[gcc r15-208] Allow flexible array members in unions and alone in structures [PR53548]

[gcc r15-208] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

https://gcc.gnu.org/g:adb1c8a0f167c3a1f7593d75f5a10eb07a5d741a

commit r15-208-gadb1c8a0f167c3a1f7593d75f5a10eb07a5d741a
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Mon May 6 16:25:04 2024 +0000

    Allow flexible array members in unions and alone in structures [PR53548]
    
    The request for GCC to accept that the C99 flexible array member can be
    in a union or alone in a structure has been made a long time ago around 2012
    for supporting several practical cases including glibc.
    
    A GCC PR has been opened for such request at that time:
    
    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
    
    However, this PR was closed as WONTFIX around 2015 due to the following reason:
    
    "there is an existing extension that makes the requested functionality possible"
    i.e GCC fully supported that the zero-length array can be in a union or alone
    in a structure for a long time. (though I didn't see any official documentation
    on such extension)
    
    It's reasonable to close PR53548 at that time since zero-length array extension
    can be used for such purpose.
    
    However, since GCC13, in order to improve the C/C++ security, we introduced
    -fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
    usages from C/C++ source code. As a result, zero-length arrays eventually
    will be replaced by C99 flexiable array member completely.
    
    Therefore, GCC needs to explicitly allow such extensions directly for C99
    flexible arrays, since flexable array member in unions or alone in structs
    are common code patterns in active use by the Linux kernel (and other projects).
    
    For example, these do not error by default with GCC:
    
    union one {
      int a;
      int b[0];
    };
    
    union two {
      int a;
      struct {
        struct { } __empty;
        int b[];
      };
    };
    
    But these do:
    
    union three {
      int a;
      int b[];
    };
    
    struct four {
      int b[];
    }
    
    Clang has supported such extensions since March, 2024
    https://github.com/llvm/llvm-project/pull/84428
    
    GCC should also support such extensions. This will allow for
    a seamless transition for code bases away from zero-length arrays without
    losing existing code patterns.
    
    gcc/ChangeLog:
    
            PR c/53548
            * doc/extend.texi: Add documentation for Flexible Array Members in
            Unions and Flexible Array Members alone in Structures.

Diff:
---
 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index e290265d68d..267fccd1512 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible Array Members.
+* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 
+@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If every member of a union is a flexible array member, the size of
+such a union is zero.
+
+@node Flexible Array Members alone in Structures
+@section Structures with only Flexible Array Members
+@cindex structures with only flexible array members
+@cindex structures with only FAMs
+
+GCC permits a C99 flexible array member (FAM) to be alone in a structure:
+
+@smallexample
+struct only_fam @{
+  int b[];
+@};
+@end smallexample
+
+The size of such a structure is zero.
+
 @node Variable Length
 @section Arrays of Variable Length
 @cindex variable-length arrays