Nonstandard extension - zero-sized array in struct/union about mbedtls

[tom-cosgrove-arm]

This is a flexible array member, a perfectly fine part of standard C99:

As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member.

The issue here is with Visual Studio 2022 not being C99-compliant

[gilles-peskine-arm]

AFAIK Visual Studio does support flexible array members, and it didn't complain about them in our CI builds which go back to VS2013. Please make sure that you compile in a C99 or C11 mode, not C90 mode.

[irwir]

Microsoft also says:

This causes a Level-2 warning when a C++ file is compiled and a Level-4 warning when a C file is compiled.

CI should not complain because of the level 4.
For me this was compiled as a part of C++ solution, and I usually prefer to see more warnings.

Closing the issue since this is valid in C99 and not an MS extension.

[SadieCat]

We're getting this issue on Clang as a library user.

 /opt/homebrew/include/psa/crypto_struct.h:229:13: fatal error: flexible array members are a C99 feature [-Wc99-extensions]
    uint8_t data[];
            ^
1 error generated.

This C feature was not included when C99 was merged into the C++ spec in C++11. This means if you are using mbedTLS from a C++ project you need to compile with non-portable vendor extensions to use mbedTLS.

https://en.wikipedia.org/wiki/Flexible_array_member#Availability

[tom-cosgrove-arm]

Hmm, these definitions should be inside extern "C" to avoid exactly this

[irwir]

extern "C" and extern "C++" function declarations
Literally, it seems the declaration allows C calling convention, not C syntax or language extenstions.

[gilles-peskine-arm]

This C feature was not included when C99 was merged into the C++ spec in C++11.

Ah, that is a problem that I was no aware of.

https://en.wikipedia.org/wiki/Flexible_array_member#Availability

Flexible array members are not officially part of C++

Hmm, is this up-to-date? https://en.cppreference.com/w/cpp/language/array (which is usually accurate in my experience, but I'm a C programmer, not a C++ programmer) suggests that uint8_t data[]; is syntactically valid, and the section “Arrays of unknown bound” suggests that it's a valid incomplete type. Can an experienced C++ programmer please let me know what the situation is here? How do C++ programs typically consume C headers that define structures with flexible array members?

[gilles-peskine-arm]

I've filed a separate issue to track the C++ incompatibility introduced in 3.6: #9020