Linking Summer 2014 COMP 2130 Intro Computer Systems Computing Science Thompson Rivers University

Linking

Summer 2014

COMP 2130 Intro Computer Systems

Computing ScienceThompson Rivers University

TRU-COMP2130 Linking 2

Course Objectives

The better knowledge of computer systems, the better programing.Computer System C Programming Language

Computer architectureCPU (Central Processing Unit)IA32 assembly language

Introduction to C language

Compiling, linking, loading, executing

Physical main memoryMMU (Memory Management Unit)

Virtual memory space

Memory hierarchyCache

Dynamic memory management

Better coding – locality

Reliable and efficient programming for power programmers(to avoid strange errors, to optimize codes, to avoid security holes, …)

Your vision? Seek with all your heart?


Course Contents

Introduction to computer systems: B&O 1 Introduction to C programming: K&R 1 – 4 Data representations: B&O 2.1 – 2.4 C: advanced topics: K&R 5.1 – 5.10, 6 – 7 Introduction to IA32 (Intel Architecture 32): B&O 3.1 – 3.8, 3.13 Compiling, linking, loading, and executing: B&O 7 (except 7.12) Dynamic memory management – Heap: B&O 9.9.1 – 9.9.2, 9.9.4 –

9.9.5, 9.11 Code optimization: B&O 5.1 – 5.6, 5.13 Memory hierarchy, locality, caching: B&O 5.12, 6.1 – 6.3, 6.4.1 –

6.4.2, 6.5, 6.6.2 – 6.6.3, 6.7 Virtual memory (if time permits): B&O 9.4 – 9.5



Unit Learning Objectives

List two jobs that linkers do. Explain what “relocatable code” means. Compare the three types of object files. Explain the purposes of .text, .rodata, .data, and .bss sections. Give examples of the three types of symbols. Compare the two types of libraries. List the advantages of using shared libraries.



Unit Contents

1. Compiler Drivers

2. Static Linking

3. Object Files

4. Relocatable Object Files

5. Symbols and Symbol Tables

6. Static Libraries

7. Loading Executable Object Files

8. Shared Libraries



Linking

Understanding linkers will help you build large programs. Understanding linkers will help you avoid dangerous programming

errors. Understanding linking will help you understand how language scoping

rules are implemented. Understanding linking will help you understand other important

systems concepts. Understanding linking will enable you to exploit shared libraries.



1. Compiler Drivers

A compiler driver invokes the language preprocessor, compiler, assembler, and linker.

the name of the compiler driver on cs.tru.ca - gcc


Example C Program

int buf[2] = {1, 2}; int main() { swap(); return 0;}

main.c swap.c

extern int buf[]; int *bufp0 = &buf[0];static int *bufp1;

void swap(){ int temp;

bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp;}

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Static Linking Programs are separately translated, and linked using a compiler driver:

$ gcc -c link1.c $ gcc -c header.c $ gcc -o out link1.o header.o ./out

Linker (ld)

Translators(cpp, cc1, as)

main.c

main.o


swap.c

swap.o

p

Source files

Separately compiled relocatable object files

Fully linked executable object file(contains code and data for all functions

defined in main.c and swap.c)

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2. Static Linking

Here, all code is contained in a single executable module. Library references are more efficient because the library procedures

are statically linked into the program. Static linking increases the file size of your program, and it may

increase the code size in memory if other applications, or other copies of your application, are running on the system.


Why Linkers? Reason 1: Modularity

Program can be written as a collection of smaller source files, rather than one monolithic mass.

Can build libraries of common functions (more on this later) e.g., Math library, standard C library

Reason 2: Efficiency Time: Separate compilation

Change one source file, compile, and then relink. No need to recompile other source files.

Space: Libraries Common functions can be aggregated into a single file... Yet executable files and running memory images contain only code for the

functions they actually use.

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What Do Linkers Do? Step 1. Symbol resolution

Programs define and reference symbols (variables and functions): void swap() {…} /* define symbol swap */ swap(); /* reference symbol swap */ int *xp = &x; /* define symbol xp, reference x */

Symbol definitions are stored (by compiler) in symbol table in the .o files. Symbol table is an array of structs. Each entry includes name, size, and relative location of symbol.

Linker associates each symbol reference with exactly one symbol definition. Symbols will be replaced with their relative locations in the .o files.

Step 2. Relocation Merges separate code and data sections in the .o files into single sections in the

a.out file. Relocates symbols from their relative locations in the .o files to their final absolute

memory locations in the executable. Updates all references to these symbols to reflect their new positions.

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3. Object Files

Relocatable object file (.o file) Contains code and data in a form that can be combined with other

relocatable object files to form executable object file. The symbols in each .o file use relative addresses. Not executable yet Each .o file is produced from exactly one source (.c) file. E.g., $ gcc –c swap.c

Executable object file (a.out file) Contains code and data in a form that can be copied directly into memory

and then executed. Instructions use absolute addresses.

Shared object file (.so file) Special type of relocatable object file that can be loaded into memory and

linked dynamically, at either load time or run-time. Called Dynamic Link Libraries (DLLs) by Windows


Information

An object file may contain five kinds of information. Header information: overall information about the file, such as the size of the code, name of the source file it was translated from, and creation date. Object code: Binary instructions and data generated by a compiler or assembler. Relocation: A list of the places in the object code that have to be fixed up when the linker changes the addresses of the object code. Symbols: Global symbols defined in this module, symbols to be imported from other modules or defined by the linker. Debugging information: Other information about the object code not needed for linking but of use to a debugger. This includes source file and line number information, local symbols, descriptions of data structures used by the object code such as C structure definitions.

TRU-COMP3710 Introduction 14


4. Relocatable Object FilesYour vision? Seek with all your heart?

Standard binary format for object files Originally proposed by AT&T System V Unix

Later adopted by BSD Unix variants and Linux ELF is flexible and extensible, and it is not bound to any particular

processor or architecture. This has allowed it to be adopted by many different operating systems on many different platforms.

One unified format for Relocatable object files (.o), Executable object files (a.out) Shared object files (.so)

Generic name: ELF binaries

Carnegie MellonExecutable and Linkable Format (ELF)

ELF header Information to parse and interpret the object

file: Word size, byte ordering, file type (.o, exec, .so), machine type, etc.

Segment header table For run time execution: Page size, virtual

addresses memory segments (sections), segment sizes.

.text section Instruction code

.rodata section Read only data: jump tables, ...

.data section Initialized global variables

.bss section Uninitialized global variables Has section header but occupies no space

ELF header

Segment header table(required for executables)

.text section

.rodata section

.bss section

.symtab section

.rel.txt section

.rel.data section

.debug section

Section header table

0

.data section

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.symtab section Symbol table Procedure and static variable names Section names and locations -> used by linker

for code relocation .rel.text section

Relocation info for .text section Addresses of instructions that will need to be

modified in the executable Instructions for modifying.

.rel.data section Relocation info for .data section Addresses of pointer data that will need to be

modified in the merged executable .debug section

Info for symbolic debugging (gcc -g) Section header table

For linking and relocation: Offsets and sizes of each section

ELF header

Segment header table(required for executables)

.text section

.rodata section

.bss section

.symtab section

.rel.txt section

.rel.data section

.debug section

Section header table

0

.data section

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Types of ELF files

ELF files come in three slightly different flavors: relocatable, executable, and shared object.

Relocatable files are created by compilers and assemblers but need to be processed by the linker before running.

Executable files have all relocation done and all symbols resolved except perhaps shared library symbols to be resolved at runtime.

Shared objects are shared libraries, containing both symbol information for the linker and directly runnable code for runtime.

TRU-COMP3710 Introduction 19


5. Symbols and Symbol Tables

Global symbols Symbols defined by module m that can be referenced by other modules. E.g.: non-static C functions and non-static global variables. (Note

that static C functions and static global variables cannot be referred from other files.)

External symbols Global symbols that are referenced by module m but defined by some

other module.

Local symbols Symbols that are defined and referenced exclusively by module m. E.g.: C functions and variables defined with the static attribute. Local linker symbols are not local program variables.


Resolving Symbols

int buf[2] = {1, 2}; int main() { swap(); return 0;} main.c

extern int buf[]; int *bufp0 = &buf[0];static int *bufp1;

void swap(){ int temp;

bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp;} swap.c

Global

External

External Local

Global

Linker knowsnothing of temp

Global

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extern void swap(); is not in main.c.[Q] Is it okay?


7. RelocationYour vision? Seek with all your heart?

Relocating Code and Data

main()

main.o

int *bufp0=&buf[0]

swap()

swap.o int buf[2]={1,2}

Headers

main()

swap()

0System code

int *bufp0=&buf[0]

int buf[2]={1,2}

System data

More system code

System data

Relocatable Object Files Executable Object File

.text

.text

.data

.text

.data

.text

.data .symtab.debug

.data

int *bufp1 .bss

System code

static int *bufp1 .bss

Even though private to swap, requires allocation in .bssbecause of lifespan

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6.2-3 Linking with Static Libraries


Packaging Commonly Used Functions

How to package functions commonly used by programmers? Math, I/O, memory management, string manipulation, etc.

Awkward, given the linker framework so far: Option 1: Put all functions into a single source file

Programmers link big object file into their programs Space and time inefficient

Option 2: Put each function in a separate source file Programmers explicitly link appropriate binaries into their programs More efficient, but burdensome on the programmer

Types of libraries: Static libraries Shared libraries

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Solution: Static Libraries Static libraries (.a archive files)

Concatenate related relocatable object files into a single file with an index (called an archive).

Enhance linker so that it tries to resolve unresolved external references by looking for the symbols in one or more archives.

If an archive member file resolves reference, link it into the executable.

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Creating Static Libraries

Translator

atoi.c

atoi.o

Translator

printf.c

printf.o

libc.a

Archiver (ar)

... Translator

random.c

random.o

unix> ar rs libc.a \ atoi.o printf.o … random.o

C standard library

Archiver allows incremental updates Recompile function that changes, and replace .o file in archive.

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Commonly Used Librarieslibc.a (the C standard library)

8 MB archive of 1392 object files. I/O, memory allocation, signal handling, string handling, data and time, random

numbers, integer math

libm.a (the C math library) 1 MB archive of 401 object files. floating point math (sin, cos, tan, log, exp, sqrt, …)

% ar -t /usr/lib/libc.a | sort …fork.o … fprintf.o fpu_control.o fputc.o freopen.o fscanf.o fseek.o fstab.o …

% ar -t /usr/lib/libm.a | sort …e_acos.o e_acosf.o e_acosh.o e_acoshf.o e_acoshl.o e_acosl.o e_asin.o e_asinf.o e_asinl.o …

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Linking with Static Libraries


main2.c

main2.o

libc.a

Linker (ld)

p2

printf.o and any other modules called by printf.o

libvector.a

addvec.o

Static libraries

Relocatableobject files

Fully linked executable object file

vector.h Archiver(ar)

addvec.o multvec.o

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Using Static Libraries Linker’s algorithm for resolving external references:

Scan .o files and .a files in the command line order. During the scan, keep a list of the current unresolved references. As each new .o or .a file, obj, is encountered, try to resolve each unresolved

reference in the list against the symbols defined in obj. If any entries in the unresolved list at end of scan, then error.

Problem: Command line order matters! Moral: put libraries at the end of the command line.

unix> gcc libtest.o -L. -lmine unix> gcc -L. -lmine libtest.o libtest.o: In function `main': libtest.o(.text+0x4): undefined reference to `libfun'

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9. Loading Executable Object Files


Loading Executable Object Files

ELF header

Program header table(required for executables)

.text section

.data section

.bss section

.symtab

.debug

Section header table(required for relocatables)

0Executable Object File Kernel virtual memory

Memory-mapped region forshared libraries

Run-time heap(created by malloc)

User stack(created at runtime)

Unused0

%esp (stack

pointer)

MemoryInvisible to user code

brk

Read/write segment(.data, .bss)

Read-only segment(.init, .text, .rodata)

Loaded from the

executable file

.rodata section

.line

.init section

.strtab

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Loaded into


10. Shared LibrariesYour vision? Seek with all your heart?

Shared Libraries Static libraries have the following disadvantages:

Duplication in the stored executables (every function need std libc) Duplication in the running executables Minor bug fixes of system libraries require each application to explicitly relink

Modern solution: Shared Libraries Object files that contain code and data that are loaded and linked into an

application dynamically, at either load-time or run-time Also called: dynamic link libraries, DLLs, .so files Shared library routines can be shared by multiple processes. Do executable files contain the code in shared libraries?

No. Dynamic linking – The symbols for the code in shared libraries will be resolved with absolute addresses at either load-time or run-time.

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Shared Libraries (cont.) Dynamic linking can occur when executable is first loaded and run (load-

time linking). Common case for Linux, handled automatically by the dynamic linker (ld-

linux.so). Standard C library (libc.so) usually dynamically linked.

Dynamic linking can also occur after program has begun (run-time linking).

The executable is not linked to a shared library at load-time, so it will not contain any stubs into the shared library.

In Linux, this is done by calls to the dlopen() interface. Distributing software. High-performance web servers. Runtime library inter-positioning.

Shared library routines can be shared by multiple processes.

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Dynamic Linking at Load-time

Translators (cpp, cc1, as)

main2.c

main2.o

libc.solibvector.so

Linker (ld)

p2

Dynamic linker (ld-linux.so)

Relocation and symbol table info

libc.solibvector.so

Code and data

Partially linked executable object file

Relocatableobject file

Fully linked executablein memory

vector.h

Loader (execve)

unix> gcc -shared -o libvector.so \ addvec.c multvec.c

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Dynamic Linking at Run-time#include <stdio.h>#include <dlfcn.h>

int x[2] = {1, 2};int y[2] = {3, 4};int z[2];

int main() { void *handle; void (*addvec)(int *, int *, int *, int); // [Q] ??? char *error;

/* dynamically load the shared lib that contains addvec() */ handle = dlopen("./libvector.so", RTLD_LAZY); // lazy binding if (!handle) {

fprintf(stderr, "%s\n", dlerror());exit(1);

}

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Dynamic Linking at Run-time ...

/* get a pointer to the addvec() function we just loaded */ addvec = dlsym(handle, "addvec"); if ((error = dlerror()) != NULL) {

fprintf(stderr, "%s\n", error);exit(1);

}

/* Now we can call addvec() just like any other function */ addvec(x, y, z, 2); printf("z = [%d %d]\n", z[0], z[1]);

/* unload the shared library */ if (dlclose(handle) < 0) {

fprintf(stderr, "%s\n", dlerror());exit(1);

} return 0;}

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Documents

Linking Summer 2014 COMP 2130 Intro Computer Systems Computing Science Thompson Rivers University