1 / 15

CSE 303 Concepts and Tools for Software Development

CSE 303 Concepts and Tools for Software Development. Richard C. Davis UW CSE – 12/1/2006 Lecture 22 – Linkers. Administravia. Any questions on HW7?. Library. Library. The Build Process. Source Files (.c, .cpp). Preprocessed Source Files. Object Files (.o). Executable. Pre-processor.

adennard
Download Presentation

CSE 303 Concepts and Tools for Software Development

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CSE 303Concepts and Tools for Software Development Richard C. DavisUW CSE – 12/1/2006 Lecture 22 – Linkers

  2. Administravia • Any questions on HW7? CSE 303 Lecture 22

  3. Library Library The Build Process Source Files (.c, .cpp) Preprocessed Source Files Object Files (.o) Executable Pre-processor Compiler Pre-processor Compiler Linker Pre-processor Compiler CSE 303 Lecture 22

  4. Library Library The Build Process Source Files (.c, .cpp) Preprocessed Source Files Object Files (.o) Executable Pre-processor Compiler Pre-processor Compiler Linker Pre-processor Compiler Today CSE 303 Lecture 22

  5. The Goal of the Linker • Compiled code (.o file) is not "runnable" • Link with other code to make executable • Where is the code for printf and new? • We only included the header files… • Need to find that code and put in executable • Normally gcc/g++ hides this from you • Use -c option to stop right before linking • We use this to produce .o files CSE 303 Lecture 22

  6. Linking Overview • C/C++ file uses undefined func./global var. • The .o file has "undefined references" • Note: declarations don't count, only definitions • Linker "patches" .o files to resolve refs. • Executable has no unresolved refs. • Ways to invoke linker • ld command • Implicitly through gcc/g++ (we'll do this) CSE 303 Lecture 22

  7. Static Linking • Static Linking: use option -static • Put all necessary code into executable • Example: "math" example program • Step 1: Compile source files • create Main.o • g++ -Wall -g -c Main.o • Step 2: Link files together • g++ -static -o math -L. Main.o -lpoly CSE 303 Lecture 22

  8. Creating a Static Library • Create with ar (stands for "archiver") • ar rc libpoly.a Polygon.o Point.o • Creates a static library named libpoly.a • Containing copies of the two object files • libpoly.a existsadds/replaces files inside • Index the archive: ranlib libpoly.a • Same as running ar with option -s • Performance during linking • Order inside the archive will no longer matter CSE 303 Lecture 22

  9. Other linker options g++ -static -o math -L. Main.o -lpoly -lpoly: links with libpoly.a -L: Specifies a directory containing libraries -v: See details • gcc/g++ automatically links executables with • libgcc.a • libc.a for C • libstdc++.a for C++ CSE 303 Lecture 22

  10. Static Linking Step-by-Step • Begin • UD Empty Set (No unresolved definitions yet) • Executable Empty (No code yet) • For each file : • .o file? Executable code • .a file? Executable code for needed definitions only • Fix references in Executable to funcs/objects defined in new file • Remove newly resolved funcs/objects from UD • Add any other unresolved funcs/objects from this file to UD • End: • UD empty? • Yes  output executable • No  error CSE 303 Lecture 22

  11. Consequences of Linking Process • Position of libs on command line matters • Discover and resolve references in order • So typically list libraries after object files • Example: switch -lpoint and -lpoly in math • Cycles • If two .a files need each other, you might need -lfoo -lbar -lfoo … CSE 303 Lecture 22

  12. Dynamic Linking • Static linking has disadvantages • More disk space • Copy portions of library for every application • More memory when programs are running • Instead, can do dynamic linking at runtime • Shared libraries (extension .so) • Saves disk space • OS can even share memory pages • Most linking is done this way now • Avoid using -static option CSE 303 Lecture 22

  13. Linking in Java • Java has the same problems a C/C++ • Must resolving undefined symbols • Java has a dynamic class loader • Loads each file when needed • From class path • From jar files • From the web • Very complicated system CSE 303 Lecture 22

  14. Summary • Main steps when building executable • Preprocessing (specific to C) • Compiling • Linking • Process gets complex for large systems • Automate the process with Makefiles • Know about potential problems • Learn how to solve as you encounter them CSE 303 Lecture 22

  15. Next Time • Readability and Robustness • STL? CSE 303 Lecture 22

More Related