Generating Programs and Linking

1. Generating Programs and Linking Professor Rick Han Department of Computer Science University of Colorado at Boulder

2. CSCI 3753 Announcements • Moodle - posted last Thursday’s lecture • Programming shell assignment 0 due Thursday at 11:55 pm, not 11 am • Introduction to Operating Systems • Read Chapters 3 and 4 in the textbook

3. Posix, Win32, Java, C libraryAPI System call API Device Manager Operating System Architecture App2 App1 App3 System Libraries and Tools (Compilers, Shells, GUIs) OS “Kernel” Scheduler VM File System CPU Memory Disk Display Mouse I/O

4. What is an Application? Program P1 • A software program consist of a sequence of code instructions and data • for now, let a simple app = a program • Computer executes the instructions line by line • code instructions operate on data Code Data

5. OS Loader Main Memory Disk Fetch Code and Data CPU P1 binary P2 binary Program P1 binary Program Counter (PC) Code Code Code Registers ALU Data Data Data Write Data Loading and Executing a Program

6. Machine Code instructions of binary executable Disk P1 binary P2 binary shift left by 2 register R1 and put in address A Code Code Code invoke low level system call n to OS: syscall n jump to address B Data Data Data Loading and Executing a Program OS Loader Main Memory Program P1 binary

7. gcc can generate any of these stages Code Generating a Program’s Binary Executable • We program source code in a high-level language like C or Java, and use tools like compilers to create a program’s binary executable Program P1’s Binary Executable file P1.c Source Code Compiler Assembler Linker P1.s P1.o Data technically, there is a preprocessing step before the compiler. “gcc -c” will generate relocatable object files, and not run linker

8. Code Linking Multiple Object Files Into an Executable P1 or P1.exe file P1.c • linker combines multiple .o object files into one binary executable file • why split a program into multiple objects and then relink them? • breaking up a program into multiple files, and compiling them separately, reduces amount of recompilation if a single file is edited • don’t have to recompile entire program, just the object file of the changed source file, then relink object files foo2.o Source Code Compiler cc1 Assembler as Linker ld P1.s P1.o Data foo3.o

9. Code Linking Multiple Object Files Into an Executable P1 or P1.exe file P1.c • in combining multiple object files, the linker must • resolve references to variables and functions defined in other object files - this is called symbol resolution • relocate each object’s internal addresses so that the executable’s combination of objects is consistent in its memory references • an object’s code and data are compiled in its own private world to start at address zero foo2.o Source Code Compiler cc1 Assembler as Linker ld P1.s P1.o Data foo3.o

10. extern void f1(...); extern int globalvar1; P1.o foo2.o the P1.o object file will contain a list of unknown symbols, e.g. f1, in a symbol table foo2.o’s symbol table lists unknown symbols, e.g. globalvar1 Linker Resolves Unknown Symbols P1.c int globalvar1=0; main(...) { ----- f1(...) ----- } foo2.c void f1(...) { ---- } void f2(...) { ---- globalvar1 = 4; ---- }

11. Linker Resolves Unknown Symbols ELF relocatable object file • ELF relocatable object file contains following sections: • ELF header (type, size, size/# sections) • code (.text) • data (.data, .bss, .rodata) • .data = initialized global variables • .bss = uninitialized global variables (does not actually occupy space on disk, just a placeholder) • symbol table (.symtab) • relocation info (.rel.text, .rel.data) • debug symbol table (.debug only if “-g” compile flag used) • line info (map C & .text line #s only if “-g”) • string table (for symbol tables) ELF header .text .rodata .data .bss .symtab .rel.text .rel.data .debug .line .strtab Section header table

12. Linker Resolves Unknown Symbols • Symbol table contains 3 types of symbols: • global symbols - defined in this object • global symbols referenced but not defined here • local symbols defined and referenced exclusively by this object, e.g. static global variables and functions • local symbols are not equivalent to local variables, which get allocated on the stack at run time

13. Linker Resolves Unknown Symbols global symbol referenced here but defined elsewhere • The symbol table informs the Linker where symbols referenced or referenceable by each object file can be found: • if another file references globalvar1, then look here for info • if this file reference f2, then another object file’s symbol table will mention f2 extern float f1(); int globalvar1=0; void f2(...) { static int x=-1; ----- } global symbols defined here “local” symbol

14. Linker Resolves Unknown Symbols • Each entry in the ELF symbol table looks like: typedef struct { int name; /* string table offset */ int value; /* section offset or VM address */ int size; /* object size in bytes */ char type:4, /* data, func, section or src file name (4 bits) */ binding:4;/* local or global (4 bits) */ char reserved; /* unused */ char section; /* section header index, ABS, UNDEF, */ } ELF_Symbol; here’s where we flag the undefined status

15. P1.o relocatableobject file P2.o P3.o Code Code Code Data Data Data defined in P2? defined in P3? No function f1() in P1.o is referenced but not defined, hence unknown .symtab .symtab .symtab Yes Linker Resolves Unknown Symbols • During linking, the linker goes through each input object file and determines if unknown symbols are defined in other object files Linker

16. Linker Resolves Unknown Symbols • What if two object files use the same name for a global variable? • Linker resolves multiply defined global symbols • functions and initialized global variables are defined as strong symbols, while uninitialized global variables are weak symbols Rule 1: multiple strong symbols are not allowed Rule 2: choose the strong symbol over the weak symbol Rule 3: given multiple weak symbols, choose any one

17. Linker Resolves Unknown Symbols • Linking with static libraries • Bundle together many related .o files together into a single file called a library or .a file • e.g. the C library libc.a contains printf(), strcpy(), random(), atoi(), etc. • library is created using the archive ar tool • the library is input to the linker as one file • linker can accept multiple libraries • linker copies only those object modules in the library that are referenced by the application program • Example: gcc main.c /usr/lib/libm.a /usr/lib/libc.a

18. Linker Resolves Unknown Symbols libfoo.a • a static library is a collection of relocatable object modules • group together related object modules • within each object, can further group related functions • if an application links to libfoo.a, and only calls a function in foo3.o, then only foo3.o will be linked into the program foo1.o foo2.o foo3.o foo4.o

19. Linker Resolves Unknown Symbols • Linker scans object files and libraries sequentiallyleft to right on command line to resolve unknown symbols • for each input file on command line, linker • updates a list of defined symbols with object’s defined symbols • tries to resolve the undefined symbols (from object and from list of previously undefined symbols) with the list of previously defined symbols • carries over the list of defined and undefined symbols to next input object file • so linker looks for undefined symbols only after they’re undefined! • it doesn’t go back over the entire set of input files to resolve the unknown symbol • if an unknown symbol becomes referenced after it was defined, then linker won’t be able to resolve the symbol! • Thus, order on the command line is important - put libraries last!

20. Linker Resolves Unknown Symbols • Example: gcc libfoo.a main.c • main.c calls a function f1 defined in libfoo.a • scanning left to right, when linker hits libfoo.a, there are no unresolved symbols, so no object modules are copied • when linker hits main.c, f1 is unresolved and gets added to unresolved list • Since there are no more input files, the linker stops and generates a linking error: /tmp/something.o: In function ‘main’: /tmp/something.o: undefined reference to ‘f1’

21. Linker Resolves Unknown Symbols • Example: gcc main.c libfoo.a • main.c calls a function f1 defined in libfoo.a • scanning left to right, when linker hits main.c, it will add f1 to the list of unresolved references • when linker next hits libfoo.a, it will look for f1 in the library’s object modules, see that it is found, and add the object module to the linked program • No errors are generated. A binary executable is generated. • Lesson #1: the order of linking can be important, so put libraries at the end of command lines • Lesson #2: an undefined symbol error can also mean that you • didn’t link in the right libraries, didn’t add right library path • forgot to define the symbol somewhere in your code

22. Linker Relocates Addresses • After resolving symbols, the linker relocates addresses when combining the different object modules • merges separate code .text sections into a single .text section • merges separate .data sections into a single .data section • each section is assigned a memory address • then each symbol reference in the code and data sections is reassigned to the correct memory address • looks at .relo.text and .relo.data to find relocation entries of references that needed address translation • these are virtual memory addresses that are translated at load time into real run-time memory addresses

23. Linked ELF Executable Object File ELF executable object file • ELF executable object file contains following sections: • ELF header (type, size, size/# sections) • segment header table • .init (program’s entry point, i.e. address of first instruction) • other sections similar • Note the absence of .rel.tex and .rel.data - they’ve been relocated! • Ready to be loaded into memory and run • only sections through .bss are loaded into memory • .symtab and below are not loaded into memory • code section is read-only • .data and .bss are read/write ELF header segment header table .init .text .rodata .data .bss .symtab .debug .line .strtab Section header table

24. Loading Executable Object Files Run-time memory • Run-time memory image • Essentially code, data, stack, and heap • Code and data loaded from executable file • Stack grows downward, heap grows upward User stack Unallocated Heap Read/write .data, .bss Read-only .init, .text, .rodata