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32 libc++ `char_traits.h` assumes `EOF` is always available · Issue #85158 · llvm/llvm-project · GitHub
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libc++ char_traits.h assumes EOF is always available #85158

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petrhosek opened this issue Mar 14, 2024 · 4 comments
Closed

libc++ char_traits.h assumes EOF is always available #85158

petrhosek opened this issue Mar 14, 2024 · 4 comments
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embedded Support for embedded development libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. wontfix Issue is real, but we can't or won't fix it. Not invalid

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@petrhosek
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libc++ assumes that EOF is always available and uses it unconditionally in the char_traits.h implementation.

In file included from /llvm_libcxx/include/chrono:830:                                                                                                                                                                                                                                                                                                                                 
In file included from /llvm_libcxx/include/__chrono/convert_to_tm.h:22:                                                                                                                                                                                                                                                                                                                
In file included from /llvm_libcxx/include/__chrono/statically_widen.h:17:                                                                                                                                                                                                                                                                                                             
In file included from /llvm_libcxx/include/__format/concepts.h:17:                                                                                                                                                                                                                                                                                                                     
In file included from /llvm_libcxx/include/__format/format_parse_context.h:16:                                                                                                                                                                                                                                                                                                         
In file included from /llvm_libcxx/include/string_view:225:                                                                                                                                                                                                                                                                                                                            
/llvm_libcxx/include/__string/char_traits.h:169:100: error: use of undeclared identifier 'EOF'                                                                                                                                                                                                                                                                                         
  169 |   static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }                                                                                                                                                                                                                                                                                                                        
      |                                                                                                    ^                                                                                                                                                                                                                                                                                                                              
/llvm_libcxx/include/__string/char_traits.h:169:66: error: no return statement in constexpr function                                                                                                                                                                                                                                                                                   
  169 |   static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }

LLVM libc doesn't currently define EOF on baremetal targets because there's no filesystem support (since baremetal targets typically don't have filesystem to begin with) which results in a compiler error.

We could extend LLVM libc to define EOF even on baremetal targets, but it's an open question whether libc++ should be able to handle the case where EOF is undefined.

This is related to issue #84879.
@petrhosek petrhosek added libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. embedded Support for embedded development labels Mar 14, 2024
petrhosek added a commit to petrhosek/llvm-project that referenced this issue Mar 14, 2024
@petrhosek
[libc] Include EOF macro in stdio.h on baremetal
5ab1ef6 
libc++ char_traits.h assumes EOF is always available

See llvm#85158 for more details.
@petrhosek petrhosek mentioned this issue Mar 14, 2024
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@philnik777
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I can't really think of a good reason a libc shouldn't provide it. Given that the main thing this might lead to are ODR violations, since libc++ would have to provide it's own if it can't find the libc's, IMO the libc should just provide it. It's not much harder than adding #define EOF (-1) to a header.

petrhosek added a commit that referenced this issue Mar 15, 2024
@petrhosek
[libc] Move EOF macro to stdio-macros.h (#85159)
d6722bc 
libc++ char_traits.h assumes EOF is always available

See #85158 for more details.
@petrhosek petrhosek closed this as not planned Won't fix, can't repro, duplicate, stale Mar 15, 2024
@EugeneZelenko EugeneZelenko added the wontfix Issue is real, but we can't or won't fix it. Not invalid label Mar 15, 2024
@ldionne
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ldionne commented Mar 27, 2024

I've come across this issue in a different setting and I had the same conclusion -- libc should provide it.

Basically, EOF is just a really unfortunately-named macro, but it is technically not tied specifically to input and output. It's literally just a macro defined to some integer, and that should be defined even when the underlying system has no notion of I/O or files. It feels really weird to have a header named <stdio.h> in a freestanding C library and to provide a macro named EOF but not any other IO related functionality, but I think that's the right thing to do.

@frederick-vs-ja
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I suspect that there should be an LWG issue - currently std::char_traits<char>::eof is required to be freestanding due to P2338R4, but it's also required to return the value of EOF which is not yet freestanding.

@jwakely
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jwakely commented Apr 8, 2024

The eof, not_eof, to_int_type and to_char_type members of char_traits are only needed by iostreams, so I see no reason for them to be present in freestanding.

In libstdc++ we make eof and not_eof depend on hosted, so they're absent for freestanding.

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