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08 Libraries and Engineering Practice

1. Static Libraries (.a / .lib)

Definition

A static library (typically .a on Linux, .lib on Windows) is an archive file that bundles multiple relocatable object files (.o or .obj) into a single file. It acts like a "container" or "storage bag" for .o files.

Creation (using ar on Linux)

The ar (archiver) utility is used to create and manage static libraries.

# Compile source files into object files
gcc -c add.c -o add.o
gcc -c sub.c -o sub.o

# Create the static library libmymath.a from object files
ar rcs libmymath.a add.o sub.o
  • r: Replace or insert files.
  • c: Create the archive.
  • s: Create a symbol table index (essential for linker efficiency).

Linker's Usage: "Extract-on-Demand"

When linking with a static library, the linker does not copy the entire library into your executable. 1. The linker consults the library's symbol index. 2. It identifies only the .o files within the .a archive that define the currently undefined symbols needed by your program. 3. Only those specific .o files are extracted and copied into your final executable. Unused .o files within the library are ignored.

Advantages

  • Ease of Distribution: Provide one .a file and one .h file.
  • Self-Contained Executables: The final program has no runtime dependencies on external .a files. It can be run on any compatible system without needing the library present.
  • Modular Compilation: Changes to one source file in the library only require recompiling that specific .o and updating the .a file.

2. Dynamic Libraries (.so / .dll)

Definition

A dynamic library (typically .so on Linux, .dll on Windows) is a file containing object code that multiple programs can use simultaneously at runtime.

Linker's Usage: "Deferred Resolution"

When linking with a dynamic library, the linker does not copy the code. 1. It resolves symbols by noting that they are defined in a specific dynamic library. 2. The executable records which dynamic libraries it needs. 3. At runtime, the operating system's dynamic loader (e.g., ld-linux.so) loads the actual library code into memory and patches the program's addresses.

Advantages

  • Reduced Executable Size: Only references to the library are stored.
  • Memory Efficiency: Multiple programs can share a single copy of the library in memory.
  • Easier Updates: Update the library, and all programs using it automatically get the new version without recompilation.

3. Specifying and Finding Libraries

When running gcc to create the executable. 1. -l<name> (Lowercase L): Specifies the library by name (e.g., -lm for libm.so or libm.a). * The linker automatically prepends lib and appends .a or .so. 2. -L<path> (Uppercase L): Specifies additional directories where the linker should search for libraries (e.g., -L/home/user/my_libs). 3. LIBRARY_PATH Environment Variable: A colon-separated list of directories for the linker to search (used by ld). 4. System Default Paths: Hardcoded paths like /lib, /usr/lib, /usr/local/lib.

When running the compiled executable. 1. RPATH / RUNPATH: Paths hardcoded into the executable during compilation using flags like -Wl,-rpath=/my/app/lib. (Highest priority). 2. LD_LIBRARY_PATH Environment Variable: A colon-separated list of directories for the dynamic loader to search. (Useful for testing, but can cause system instability if set globally). 3. /etc/ld.so.cache: A cache of library locations built from /etc/ld.so.conf (and /etc/ld.so.conf.d/). Updated by sudo ldconfig. 4. System Default Paths: /lib, /usr/lib.

The linker processes files from left to right. * Correct Order: gcc my_program.c -lmy_library * my_program.c (caller) \(\to\) libmy_library (callee). * Incorrect Order (often leads to undefined reference): gcc -lmy_library my_program.c * The linker sees libmy_library first, may discard it if no immediate need, then finds my_program.c needing symbols from it.

Headaches with Debugging Linker Errors

  • undefined reference to 'symbol': The linker couldn't find the definition. Check -l flags, -L paths, and the linking order.
  • error while loading shared libraries: libxxx.so: cannot open...: Runtime error. The dynamic loader can't find the library. Check LD_LIBRARY_PATH, RPATH, or system installation.
  • ldd my_program: Use this command to see which dynamic libraries an executable depends on and where they are located at runtime.