnu-variable-sized-type-not-at-end.
gcc/c/ChangeLog:
PR c/77650
* c-decl.cc (finish_struct): Issue warnings for new option.
gcc/ChangeLog:
PR c/77650
* doc/extend.texi: Document GCC extension on a structure containing
a flexible array member to be a member of another structure.
gcc/testsuite/ChangeLog:
PR c/77650
* gcc.dg/variable-sized-type-flex-array.c: New test.
---
gcc/c-family/c.opt| 5 ++
gcc/c/c-decl.cc | 7 +++
gcc/doc/extend.texi | 58 ++-
.../gcc.dg/variable-sized-type-flex-array.c | 31 ++
4 files changed, 100 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index c0fea56a8f5..fd720538800 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -737,6 +737,11 @@ Wformat-truncation=
C ObjC C++ LTO ObjC++ Joined RejectNegative UInteger Var(warn_format_trunc)
Warning LangEnabledBy(C ObjC C++ LTO ObjC++,Wformat=, warn_format >= 1, 0)
IntegerRange(0, 2)
Warn about calls to snprintf and similar functions that truncate output.
+Wgnu-variable-sized-type-not-at-end
+C C++ Var(warn_variable_sized_type_not_at_end) Warning
+Warn about structures or unions with C99 flexible array members are not
+at the end of a structure.
+
Wif-not-aligned
C ObjC C++ ObjC++ Var(warn_if_not_aligned) Init(1) Warning
Warn when the field in a struct is not aligned.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 741a37560b0..041df4355da 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9289,6 +9289,13 @@ finish_struct (location_t loc, tree t, tree fieldlist,
tree attributes,
&& is_last_field)
TYPE_INCLUDE_FLEXARRAY (t) = true;
+ if (warn_variable_sized_type_not_at_end
+ && !is_last_field
+ && TYPE_INCLUDE_FLEXARRAY (TREE_TYPE (x)))
+ warning_at (DECL_SOURCE_LOCATION (x),
+ OPT_Wgnu_variable_sized_type_not_at_end,
+ "variable sized type not at the end of a struct");
+
if (DECL_NAME (x)
|| RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
saw_named_field = true;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 5a026c4b48c..737228b35ac 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1748,7 +1748,63 @@ Flexible array members may only appear as the last
member of a
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
-permitted by GCC as extensions.)
+permitted by GCC as extensions, see details below.)
+@end itemize
+
+GCC extension accepts a structure containing a ISO C99 @dfn{flexible array
+member}, or a union containing such a structure (possibly recursively)
+to be a member of a structure.
+
+There are three situations:
+
+@itemize @bullet
+@item
