Segmentation fault (core dumped)

Segmentation fault (core dumped) means your program tried to access memory it does not have permission to access. The OS memory protection unit killed the process.

What It Means

The CPU triggered a segmentation fault when your program attempted to read from or write to an invalid memory address. The “core dumped” part means the OS saved a core dump file for debugging.

Why It Happens

• Dereferencing a NULL or uninitialized pointer
• Writing past the end of an array (buffer overflow)
• Using a dangling pointer that points to freed memory
• Recursive function calls causing stack overflow
• Modifying a string literal (which is read-only)

How to Fix It

Step 1: Check for NULL pointer dereference

// bad.cpp
int* ptr = nullptr;
*ptr = 42; // segmentation fault!

// fixed.cpp
int* ptr = nullptr;
if (ptr != nullptr) {
    *ptr = 42;
} else {
    ptr = new int(42);
}

Step 2: Avoid buffer overflows

// bad.cpp
int arr[5];
for (int i = 0; i <= 5; ++i) { // off-by-one: writes past the end
    arr[i] = i * 10;
}

// fixed.cpp
int arr[5];
for (int i = 0; i < 5; ++i) {
    arr[i] = i * 10;
}

Step 3: Fix dangling pointers

// bad.cpp
int* ptr = new int(42);
delete ptr;
*ptr = 100; // dangling pointer — undefined behavior

// fixed.cpp
int* ptr = new int(42);
delete ptr;
ptr = nullptr; // good practice
// Do not use ptr again without reassigning

Step 4: Debug with GDB

# Compile with debug symbols
g++ -g crash.cpp -o crash
# Run with GDB
gdb ./crash
(gdb) run
(gdb) bt      # backtrace — shows the call stack
(gdb) info locals  # show local variable values
(gdb) list    # show source code around the crash

Step 5: Detect memory errors with Valgrind

g++ -g crash.cpp -o crash
valgrind --leak-check=full ./crash

Valgrind reports invalid reads/writes, use-after-free, and memory leaks with exact line numbers.

Step 6: Use address sanitizer

g++ -g -fsanitize=address crash.cpp -o crash
./crash

The address sanitizer catches buffer overflows, use-after-free, and memory leaks at runtime with detailed diagnostics.