Example C Program 15 213 The course that gives CMU its Zip main c Linking October 11 2006 Topics Static linking Dynamic linking Case study Library interpositioning 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 Static Linking Why Linkers Programs are translated and linked using a compiler driver driver 15 213 F 06 2 class13 ppt unix gcc O2 g o p main c swap c unix p main c swap c Translators cpp cc1 as Reason 1 Modularity swap o 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 Source files Translators cpp cc1 as main o z e g Math library standard C library Separately compiled relocatable object files Linker ld p 3 Fully linked executable object file contains code and data for all functions defined in main c and swap c 15 213 F 06 4 15 213 F 06 Why Linkers cont What Do Linkers Do Reason 2 Efficiency Step 1 Symbol resolution Time Separate Compilation z Change one source file compile and then relink Programs define and reference symbols variables and functions z void swap z No need to recompile other source files z swap z int xp x define symbol swap reference symbol a define symbol xp reference x Space Libraries z Common functions can be aggregated into a single file z Yet executable files and running memory images contain only Symbol definitions are stored by compiler in symbol table z Symbol table is an array of structs code for the functions they actually use z Each entry includes name size and location of symbol 15 213 F 06 5 Linker associates each symbol reference with exactly one symbol definition 15 213 F 06 6 What Do Linkers Do cont Three Kinds of Object Files Modules Step 2 Relocation 1 Relocatable object file o o file 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 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 combined with other relocatable object files to form executable object file Updates all references to these symbols to reflect their new positions Contains code and data in a form that can be copied directly into memory and then executed 3 Shared object file so so file 7 15 213 F 06 8 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 15 213 F 06 Executable and Linkable Format ELF ELF Object File Format Elf header Standard binary format for object files Originally proposed by AT T System V Unix Segment header table Later adopted by BSD Unix variants and Linux One unified format for Relocatable object files o Executable object files Shared object files so Magic number type o exec so machine byte ordering etc Page size virtual addresses memory segments sections segment sizes text section Code data section Initialized global variables bss section Generic name ELF binaries 15 213 F 06 9 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 text section data section bss section symtab section rel txt section rel data section debug section Section header table Linker Symbols ELF header 0 Global symbols Segment header table required for executables text section data section symtab section rel text section Symbols defined by module m that can be referenced by other modules Ex non static C functions and non static global variables External symbols bss section Global symbols that are referenced by module m but defined by some other module Local symbols rel data section debug section Section header table 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 Section header table 11 Segment header table required for executables 15 213 F 06 10 ELF Object File Format cont symtab section Uninitialized global variables Block Started by Symbol Better Save Space Has section header but occupies no space ELF header Offsets and sizes of each section 15 213 F 06 12 15 213 F 06 0 Relocating Code and Data Resolving Symbols Ref to external symbol buf Def of global symbol buf Relocatable Object Files swap c extern int buf main c int buf 2 1 2 static int bufp0 buf 0 static int bufp1 int main swap return 0 void swap int temp Def of local symbol bufp0 Ref to external symbol swap Def of global symbol swap Linker knows nothing of temp main int main swap return 0 31 c0 89 ec 5d c3 Headers System code text swap text More system code System data int buf 2 1 2 int bufp0 buf 0 Uninitialized data symtab debug swap o swap text int bufp0 buf 0 data bss int bufp1 15 213 F 06 b d f 10 0 int buf 2 1 2 data 0000000 main 0 55 1 89 e5 3 83 ec 08 6 e8 fc ff ff ff System data data main data bss 15 213 F 06 14 main o Relocation Info int buf 2 1 2 text main o bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 13 System code Executable Object File swap o Relocation Info text 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 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 Disassembly of section data 00000000 buf 0 01 00 00 00 02 00 00 00 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 Source objdump 15 15 213 F 06 16 15 213 F 06 Executable After …
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