Fix missing C stderr output (fprintf(stderr, ...) not shown)

[anutosh491]

Description

Fixes #381

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[anutosh491]

The Problem here

C++ error output via std::cerr appeared correctly because xeus-cpp redirects the iostream buffers to its xoutput_buffer. However, C-level error output via fprintf(stderr, ...) did not appear. Only printf(...) and fprintf(stdout, ...) worked, because StreamRedirectRAII forwarded the captured stdout stream but silently dropped the captured stderr stream during normal execution.

Hence causing this

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.05%. Comparing base (9a4af34) to head (10d6693).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
- Coverage   82.29%   82.05%   -0.25%     
==========================================
  Files          21       21              
  Lines         853      858       +5     
  Branches       87       89       +2     
==========================================
+ Hits          702      704       +2     
- Misses        151      154       +3     

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	90.85% <100.00%> (+0.28%)	⬆️

... and 1 file with indirect coverage changes

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	90.85% <100.00%> (+0.28%)	⬆️

... and 1 file with indirect coverage changes

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[github-actions]

clang-tidy made some suggestions

[anutosh491]

Fix introduced

• StreamRedirectRAII now:

  • accepts two references (out, err)
  • begins capture in the correct order (stdout → stderr)
  • ends capture in LIFO order (stderr → stdout)
  • forwards neither inside the destructor

• execute_request_impl now:

  • emits the captured out → std::cout and captured err → std::cerr once
  • flushes them immediately.

This fixes what we want

And C++ based streams worked already

[mcbarton]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

[anutosh491]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

Thank you, I shall do that soon (afk for sometime)

We technically need to track that we should test such fixes in favour of the C Kernel whenever we end up with a framework for that. I haven't gone through all the discussion but as far as I remember, cppinterop needs to provide us with a utility to differentiate between C++ and C snippets isn't it ?

[mcbarton]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

Thank you, I shall do that soon (afk for sometime)

We technically need to track that we should test such fixes in favour of the C Kernel whenever we end up with a framework for that. I haven't gone through all the discussion but as far as I remember, cppinterop needs to provide us with a utility to differentiate between C++ and C snippets isn't it ?

Yes, that is exactly what needs to be done to enable c kernel tests. I have already tested the other small change which would be needed in test_xcpp_kernel.py to run the c kernel tests. If you mention all the things you think the c kernel tests should be testing in the issue already open about adding c tests #394 then I can add them once the cppinterop utility is available.

[github-actions]

clang-tidy made some suggestions

[anutosh491]

Technically this change was made in favour of the C Kernel, but I guess we can check some redirection through a simple test like this through tests that cater to our C++ kernel.

Fails on masters but now works as can be seen through the CI.

(xeus-cpp) anutosh491@Anutoshs-MacBook-Air test % ./test_xeus_cpp 
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
Failed to detect the resource-dir
clang version 20.1.7
Target: arm64-apple-darwin24.5.0
Thread model: posix
InstalledDir: /Users/anutosh491/work/xeus-cpp/build/test
ignoring nonexistent directory "/Users/anutosh491/work/xeus-cpp/build/test/../include/c++/v1"
 "/Users/anutosh491/work/xeus-cpp/build/test/test_xeus_cpp" -cc1 -triple arm64-apple-macosx15.0.0 -Wundef-prefix=TARGET_OS_ -Werror=undef-prefix -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -E -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name "<<< inputs >>>" -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -ffp-contract=on -fno-rounding-math -funwind-tables=1 -target-sdk-version=15.2 -fcompatibility-qualified-id-block-type-checking -fvisibility-inlines-hidden-static-local-var -fdefine-target-os-macros -fno-modulemap-allow-subdirectory-search -target-cpu apple-m1 -target-feature +zcm -target-feature +zcz -target-feature +v8.4a -target-feature +aes -target-feature +altnzcv -target-feature +ccdp -target-feature +ccpp -target-feature +complxnum -target-feature +crc -target-feature +dotprod -target-feature +flagm -target-feature +fp-armv8 -target-feature +fp16fml -target-feature +fptoint -target-feature +fullfp16 -target-feature +jsconv -target-feature +lse -target-feature +neon -target-feature +pauth -target-feature +perfmon -target-feature +predres -target-feature +ras -target-feature +rcpc -target-feature +rdm -target-feature +sb -target-feature +sha2 -target-feature +sha3 -target-feature +specrestrict -target-feature +ssbs -target-abi darwinpcs -debugger-tuning=lldb -fdebug-compilation-dir=/Users/anutosh491/work/xeus-cpp/build/test -target-linker-version 951.9 -v -fcoverage-compilation-dir=/Users/anutosh491/work/xeus-cpp/build/test -resource-dir /Users/anutosh491/micromamba/envs/xeus-cpp -isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1 -isystem /Library/Developer/CommandLineTools/usr/lib/clang/16/include -isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -isystem /Library/Developer/CommandLineTools/usr/include -isystem "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)" -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1 -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /Users/anutosh491/micromamba/envs/xeus-cpp/include -internal-externc-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -std=c++14 -fdeprecated-macro -ferror-limit 19 -stack-protector 1 -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -fmax-type-align=16 -fcolor-diagnostics -fincremental-extensions -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o - -x c++ "<<< inputs >>>"
clang -cc1 version 20.1.7 based upon LLVM 20.1.7 default target arm64-apple-darwin24.5.0
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/SubFrameworks"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/Library/Frameworks"
ignoring duplicate directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1"
ignoring duplicate directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include"
#include "..." search starts here:
#include <...> search starts here:
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1
 /Library/Developer/CommandLineTools/usr/lib/clang/16/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/usr/include
 /Users/anutosh491/micromamba/envs/xeus-cpp/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.
===============================================================================
[doctest] test cases: 1 | 0 passed | 1 failed | 0 skipped
[doctest] assertions: 3 | 2 passed | 1 failed |
[doctest] Status: FAILURE!

[anutosh491]

Last commit satisfies clang format but if it's over the top, can be reverted.

[anutosh491]

cc @davy39 @dannluciano

Possibly this fix can help y'all. Hence bringing to your notice