C++ Program Loading

Program Loading & Execution

When you run a C++ program, the OS performs complex steps to transform the executable file into a running process.

Memory Layout

Once loaded, your program has a well-defined memory structure:

Loading Steps

1. Read ELF headers

   • Validate executable format
   • Check architecture compatibility
   • Find entry point

2. Create process

   • Allocate Process Control Block (PCB)
   • Assign Process ID (PID)
   • Create virtual address space

3. Map segments

   • Map code (.text) as read-execute
   • Map data (.data, .bss) as read-write
   • Set up memory protections

4. Load dynamic linker

   • If dynamically linked, load ld.so
   • Resolve library dependencies
   • Perform relocations

5. Initialize runtime

   • Set up stack and heap
   • Run global constructors
   • Initialize Thread Local Storage

6. Transfer control

   • Jump to _start (not main!)
   • C runtime setup
   • Finally call main()

Memory Regions

Stack & Heap

• Stack: Local variables, function calls
• Heap: Dynamic allocations

Code & Data

• .text: Executable instructions
• .data: Initialized globals
• .bss: Uninitialized globals
• .rodata: Constants

Memory Mapping

• Shared libraries
• Memory-mapped files
• Thread stacks

Before main()

Your code doesn't run first:

// Global constructors run before main
class Global {
    Global() { 
        std::cout << "Before main!\n"; 
    }
} g;

int main() {
    std::cout << "In main\n";
    return 0;
}

// Output:
// Before main!
// In main

Runtime Environment

Process Information

# View process memory
cat /proc/self/maps

# Check memory usage
pmap <pid>

# Monitor runtime
strace ./program

Environment Variables

# Dynamic linker paths
LD_LIBRARY_PATH=/custom/lib

# Preload libraries
LD_PRELOAD=./hook.so

# Debug loading
LD_DEBUG=all ./program

Loading Topics

Explore specific aspects:

• 📊 Stack vs Heap - Memory allocation strategies
• 🔄 Dynamic Loading - Runtime library loading
• 🗺️ Virtual Memory - Address translation

Related Topics

The complete journey:

1. Compilation - Source to object files
2. Linking - Object files to executable
3. Loading - Executable to process (you are here)

Advanced Topics

• Smart Pointers - Modern memory management
• RAII - Resource management
• Thread Safety - Concurrent execution