ELF and Dynamic Linking
Executable and Linkable Format (ELF) is the standard binary file format on Linux, most BSDs, and many embedded systems. It replaces older a.out and COFF formats.
| Aspect | Summary |
| ————————– | ——————————————————————————————————————————————————————————————————————————————————————————————————————————————————- |
| Purpose | Encapsulate everything the OS or linker needs to load an executable, shared library (.so), object file (.o), core dump, or kernel module. |
| Key parts (simplified) | ELF Header → tells the loader “I’m 64-bit little-endian, entry point is 0x401000, program-header table starts at …”
Program Header Table → runtime view: segments to map (text, data, dynamic info).
Section Header Table (absent in stripped binaries) → link-time view: named sections like .text, .rodata, .dynsym, .debug_info. |
| Dynamic linking | One segment (PT_DYNAMIC) lists required shared objects and relocation info; at runtime the dynamic loader (ld.so) walks this table to satisfy libm.so.6, libstdc++.so.6, etc. Tools such as ldd simply read that table and show you what the loader will do. |
| Extensibility | New segment or section types can be added without breaking older loaders; that’s how features like ASLR hints, GNU RELRO, and CET notes slid in over the years. |
| Contrast | Windows uses PE/COFF; macOS uses Mach-O. The roles are analogous but the layout and metadata differ. |
ELF is both the link-time and runtime container, it covers static executables, position-independent shared-libraries, tiny bootloaders, and even kernel images.
ldd program- reads ELF’s dynamic section to list library dependenciesreadelf -a fileorobjdump -p file- dumps headers symbol tables, relocation entries
ldd - Inspect Shared-Library Dependencies
ldd some_executable asks the dynamic linker to resolve each shared object name (they are loaded during runtime), and prints libsomething.so.1 => /actual/path/libsomething.so.1 (0xaddress)
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└─ $ ldd ./lib/libbehavior_executor.so
linux-vdso.so.1 (0x0000728663d27000)
libdiagnostic_msgs__rosidl_typesupport_cpp.so => /opt/ros/humble/lib/libdiagnostic_msgs__rosidl_typesupport_cpp.so (0x0000728663aaa000)
