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

TRU-COMP2130 Linking2 Course Objectives The better knowledge of computer systems, the better programing. Computer SystemC Programming Language Computer architecture CPU (Central Processing Unit) IA32 assembly language Introduction to C language Compiling, linking, loading, executing Physical main memory MMU (Memory Management Unit) Virtual memory space Memory hierarchy Cache 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?

TRU-COMP2130 Linking3 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 – 9.9.2, – 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, – 6.4.2, 6.5, – 6.6.3, 6.7 Virtual memory (if time permits): B&O 9.4 – 9.5 Your vision? Seek with all your heart?

TRU-COMP2130 Linking4 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. Your vision? Seek with all your heart?

TRU-COMP2130 Linking5 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 Your vision? Seek with all your heart?

TRU-COMP2130 Linking6 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. Your vision? Seek with all your heart?

TRU-COMP2130 Linking7 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 Your vision? Seek with all your heart?

Example C Program int buf[2] = {1, 2}; int main() { swap(); return 0; } main.cswap.c extern int buf[]; int *bufp0 = &buf[0]; static int *bufp1; void swap() { int temp; bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } Carnegie Mellon

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 Translators (cpp, cc1, as) 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 ) Carnegie Mellon

TRU-COMP2130 Linking10 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. Your vision? Seek with all your heart?

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. Carnegie Mellon

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. Carnegie Mellon

TRU-COMP2130 Linking13 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 Your vision? Seek with all your heart?

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 Introduction14

TRU-COMP2130 Linking15 4. Relocatable Object Files Your 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 Mellon Executable 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 Carnegie Mellon

.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 Carnegie Mellon

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 Introduction19

TRU-COMP2130 Linking20 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. Your vision? Seek with all your heart?

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 Local Global Linker knows nothing of temp Global Carnegie Mellon extern void swap(); is not in main.c. [Q] Is it okay?

TRU-COMP2130 Linking22 7. Relocation Your 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() 0 System code int *bufp0=&buf[0] int buf[2]={1,2} System data More system code System data Relocatable Object FilesExecutable Object File.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.bss because of lifespan Carnegie Mellon

TRU-COMP2130 Linking Linking with Static Libraries Your vision? Seek with all your heart?

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 Carnegie Mellon

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. Carnegie Mellon

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. Carnegie Mellon

Commonly Used Libraries libc.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 … Carnegie Mellon

Linking with Static Libraries Translators ( cpp, cc1, as ) 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 Relocatable object files Fully linked executable object file vector.h Archiver ( ar ) addvec.o multvec.o Carnegie Mellon

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' Carnegie Mellon

TRU-COMP2130 Linking31 9. Loading Executable Object Files Your vision? Seek with all your heart?

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) 0 Executable Object File Kernel virtual memory Memory-mapped region for shared libraries Run-time heap (created by malloc ) User stack (created at runtime) Unused 0 %esp (stack pointer) Memory Invisible 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 Carnegie Mellon Loaded into

TRU-COMP2130 Linking Shared Libraries Your 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. Carnegie Mellon

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. Carnegie Mellon

Dynamic Linking at Load-time Translators ( cpp, cc1, as ) main2.c main2.o libc.so libvector.so Linker ( ld ) p2 Dynamic linker ( ld-linux.so ) Relocation and symbol table info libc.so libvector.so Code and data Partially linked executable object file Relocatable object file Fully linked executable in memory vector.h Loader ( execve ) unix> gcc -shared -o libvector.so \ addvec.c multvec.c Carnegie Mellon

Dynamic Linking at Run-time #include 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); } Carnegie Mellon

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; } Carnegie Mellon