15 213 Linking October 10 2007 Topics class13 ppt Static linking Dynamic linking Case study Library interpositioning 15 213 F 07 Example C Program main c swap c int buf 2 1 2 extern int buf int main swap return 0 static int bufp0 buf 0 static int bufp1 void swap int temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 2 15 213 F 07 Static Linking Programs are translated and linked using a compiler driver unix gcc O2 g o p main c swap c unix p main c swap c Translators cpp cc1 as Source files Translators cpp cc1 as main o swap o Separately compiled relocatable object files Linker ld p Fully linked executable object file contains code and data for all functions defined in main c and swap c 3 15 213 F 07 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 z e g Math library standard C library 4 15 213 F 07 Why Linkers cont Reason 2 Efficiency Time Separate Compilation z Change one source file compile and then relink z No need to recompile other source files Space Libraries z Common functions can be aggregated into a single file z Yet executable files and running memory images contain only code for the functions they actually use 5 15 213 F 07 What Do Linkers Do Step 1 Symbol resolution Programs define and reference symbols variables and functions z void swap z swap z int xp x define symbol swap reference symbol a define symbol xp reference x Symbol definitions are stored by compiler in symbol table z Symbol table is an array of structs z Each entry includes name size and location of symbol Linker associates each symbol reference with exactly one symbol definition 6 15 213 F 07 What Do Linkers Do cont Step 2 Relocation Merges separate code and data sections into single sections 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 7 15 213 F 07 Three Kinds of Object Files Modules 1 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 z Each o file is produced from exactly one source c file 2 Executable object file Contains code and data in a form that can be copied directly into memory and then executed 3 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 runtime Called Dynamic Link Libraries DLLs by Windows 8 15 213 F 07 Executable and Linkable Format ELF Standard binary format for object files Originally proposed by 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 9 15 213 F 07 ELF Object File Format Elf header Magic number type o exec so machine byte ordering etc Segment header table Page size virtual addresses memory segments sections segment sizes text section Code data section Initialized global variables bss section Uninitialized global variables Block Started by Symbol Better Save Space Has section header but occupies no space ELF header Segment header table required for executables text section data section bss section symtab section rel txt section rel data section debug section Section header table 10 15 213 F 07 0 ELF Object File Format cont symtab section Symbol table Procedure and static variable names Section names and locations 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 ELF header Segment header table required for executables text section data section bss section symtab section rel text section rel data section debug section Section header table Section header table Offsets and sizes of each section 11 15 213 F 07 0 Linker Symbols Global symbols Symbols defined by module m that can be referenced by other modules Ex non static C functions and non static global variables 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 Ex C functions and variables defined with the static attribute Key Point Local linker symbols are not local program variables 12 15 213 F 07 Resolving Symbols Ref to external symbol buf Def of global symbol buf main c swap c extern int buf int buf 2 1 2 static int bufp0 buf 0 static int bufp1 int main swap return 0 Ref to external symbol swap void swap int temp Def of local symbol bufp0 Def of global symbol swap Linker knows nothing of temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 13 15 213 F 07 Relocating Code and Data Relocatable Object Files System code text System data data Executable Object File 0 Headers System code main text main o main text int buf 2 1 2 data swap o swap text int bufp0 buf 0 data bss int bufp1 swap More system code System data int buf 2 1 2 int bufp0 buf 0 Uninitialized data symtab debug 14 15 213 F 07 data bss main o Relocation Info int buf 2 1 2 int main swap return 0 0000000 main 0 55 1 89 e5 3 83 ec 08 6 e8 fc ff ff ff b d f 10 31 c0 89 ec 5d c3 push ebp mov esp ebp sub 0x8 esp call 7 main 0x7 7 R 386 PC32 swap xor eax eax mov ebp esp pop ebp ret Disassembly of section data 00000000 buf 0 01 00 00 00 02 00 00 00 Source objdump 15 15 213 F 07 swap o Relocation Info text extern int buf Disassembly of section text static int bufp0 buf 0 static int bufp1 00000000 swap 0 55 1 8b 15 00 00 00 00 7 a1 void swap int temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp c 89 e c7 15 00 18 1a 1c 1e 89 8b 89 a1 23 89 25 5d 26 c3 push ebp mov 0x0 edx 3 R 386 32 bufp0 0 00 00 00 mov 0x4 eax 8 R 386 32 buf e5 mov esp ebp 05 00 00 00 00 04movl 0x4 0x0 00 00 10 R 386 32 bufp1 14 R 386 32 buf ec mov ebp esp 0a mov edx ecx 02 mov eax edx 00 00 00 00 mov 0x0 eax 1f R 386 32 bufp1 08 mov ecx eax pop ebp ret 16 15 213 F 07 swap o Relocation Info data extern int buf …
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