Fix: Add nullptr guard for sharedData_create and shmctl segment size verification

Address Copilot review feedback:
- Add sharedData_create != nullptr check before strncpy/null-termination
  in all write_SM() overloads to prevent null pointer dereference when
  shmat() fails in createSharedMemory()
- Add shmctl(IPC_STAT) verification in createSharedMemory() to detect
  stale segments smaller than SHM_SIZE (shmget won't resize existing
  segments); removes and recreates the segment when too small
- Add early return in concore.hpp createSharedMemory() on shmget failure
  (was missing, unlike concoredocker.hpp which already had it)

Author: GaneshPatil7517

concore.hpp

@@ -265,6 +265,20 @@ class Concore{
 
         if (shmId_create == -1) {
             std::cerr << "Failed to create shared memory segment." << std::endl;
+            return;
+        }
+
+        // Verify the segment is large enough (shmget won't resize an existing segment)
+        struct shmid_ds shm_info;
+        if (shmctl(shmId_create, IPC_STAT, &shm_info) == 0 && shm_info.shm_segsz < SHM_SIZE) {
+            std::cerr << "Shared memory segment too small (" << shm_info.shm_segsz
+                      << " bytes, need " << SHM_SIZE << "). Removing and recreating." << std::endl;
+            shmctl(shmId_create, IPC_RMID, nullptr);
+            shmId_create = shmget(key, SHM_SIZE, IPC_CREAT | 0666);
+            if (shmId_create == -1) {
+                std::cerr << "Failed to recreate shared memory segment." << std::endl;
+                return;
+            }
         }
 
         // Attach the shared memory segment to the process's address space
@@ -660,6 +674,8 @@ class Concore{
         try {
             std::ostringstream outfile;
             if(shmId_create != -1){
+                if (sharedData_create == nullptr)
+                    throw 506;
                 val.insert(val.begin(),simtime+delta);
                 outfile<<'[';
                 for(int i=0;i<val.size()-1;i++)
@@ -697,6 +713,8 @@ class Concore{
         this_thread::sleep_for(timespan);
         try {
             if(shmId_create != -1){
+                if (sharedData_create == nullptr)
+                    throw 506;
                 if (val.size() >= SHM_SIZE) {
                     std::cerr << "ERROR: write_SM payload (" << val.size()
                               << " bytes) exceeds " << SHM_SIZE - 1

concoredocker.hpp

@@ -240,6 +240,20 @@ class Concore {
             std::cerr << "Failed to create shared memory segment.\n";
             return;
         }
+
+        // Verify the segment is large enough (shmget won't resize an existing segment)
+        struct shmid_ds shm_info;
+        if (shmctl(shmId_create, IPC_STAT, &shm_info) == 0 && shm_info.shm_segsz < SHM_SIZE) {
+            std::cerr << "Shared memory segment too small (" << shm_info.shm_segsz
+                      << " bytes, need " << SHM_SIZE << "). Removing and recreating.\n";
+            shmctl(shmId_create, IPC_RMID, nullptr);
+            shmId_create = shmget(key, SHM_SIZE, IPC_CREAT | 0666);
+            if (shmId_create == -1) {
+                std::cerr << "Failed to recreate shared memory segment.\n";
+                return;
+            }
+        }
+
         sharedData_create = static_cast<char*>(shmat(shmId_create, NULL, 0));
         if (sharedData_create == reinterpret_cast<char*>(-1)) {
             std::cerr << "Failed to attach shared memory segment.\n";
@@ -421,6 +435,8 @@ class Concore {
         try {
             if (shmId_create == -1)
                 throw 505;
+            if (sharedData_create == nullptr)
+                throw 506;
             val.insert(val.begin(), simtime + delta);
             std::ostringstream outfile;
             outfile << '[';