Dynamic linker

Dynamic Linker

The dynamic linker is a key component of an operating system's runtime environment, responsible for loading and linking shared libraries into a program at runtime, rather than at compile time. This process allows for the efficient use of memory and reduces the size of executables by sharing common library code among multiple programs.

Overview

Dynamic linking involves the resolution of symbols - such as functions and variables - that are referenced by an executable but defined in a separate shared library. The dynamic linker defers the resolution of these symbols until a program is executed, which contrasts with static linking, where all symbols are resolved at compile time and the libraries are copied into the executable.

Operation

When a program starts, the dynamic linker is invoked by the operating system. It reads the program's header to determine which shared libraries are needed. The linker then loads these libraries into memory, if they are not already loaded, and resolves the symbols by matching them with the correct addresses of functions and variables in the shared libraries.

Benefits

• Efficiency: Since shared libraries are loaded into memory only once, multiple programs can use the library code without the need for redundant copies.
• Flexibility: Libraries can be updated without the need to recompile dependent programs, as long as the interface of the library remains compatible.
• Memory Savings: Dynamic linking reduces the overall memory footprint of programs by sharing common library code.

Challenges

• Startup Time: Programs may take longer to start due to the overhead of loading and linking the required libraries at runtime.
• Complexity: Managing dependencies and ensuring compatibility between different library versions can be complex.
• Security: Dynamic linking introduces potential security vulnerabilities, such as dependency hijacking or the use of outdated libraries with known vulnerabilities.

Platforms

Different operating systems implement dynamic linking through various mechanisms. For example, in UNIX-like systems, the dynamic linker is often called ld-linux.so (on Linux), while Windows uses the Portable Executable (PE) format and a dynamic-link library (DLL) mechanism.

See Also

• Static Linking
• Shared Library
• Executable and Linkable Format (ELF)
• Portable Executable (PE)
• Library (computing)