Computer Organization: A Programmer's Perspective
Linking A Simplistic Program Translation Scheme
m.c ASCII source file
Translator
Binary executable object file p (memory image on disk)
Problems: • Efficiency: small change requires complete recompilation • Modularity: hard to share common functions (e.g. printf) Solution: • Static linker (or linker)
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 2 A Better Scheme Using a Linker
m.c a.c
Translators Translators
Separately compiled m.o a.o relocatable object files
Linker (ld)
Executable object file (contains code and p data for all functions defined in m.c and a.c)
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 3 Translating the Example Program
Compiler driver coordinates all steps in the translation and linking process. Typically included with each compilation system (e.g., gcc) Invokes preprocessor (cpp), compiler (cc1), assembler (as), and linker (ld). Passes command line arguments to appropriate phases Example: create executable p from m.c and a.c:
> gcc -O2 -v -o p m.c a.c cpp [args] m.c /tmp/cca07630.i cc1 /tmp/cca07630.i m.c -O2 [args] -o /tmp/cca07630.s as [args] -o /tmp/cca076301.o /tmp/cca07630.s ...
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 4 What Does a Linker Do?
Merges multiple relocatable (.o) object files Create a single executable object file Executable can loaded and executed by the loader Resolves external references External reference: to a symbol defined in another object file Relocates symbols From their relative locations in the .o files To new absolute positions in the executable Updates all references to reflect their new positions. References can be in either code or data » code: a(); /* reference to symbol a */ » data: int *xp=&x; /* reference to symbol x */
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 5 Why Linkers? 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 Efficiency Time: Change one source file, compile, and then relink. No need to recompile other source files. Space: Libraries of 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.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 6 Executable and Linkable Format (ELF)
Standard binary format for object (.o) files Derives from AT&T System V Unix Later adopted by BSD Unix variants and Linux One unified format for Relocatable object files (.o), Executable object files Shared object files (.so) Generic name: ELF binaries Better support for shared libraries than old a.out formats.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 7 ELF Object File Format Elf header 0 Magic number, type (.o, exec, .so), ELF header machine, byte ordering, etc. Program header table Program header table (required for executables) Page size, virtual addresses memory .text section segments (sections), segment sizes. .data section .text section .bss section Code .symtab .data section Initialized (static) data .rel.txt .rel.data .bss section Uninitialized (static) data .debug “Block Started by Symbol” Section header table (required for relocatables) “Better Save Space” Has section header but occupies no space
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 8 ELF Object File Format (cont)
.symtab section 0 ELF header Symbol table Procedure and static variable names Program header table (required for executables) Section names and locations .text section .rel.text section .data section Relocation info for .text section Addresses of instructions that will need to .bss section be modified in the executable .symtab Instructions for modifying. .rel.text
.rel.data section .rel.data Relocation info for .data section .debug Addresses of pointer data that will need to be modified in the merged executable Section header table (required for relocatables) .debug section Info for symbolic debugging (gcc -g)
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 9 Example C Program
m.c a.c int e=7; extern int e;
int main() { int *ep=&e; int r = a(); int x=15; exit(0); int y; } int a() { return *ep+x+y; }
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 10 Merging Relocatable Object Files into an Executable Object File Relocatable Object Files Executable Object File 0 system code .text headers .data system data system code
main() .text
a() main() .text m.o int e = 7 .data more system code system data int e = 7 .data a() .text int *ep = &e int x = 15 a.o int *ep = &e .data uninitialized data .bss int x = 15 .symtab int y .bss .debug Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 11 Relocating Symbols and Resolving External References Symbols are lexical entities that name functions and variables. Each symbol has a value (typically a memory address). Code consists of symbol definitions and references. References can be either local or external. m.c a.c int e=7; extern int e; Ref to Def of local external symbol e int main() { int *ep=&e; symbol e int r = a(); int x=15; exit(0); int y; } Def of int a() { local return *ep+x+y; Defs of symbol } local Ref to external ep symbols x symbol exit Ref to external and y (defined in symbol a Def of libc.so) local Refs of local symbol a symbols ep,x,y Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 12 m.o Relocation Info m.c int e=7; Disassembly of section .text:
int main() { 00000000
Disassembly of section .data:
00000000
source: objdump Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 13 a.o Relocation Info (.text) a.c extern int e; Disassembly of section .text:
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 14 a.o Relocation Info (.data) a.c extern int e; Disassembly of section .data:
int *ep=&e; 00000000
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 15 Executable After Relocation and External Reference Resolution (.text)
08048530
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 16 Executable After Relocation and External Reference Resolution(.data)
m.c int e=7; Disassembly of section .data:
int main() { 0804a018
0804a01c
extern int e;
int *ep=&e; 0804a020
int a() { return *ep+x+y; }
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 17 Strong and Weak Symbols
Program symbols are either strong or weak strong: procedures and initialized globals weak: uninitialized globals
p1.c p2.c strong int foo=5; int foo; weak
strong p1() { p2() { strong } }
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 18 Linker’s Symbol Rules
Rule 1. A strong symbol can only appear once.
Rule 2. A weak symbol can be overridden by a strong symbol of the same name. references to the weak symbol resolve to the strong symbol.
Rule 3. If there are multiple weak symbols, the linker can pick an arbitrary one.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 19 Linker Puzzles
int x; p1() {} p1() {} Link time error: two strong symbols (p1)
int x; int x; References to x will refer to the same p1() {} p2() {} uninitialized int. Is this what you really want?
int x; double x; int y; p2() {} Writes to x in p2 might overwrite y! Evil! p1() {}
int x=7; double x; Writes to x in p2 will overwrite y! Nasty! int y=5; p2() {} p1() {}
int x=7; int x; References to x will refer to the p1() {} p2() {} same initialized variable. Nightmare scenario: two identical weak structs, compiled by different compilers with different alignment rules. Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 20 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 in 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 Solution: static libraries (linux: .a archive files, win: .lib 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 into executable. Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 21 Static Libraries (archives)
p1.c p2.c
Translator Translator
static library (archive) of p1.o p2.o libc.a relocatable object files concatenated into one file.
Linker (ld)
executable object file (only contains code and p data for libc functions that are called from p1.c and p2.c)
Further improves modularity and efficiency by packaging commonly used functions [e.g., C standard library (libc), math library (libm)]
Linker selectively only the .o files in the archive that are actually needed by the program.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 22 Creating Static Libraries
atoi.c printf.c random.c
Translator Translator ... Translator
atoi.o printf.o random.o
Archiver (ar)
libc.a C standard library Command-line: ar rs libc.a atoi.o printf.o … random.o Archiver allows incremental updates: • Recompile function that changes and replace .o file in archive.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 23 Commonly Used Libraries libc.a (the C standard library) A few MB archive of 900 object files. I/O, memory allocation, signal handling, string handling, data and time, random numbers, integer math libm.a (the C math library) Archive of 226 object files. floating point math (sin, cos, tan, log, exp, sqrt, …)
% ar -t /usr/lib/libc.a | sort % ar -t /usr/lib/libm.a | sort … … fork.o e_acos.o … e_acosf.o fprintf.o e_acosh.o fpu_control.o e_acoshf.o fputc.o e_acoshl.o freopen.o e_acosl.o fscanf.o e_asin.o fseek.o e_asinf.o fstab.o e_asinl.o … …
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 24 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 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.
bass> gcc -L. libtest.o -lmine bass> gcc -L. -lmine libtest.o libtest.o: In function `main': libtest.o(.text+0x4): undefined reference to `libfun'
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 25 Loading Executable Binaries Executable object file for example program p 0 ELF header Virtual addr Process image Program header table (required for executables) 0x080483e0 init and shared lib .text section segments
.data section 0x08048494 .bss section .text segment (r/o) .symtab
.rel.text 0x0804a010 .data segment .rel.data (initialized r/w)
.debug 0x0804a3b0 Section header table .bss segment (required for relocatables) (uninitialized r/w)
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 26 Static Libraries: Disadvantages
Potential for duplicating lots of common code in the executable files on a filesystem. e.g., every C program needs the standard C library Potential for duplicating lots of code in the virtual memory space of many processes. Minor bug fixes of system libraries require each application to explicitly relink
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 27 Dynamic Libraries
Dynamically- linked libraries (linux: shared libraries/objects .so, win:.DLL): Members are dynamically loaded into memory and linked into an application During run-time Based on the dynamic linker search path Dynamic linking can occur when executable is first loaded and run. Common case for Linux, handled automatically by ld-linux.so. Dynamic linking can also occur after program has begun. In Linux, this is done explicitly by user with dlopen(). Basis for exchanging code while program running. Shared library routines can be shared by multiple processes.
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 28 Dynamically Linked Shared Libraries
m.c a.c
Translators Translators (cc1, as) (cc1,as)
m.o a.o
Linker (ld)
Partially linked executable p libc.so Shared library of dynamically p relocatable object files (on disk)
Loader/Dynamic Linker (ld-linux.so) libc.so functions called by m.c and a.c are loaded, linked, and (potentially) shared among Fully linked executable processes. p’ (in memory) P’ Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 29 The Complete Picture
m.c a.c
Translator Translator
m.o a.o libwhatever.a
Static Linker (ld)
p libc.so libm.so
Loader/Dynamic Linker (ld-linux.so)
p’
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 30 ?שאלות
Computer Organization: A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 31