Lecture 4

Libraries and binary file format

Lecture

Slides (PDF, PPTX).

Outline

• Linking
• Relocatable object files
• Executable and Linkable Format (ELF)
• Shared libraries
• Linked libraries
• Library interpositioning

Tools for Manipulating Object Files

• ar - creates static libraries, and inserts, deletes, lists, and extracts members.
• strings - lists all of the printable strings contained in an object file.
• strip - deletes symbol table information from an object file.
• nm - lists the symbols defined in the symbol table of an object file.
• size - lists the names and sizes of the sections in an object file.
• readelf - displays the complete structure of an object file, including all of the information encoded in the ELF header. Subsumes the functionality of size and nm.
• objdump - the mother of all binary tools. Can display all of the information in an object file. Its most useful function is disassembling the binary instructions in the .text section.
• ldd - lists the shared libraries that an executable needs at run time.

Library Interpositioning

Examples:

• Compile-time
• Link-time
• Load-time/run-time

Workshop

Outline

• Creating static libraries
• Creating shared librarires
• Library interpositioning
• Make scripts

Practice

• Part 1.
• Part 2.

Homework

1. Improve Task 1 from Part 1:
   • Provide link-time interpositioning for the fred and john functions
   • Provide load-time interpositioning for the bill and sam functions
   • Create a Make script to build the program

Note: interposed functions can just print a message like “fred is called”.

References

• Linking. Chapter 7 in [CSPP].
• Section 2.6. in [OSC].
• Chapters 41 and 42 in [TLPI].