CSU0027 - System Programming, NTNU CSIE1 / 99 Linkers and Libraries Advisor: Dr. Gwan-Hwan Hwang...

CSU0027 - System Programming, NTNU CSIE 1 / 99

Linkers and Libraries

Advisor: Dr. Gwan-Hwan Hwang

Lecturer: Che-Sheng Lin


Agenda

• Introduction• Static-Linking Libraries: Build &

Usage• Dynamic-Linking and ELF• Dynamic-Linking Libraries: Build

& Usage• Windows Platform Approach


Libraries

• A library is a collection of subprograms used to develop software.– Allows code and data to be reused,

shared and changed in a modular fashion.

– Linking: A linker resolves the references between executables and libraries.


Benefits of Using Libraries

• Software Engineering Perspective:– Increasing the reusability of common

routines.– Easy to upgrade by changing the

libraries only.• System Utilization Perspective:

– The code segment can be sharing at runtime; decrease the consume of memory and disk space.


Naming and Binding

• Name Binding: The association of values with identifiers.– An identifier bound to a value is said

to reference that value.– Reference Resolving: Retrieving the

value/address by a reference.– Note: Different to the reference

resolving, memory-mapping is retrieving the memory address by another memory address.


Linkers

• Module• Find the library routines and

determine the addresses at runtime.


Print Shared Library Dependencies

• UNIX Platform> ldd /bin/bash linux-gate.so.1 => (0xffffe000) libncurses.so.5 => /lib/libncurses.so.5 (0xb7f1d000) libdl.so.2 => /lib/libdl.so.2 (0xb7f19000) libc.so.6 => /lib/libc.so.6 (0xb7dfb000) /lib/ld-linux.so.2 (0xb7f61000)

• Windows Platform– PE Viewer or PE Explorer– PE: Portable Executable


Categories of Libraries(by linking time)

• Static linking libraries• Dynamic linking libraries

– Run-Time Environment libraries– Programming Language libraries


From Source to Execution

SourceProgram

Preprocessorcpp / gcc -E

Compilercc1 / gcc -S

AssemblyCode Assembler

as / gcc -c

ObjectProgram

Linkerld / gcc -l

ExecutablesLoader

static-linking

dynamic-

linking


Static Linking Libraries

• The code segments will be copy to each executables.

• Pros:– Easy to use; no dependency problem

after compilation.• Cons:

– The executable size will be larger.– Require re-linking when libraries

changed.


Dynamic Linking Libraries (1/2)

• Allow multiple processes to share the same code segment.

• Pros:– Greater flexibility– Possible support for plugins.

• Cons:– Slow application at start time.– Dependent on the libraries when

execution.


Dynamic Linking Libraries (2/2)

• The references can be resolved either at:– Load-time– Run-time

• UNIX Platform– “shared-object”: *.so

• Windows Platform– “dynamic-linking library”: *.dll


Location of Libraries

• UNIX Platform– /lib: runtime environment libraries– /usr/lib: for program development

• Windows Platform– C:\WINDOWS\system32\– The libraries for program

development will be accompanies with compiler, like: Visual C++.


Linking with C Runtime Libraries

• Static Linking– gcc –static –o hello-s helloworld.c

/usr/lib/libc.a– hello-s: 521,238 bytes

• Dynamic Linking– gcc –o hello-d helloworld.c– hello-d: 6,020 bytes

• The CRT libraries consume 515,218 bytes.


End of Introduction


Static-Linking Libraries

• Build– gcc –c mystaticlib.c– ar –r mystaticlib.a mystaticlib.o

• Usage– (same as using C runtime libraries)


GNU Binary Utilities

• strings: display all printable characters.

• ar: create static-linking libraries.• nm: list symbols from object files.• size: list section sizes and total sizes.• readelf:• objdump:


Unix a.out Format

• Hardware MMU is required.• No relocation at load time.• Separate address space for code

(I space) and data (D space), 64KB of each, in the later model of PDP-11 line.

• Since a program couldn’t change its own I space, multiple copies of a single program could share a single copy of a program’s code.– Sharing at the runtime.

a.out header

text section

data section

text relocation

data relocation

symbol table

string table

ˇ

ˇ


ELF: Executable and Linking Format

• To support cross-compilation, dynamic linking and other modern system features.

• ELF has an associated debugging format called DWARF.

• Three slightly different flavors: relocatable, executable, and shared object.

• Dual natures:– A set of logical sections described by a section

table. (for compilers, assemblers, and linkers)– A set of segments described by a program

header table. (for loaders)


Two Views of an ELF File

ELF HeaderProgram Header

Table

read/write

.got

.data

.bss

Linkable Sections

Executable

Segments

Section Header Table

.pltread only.text

… …sections

(describes sections)

(optional, ignored)

(optional, ignored)

(describes segments)

segments


ELF Sections

• .text: Equivalent to the a.out text segment.• .data: Equivalent to the a.out data

segment.• .bss: Takes no space in the file but is

allocated at runtime.• .init and .finit: Code to be executed when

the program starts up or terminates, respectively.– Useful for C++ which has global data with

initializers and finalizers.

• .symtab


ELF SectionsThere are 32 section headers, starting at offset 0xabc:

Section Headers: [Nr] Name Type Addr Off Size ES Flg Lk Inf Al [ 0] NULL 00000000 000000 000000 00 0 0 0 [ 1] .interp PROGBITS 08048134 000134 000013 00 A 0 0 1 [ 2] .note.ABI-tag NOTE 08048148 000148 000020 00 A 0 0 4 [ 3] .hash HASH 08048168 000168 00002c 04 A 4 0 4 [ 4] .dynsym DYNSYM 08048194 000194 000060 10 A 5 1 4 [ 5] .dynstr STRTAB 080481f4 0001f4 00005e 00 A 0 0 1 [ 8] .rel.dyn REL 08048280 000280 000008 08 A 4 0 4 [ 9] .rel.plt REL 08048288 000288 000018 08 A 4 11 4 [10] .init PROGBITS 080482a0 0002a0 000017 00 AX 0 0 4 [11] .plt PROGBITS 080482b8 0002b8 000040 04 AX 0 0 4 [12] .text PROGBITS 08048300 000300 000184 00 AX 0 0 16 [13] .fini PROGBITS 08048484 000484 00001c 00 AX 0 0 4 [14] .rodata PROGBITS 080484a0 0004a0 000015 00 A 0 0 4 [15] .eh_frame PROGBITS 080484b8 0004b8 000004 00 A 0 0 4 [16] .ctors PROGBITS 080494bc 0004bc 000008 00 WA 0 0 4 [17] .dtors PROGBITS 080494c4 0004c4 000008 00 WA 0 0 4 [18] .jcr PROGBITS 080494cc 0004cc 000004 00 WA 0 0 4 [19] .dynamic DYNAMIC 080494d0 0004d0 0000c8 08 WA 5 0 4 [20] .got PROGBITS 08049598 000598 000004 04 WA 0 0 4 [21] .got.plt PROGBITS 0804959c 00059c 000018 04 WA 0 0 4 [22] .data PROGBITS 080495b4 0005b4 00000c 00 WA 0 0 4 [23] .bss NOBITS 080495c0 0005c0 000004 00 WA 0 0 4 [29] .shstrtab STRTAB 00000000 0009ae 00010c 00 0 0 1 [30] .symtab SYMTAB 00000000 000fbc 0004e0 10 31 55 4 [31] .strtab STRTAB 00000000 00149c 0002e8 00 0 0 1


Dynamic Linking

• Static Shared Libraries• Dynamic Shared Libraries• (Propose the problems when using

dynamic linking.)• Shared code in the runtime• Virtual Addressing Space• Position-Independent Code

– cf: relocatable


Dynamic Linking Library:File Header (/lib/libc.so.6)

ELF Header: Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 Class: ELF32 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: DYN (Shared object file) Machine: Intel 80386 Version: 0x1 Entry point address: 0x159c0 Start of program headers: 52 (bytes into file) Start of section headers: 1194540 (bytes into file) Flags: 0x0 Size of this header: 52 (bytes) Size of program headers: 32 (bytes) Number of program headers: 11 Size of section headers: 40 (bytes) Number of section headers: 66 Section header string table index: 65


Position-Independent Code

• The text pages of the file need not be relocated and thus can be shared among multiple process.

• ELF linkers supports PIC with a global offset table (GOT) in each shared library.– GOT contains pointer to all the static

data reference in the program.


Position-Independent Code

main: ... call printf

0x08048f44 printf: ... ret

0x08048000

program

libc

main: ... call printf

0x40001234 printf: ... ret

0x08048000

program

libc

printf: call GOT[5]

... [5]: &printf ...

PLT(r/o code)

GOT(r/w data)Static Libraries

Dynamic Shared Libraries


Global Offset Table

• To be an indirection table and accessed when PIC calls functions and operates on data variables.

• It always consumes no space in executables and allocate memory when loading.

• The values in GOT will be evaluated at runtime by linker.


Procedure Linkage Table

• For procedure only.• To support dynamic linking, each

ELF shared libraries or executables that uses shared libraries has a PLT.

• Adds a level of indirection for function calls analogous to that provided by the GOT for data.


Dynamic Linking Library:Section Headers (/lib/libc.so.6)

> readelf –S /lib/libc.so.6There are 66 section headers, starting at offset 0x123a2c:

Section Headers: [Nr] Name Type Addr Off Size ES Flg Lk Inf Al [ 0] NULL 00000000 000000 000000 00 0 0 0 [ 1] .note.ABI-tag NOTE 00000194 000194 000020 00 A 0 0 4 [ 2] .hash HASH 000001b4 0001b4 0032d8 04 A 3 0 4 [ 3] .dynsym DYNSYM 0000348c 00348c 008b50 10 A 4 19 4 [ 4] .dynstr STRTAB 0000bfdc 00bfdc 0054e8 00 A 0 0 1 [ 8] .rel.dyn REL 00012904 012904 002c98 08 A 3 0 4 [ 9] .rel.plt REL 0001559c 01559c 000040 08 A 3 10 4 [10] .plt PROGBITS 000155dc 0155dc 000090 04 AX 0 0 4 [11] .text PROGBITS 00015670 015670 0de440 00 AX 0 0 16 [27] .data.rel.ro PROGBITS 00118060 117060 001cfc 00 WA 0 0 32 [28] .dynamic DYNAMIC 00119d5c 118d5c 0000f0 08 WA 4 0 4 [29] .got PROGBITS 00119e4c 118e4c 0001a8 04 WA 0 0 4 [30] .got.plt PROGBITS 00119ff4 118ff4 00002c 04 WA 0 0 4 [31] .data PROGBITS 0011a020 119020 00095c 00 WA 0 0 32 [32] .bss NOBITS 0011a980 11997c 002bfc 00 WA 0 0 32

Key to Flags: A (alloc), X (execute), W (write)


Dynamic Linking Library

• Build– gcc –fPIC –c –o myprnt.c myprnt.c– gcc –shared –o libmyprnt.so myprnt.o

• Usage– gcc –c main.c– gcc –o main.out main.o –L. –lmyprnt

• Runtime Environment Variable– export LD_LIBRARY_PATH =

$LD_LIBRARY_PATH : /path/


GCC – GNU Compiler Collection

• cpp: preprocess macros• cc1: perform semantic routines and

translate into assembly language• as: assemble to relocatable object

files• ld: linking• To view the commands executed to

run the stages of compilation.– gcc -v


From Source to Execution

SourceProgram

Preprocessorcpp / gcc -E

Compilercc1 / gcc -S

AssemblyCode Assembler

as / gcc -c

ObjectProgram

Linkerld / gcc -l

ExecutablesLoader


References

• Wikipedia• Linkers and Loaders• Google

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