Fix: Replace hardcoded 256-byte shared memory buffer with 4096-byte

[GaneshPatil7517]

The C++ shared memory implementation in concore.hpp and concoredocker.hpp allocates a fixed 256-byte buffer for inter-node data exchange and silently truncates any payload exceeding 255 characters with no warning, no error, and no log output. Downstream nodes receive incomplete data and produce wrong results.

Closes #514

Root Cause
Three hardcoded 256 values across shmget(), strncpy(), and strnlen() calls limited the shared memory buffer to 256 bytes. The wire format [simtime, val1, val2, ..., valN] uses ~8–18 characters per double, so payloads with 25+ values silently overflow. This is a critical data corruption bug for realistic neuromodulation studies with 32+ sensor channels (EEG, multi-electrode arrays).

Changes
In both concore.hpp and concoredocker.hpp:

1. Added static constexpr size_t SHM_SIZE = 4096 replaces all hardcoded 256 with a single named constant. Buffer now supports ~200+ double values (up from ~25).
2. Replaced all hardcoded 256 in:
   • shmget(key, 256, ...) → shmget(key, SHM_SIZE, ...)
   • strnlen(sharedData_get, 256) → strnlen(sharedData_get, SHM_SIZE)
   • strncpy(sharedData_create, ..., 256 - 1) → strncpy(sharedData_create, ..., SHM_SIZE - 1)
3. Added overflow detection in write_SM() prints a clear stderr error message when payload exceeds SHM_SIZE, instead of silently truncating.
4. Added explicit null termination sharedData_create[SHM_SIZE - 1] = '\0' after every strncpy() to prevent read overruns (defense-in-depth).

Affected Files

File	Changes
concore.hpp	SHM_SIZE constant + 7 replacements + 2 overflow checks + 2 null terminators
concoredocker.hpp	SHM_SIZE constant + 4 replacements + 1 overflow check + 1 null terminator

Note: sample/src/concore.hpp has the same issue but is not tracked in git. The fix pattern is identical and can be applied when that file is added to the repository.

Testing

• All 164 existing Python tests pass (pytest tests/ -v → 164 passed)
• C++ compilation verified on Windows (MinGW g++ 6.3.0) — shared memory code is #ifdef __linux__ guarded, compiles cleanly
• Manual verification: 3000-byte payload fits without truncation (previously would have been silently cut at 255 bytes)

Before / After

Scenario	Before (256 bytes)	After (4096 bytes)
10 doubles (~120 bytes)	Works	Works
25 doubles (~280 bytes)	Silently truncated	Works
50 doubles (~600 bytes)	Silently truncated	Works
200 doubles (~2400 bytes)	Silently truncated	Works
>200 doubles (>4095 bytes)	Silently truncated	Error message printed + truncated

[Copilot]

Pull request overview

This PR fixes silent data truncation in the Linux shared-memory transport used by the C++ Concore implementations by replacing the hardcoded 256-byte buffer limit with a named 4096-byte constant and adding explicit overflow signaling.

Changes:

• Introduces SHM_SIZE = 4096 and replaces previous hardcoded 256 usages in shmget(), strnlen(), and strncpy().
• Adds overflow detection in write_SM() that emits an stderr error when the payload exceeds the shared-memory capacity.
• Adds explicit null termination after strncpy() to ensure safe reads.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
concore.hpp	Uses SHM_SIZE for SHM allocation/read/write; adds overflow warning + explicit null termination on SHM writes.
concoredocker.hpp	Same SHM sizing + overflow warning + explicit null termination for Docker SHM path.

Comments suppressed due to low confidence (2)

concoredocker.hpp:243

• shmget(key, SHM_SIZE, IPC_CREAT|0666) will not resize an existing shared-memory segment for the same key; it will return the existing segment even if it was created with a smaller size. That can defeat the intention of increasing the buffer after upgrades or after a crash leaves the segment behind. Consider checking shm_segsz via shmctl(IPC_STAT) and recreating or failing with a clear error if the segment is smaller than SHM_SIZE.

        shmId_create = shmget(key, SHM_SIZE, IPC_CREAT | 0666);
        if (shmId_create == -1) {
            std::cerr << "Failed to create shared memory segment.\n";
            return;
        }
        sharedData_create = static_cast<char*>(shmat(shmId_create, NULL, 0));

concore.hpp:272

• In createSharedMemory(), if shmget() fails (shmId_create == -1) the code still calls shmat(shmId_create, ...). This can end up calling shmat(-1) and masking the real failure, and later writes may still proceed with shmId_create set to -1/invalid. Return early (or throw) when shmget() fails before attempting shmat().

        shmId_create = shmget(key, SHM_SIZE, IPC_CREAT | 0666);

        if (shmId_create == -1) {
            std::cerr << "Failed to create shared memory segment." << std::endl;
        }

        // Attach the shared memory segment to the process's address space
        sharedData_create = static_cast<char*>(shmat(shmId_create, NULL, 0));
        if (sharedData_create == reinterpret_cast<char*>(-1)) {

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