Generating Programs and Linking

Professor Rick Han

Department of Computer Science

University of Colorado at Boulder

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

System Libraries and Tools(Compilers, Shells, GUIs)

Operating System Architecture

App3

DiskMemoryCPU Display Mouse

App2App1

I/O

Scheduler VMFile

System OS

“Kernel”

Posix, Win32,Java, C library APISystem call API

DeviceManager

What is an Application?

• 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

Program P1

CPU

ProgramCounter (PC)

Registers

ALU

Fetch Codeand Data

Write Data

Code

Data

MainMemory

OS Loader

ProgramP1

binary

Loading and Executing a Program

Code

Data

Code

Data

P1binary

P2binary

Disk

Loading and Executing a Program

Code

Data

Code

Data

Code

Data

P1binary

P2binary

Disk

MainMemory

OS Loader

ProgramP1

binary

shift left by 2 register R1and put in address A

invoke low level systemcall n to OS: syscall n

jump to address B

Machine Code instructionsof binary executable

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

Code

Program P1’sBinary

Executable

SourceCode

Compiler

file P1.c

Assembler Linker

Data

gcc can generate any of these stages

P1.s P1.o

technically, there is a preprocessing step before the compiler.“gcc -c” will generate relocatable object files, and not run linker

Linking Multiple Object Files Into an Executable

• 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

Code

P1 or P1.exe

SourceCode

Compilercc1

file P1.c

Assembleras

Linkerld

Data

P1.s P1.o

foo2.o

foo3.o

Linking Multiple Object Files Into an Executable

• 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

Code

P1 or P1.exe

SourceCode

Compilercc1

file P1.c

Assembleras

Linkerld

Data

P1.s P1.o

foo2.o

foo3.o

Linker Resolves Unknown SymbolsP1.c

int globalvar1=0;

main(...) { ----- f1(...) -----}

foo2.c

void f1(...) { ----}

void f2(...) { ---- globalvar1 = 4; ----}

extern void f1(...); extern int globalvar1;

P1.o

the P1.o object file will contain a list ofunknown symbols, e.g. f1, in a symbol table

foo2.o

foo2.o’s symbol table listsunknown symbols, e.g. globalvar1

Linker Resolves Unknown Symbols

• 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

ELF relocatable object file

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

Linker Resolves Unknown Symbols

extern float f1();

int globalvar1=0;

void f2(...) {

static int x=-1; ----- }

global symbols defined here

global symbol referenced herebut defined elsewhere

“local” symbol

• 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

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

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

Code

Data

.symtab

P1.o relocatableobject file

Code

Data

.symtab

P2.o

Code

Data

.symtab

P3.o

function f1() in P1.ois referenced butnot defined, henceunknown

definedin P2?

No defined inP3?

Yes

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

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

Linker Resolves Unknown Symbols

• 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

libfoo.a

foo1.o

foo2.o

foo3.o

foo4.o

Linker Resolves Unknown Symbols

• Linker scans object files and libraries sequentially left 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!

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’

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

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

Linked 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 headersegment header table

.init.text

.rodata.data.bss

.symtab.debug

.line.strtab

Section header table

ELF executable object file

Loading Executable Object Files

• 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

Heap

Read/write .data, .bss

Read-only .init, .text, .rodata

Unallocated

Run-time memory