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Qing
> On Feb 10, 2023, at 7:50 PM, Qing Zhao <qing.z...@oracle.com> 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
> +
> +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
> +The structure with a C99 flexible array member is the last field of another
> +structure, for example:
> +
> +@smallexample
> +struct flex @{ int length; char data[]; @};
> +
> +struct out_flex @{ int m; struct flex flex_data; @};
> +@end smallexample
> +
> +In the above, @code{flex_data.data[]} is considered as a flexible array too.
> +
> +@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; @}
> +@end smallexample
> +
> +In the above, @code{flex_data1.data1[]} or @code{flex_data2.data2[]}
> +is considered as flexible arrays too.
> +
> +@item
> +The structure with a C99 flexible array member is the middle field of another
> +structure, for example:
> +
> +@smallexample
> +struct flex @{ int length; char data[]; @};
> +
> +struct mid_flex @{ int m; struct flex flex_data; int n; @};
> +@end smallexample
> +
> +In the above, @code{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 @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.
> @end itemize
>
> Non-empty initialization of zero-length
> diff --git a/gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
> b/gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
> new file mode 100644
> index 00000000000..e3f65c5ed07
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/variable-sized-type-flex-array.c
> @@ -0,0 +1,31 @@
> +/* Test for -Wgnu-variable-sized-type-not-at-end on structure/union with
> + C99 flexible array members being embedded into another structure. */
> +/* { dg-do compile } */
> +/* { dg-options "-Wgnu-variable-sized-type-not-at-end" } */
> +
> +struct flex { int n; int data[]; };
> +struct out_flex_end { int m; struct flex flex_data; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct out_flex_mid { struct flex flex_data; int m; }; /* { dg-warning
> "variable sized type not at the end of a struct" } */
> +/* since the warning has been issued for out_flex_mid, no need to
> + issue warning again when it is included in another structure/union. */
> +struct outer_flex_mid { struct out_flex_mid out_flex_data; int p; }; /* {
> dg-bogus "variable sized type not at the end of a struct" } */
> +union flex_union_mid { int a; struct outer_flex_mid b; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +
> +
> +struct flex0 { int n; int data[0]; };
> +struct out_flex_end0 { int m; struct flex0 flex_data; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct out_flex_mid0 { struct flex0 flex_data; int m; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct outer_flex_mid0 { struct out_flex_mid0 out_flex_data; int p; }; /* {
> dg-bogus "variable sized type not at the end of a struct" } */
> +union flex_union_mid0 { int a; struct outer_flex_mid0 b; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +
> +struct flex1 { int n; int data[1]; };
> +struct out_flex_end1 { int m; struct flex1 flex_data; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct out_flex_mid1 { struct flex1 flex_data; int m; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct outer_flex_mid1 { struct out_flex_mid1 out_flex_data; int p; }; /* {
> dg-bogus "variable sized type not at the end of a struct" } */
> +union flex_union_mid1 { int a; struct outer_flex_mid1 b; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +
> +struct flexn { int n; int data[8]; };
> +struct out_flex_endn { int m; struct flexn flex_data; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> +struct out_flex_midn { struct flexn flex_data; int m; }; /* {
> dg-bogus"variable sized type not at the end of a struct" } */
> +struct outer_flex_midn { struct out_flex_midn out_flex_data; int p; }; /* {
> dg-bogus"variable sized type not at the end of a struct" } */
> +union flex_union_midn { int a; struct outer_flex_midn b; }; /* { dg-bogus
> "variable sized type not at the end of a struct" } */
> --
> 2.31.1
>
