DOCSLIB.ORG

Dynamic Linker and Debugging/Tracing Interface for Helenos

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Dynamic Linker and Debugging/Tracing Interface for Helenos Charles University in Prague Faculty of Mathematics and Physics MASTER THESIS Jiří Svoboda Dynamic linker and debugging/tracing interface for HelenOS Department of Software Engineering Supervisor: Mgr. Martin Děcký Study program: Computer science, Software systems 2 I would like to thank my supervisor Mgr. Martin Děcký for reading through the preliminary versions of this document, for valuable suggestions and for kind words of support. I am also indebted to him for introducing me to HelenOS. I would also like to thank Jakub Jermář for countless hours of discussion, for all his advice, constant encouragement and for persuading me to find better solutions to problems. I also owe him for implementing VFS just in time for me to use it and for conceiving HelenOS in the first place. Last, but certainly not least, I must thank Jakub for being a great friend. Thank you. I hereby declare that I have written this thesis myself, on my own and solely using the cited sources. I give permission to loan this document. Prohlašuji, že jsem svou diplomovou práci napsal samostatně a výhradně s použitím citovaných pramenů. Souhlasím se zapůjčováním práce. V Praze dne 11. 12. 2008 Jiří Svoboda .............. 3 4 Contents Cover Page 1 Contents 8 List of Tables .................................. 9 List of Figures .................................. 9 1 Introduction 11 1.1 Motivation ................................. 11 1.2 Goals .................................... 11 1.3 Getting the Source Code ......................... 12 1.4 How to Read This Document ...................... 13 1.4.1 Organization of This Document ................. 13 1.4.2 Conventions Used in This Document .............. 13 2 HelenOS Overview 15 2.1 History of SPARTAN and HelenOS ................... 15 2.2 Multiprocessing Support ......................... 15 2.2.1 Scheduling ............................. 15 2.2.2 Synchronization .......................... 16 2.3 Memory Management ........................... 17 2.4 User-Space Tasks ............................. 17 2.4.1 Creation of a Task ........................ 17 2.4.2 Creating New Tasks from User Space .............. 18 2.4.3 Identifying Kernel Resources ................... 18 2.5 IPC Subsystem .............................. 18 2.5.1 Low-Level IPC .......................... 18 2.5.2 System-Call IPC Layer ...................... 19 2.5.3 Message Processing ........................ 19 2.5.4 Asynchronous Library ...................... 20 2.5.5 Connections and the Naming Service .............. 21 2.6 Further Reading .............................. 21 2.7 SMC Coherency .............................. 21 2.7.1 Why Self-Modifying Code? .................... 21 2.7.2 Contemporary Memory Architecture .............. 22 2.7.3 Coherency Problems ....................... 22 2.7.4 Instruction Memory Barriers ................... 23 2.7.5 SMC Coherency in HelenOS ................... 23 5 3 Debugging and Tracing Overview 27 3.1 Bugs and Observability .......................... 27 3.1.1 Hunting Bugs ........................... 27 3.1.2 Observability ........................... 27 3.1.3 Impact ............................... 27 3.1.4 Common Debugging Methods .................. 28 3.1.5 Common Techniques ....................... 28 3.1.6 Debugging Software ........................ 29 3.1.7 Data for Post-Mortem Analysis ................. 30 3.1.8 Methods for Static Analysis ................... 30 3.2 Breakpoint Debugging Support in Processors .............. 30 3.3 Network Packet Analysis ......................... 31 4 Debugging and Tracing Design and Implementation 33 4.1 Design Overview ............................. 33 4.2 Supported Architectures ......................... 34 4.3 Udebug Interface ............................. 34 4.3.1 Interface Form .......................... 34 4.3.2 Connecting ............................ 34 4.3.3 Debugging Message Format ................... 35 4.3.4 Debugging Methods ....................... 35 4.3.5 Typical Debugging Session .................... 37 4.3.6 Events ............................... 38 4.4 Udebug Implementation ......................... 39 4.4.1 Implementation Overview .................... 39 4.4.2 Suspending and Resuming Threads ............... 40 4.4.3 Hooks ............................... 43 4.4.4 Task Memory Access ....................... 44 4.4.5 Kbox Thread Benefits ...................... 46 4.4.6 Register State Access ....................... 46 4.4.7 Synchronization and State Management ............ 48 4.5 System Call/IPC Tracer ......................... 51 4.5.1 Overview ............................. 51 4.5.2 End-User Perspective ....................... 51 4.5.3 Under the Hood .......................... 52 4.6 Breakpoint Debugger ........................... 53 4.6.1 Overview ............................. 53 4.6.2 End-User Perspective ....................... 53 4.6.3 Under the Hood .......................... 54 4.7 Future Work ................................ 55 5 Dynamic Linking Overview 57 5.1 Basic Concepts .............................. 57 5.1.1 Separate Compilation ....................... 57 5.1.2 Symbols .............................. 57 5.1.3 Object Files ............................ 58 5.1.4 Sections .............................. 59 5.1.5 Executable Files ......................... 59 5.1.6 Loader ............................... 59 6 5.1.7 Linker ............................... 59 5.1.8 Libraries .............................. 59 5.1.9 Dynamically-Linked Libraries .................. 60 5.1.10 Loading a Library at Run Time ................. 60 5.1.11 ELF and the System V ABI ................... 61 5.1.12 Library ABI and Versioning ................... 61 5.1.13 Thread-Local Storage ....................... 62 5.2 Executable and Linking Format (ELF) ................. 63 5.2.1 Features .............................. 63 5.2.2 File Structure Overview ..................... 63 5.2.3 ELF Header ............................ 63 5.2.4 Sections .............................. 63 5.2.5 Segments ............................. 64 5.2.6 String Table ............................ 64 5.2.7 Symbol Table ........................... 65 5.2.8 Relocation Table ......................... 65 5.3 ELF Dynamic Linking .......................... 65 5.3.1 Base Address ........................... 65 5.3.2 Program Interpreter ....................... 66 5.3.3 Dynamic Section ......................... 66 5.3.4 Shared Library Dependencies .................. 66 5.3.5 Global Offset Table ........................ 67 5.3.6 Procedure Linkage Table ..................... 67 5.3.7 Hash Table ............................ 68 5.3.8 Initialization and Termination Functions ............ 68 5.4 GNU Linker ................................ 69 5.5 ELF Thread-Local Storage ........................ 69 6 Dynamic Linking Design and Implementation 71 6.1 Overview .................................. 71 6.2 Supported Architectures ......................... 71 6.3 Building .................................. 71 6.4 Program Loader .............................. 72 6.4.1 Interim Solution .......................... 72 6.4.2 Cracking the Chicken and Egg Problem ............. 72 6.4.3 Kernel Infrastructure ....................... 74 6.4.4 Entry-Point Interface ....................... 74 6.4.5 Program Control Block ...................... 75 6.4.6 IPC Communication Protocol .................. 75 6.4.7 Library API ............................ 76 6.4.8 Program Loader Implementation ................ 77 6.4.9 SMC Coherency .......................... 77 6.5 Dynamic Linker .............................. 77 6.5.1 The Big Picture .......................... 77 6.5.2 Design Considerations ...................... 78 6.5.3 Source Code Structure ...................... 80 6.5.4 Dynamic Linker Operation in a Nutshell ............ 82 6.5.5 Building the Shared C Library .................. 84 6.5.6 Trying out the Dynamic Linker ................. 85 7 6.5.7 Future Work ............................ 85 7 Related Work 87 7.1 Debugging on UNIX System V ..................... 87 7.2 Debugging on Microsoft Windows .................... 87 7.3 Dynamic Linking on UNIX Systems ................... 88 7.4 Dynamic Linking on Windows ...................... 88 7.5 Debugging on Linux and Solaris ..................... 88 7.6 QNX Neutrino .............................. 89 7.7 OKL4 ................................... 89 7.8 MINIX ................................... 89 7.9 GNU Hurd ................................. 90 8 Conclusion 91 8.1 Achievements ............................... 91 8.2 Contributions ............................... 91 8.3 Perspectives ................................ 91 Bibliography 93 8 List of Tables 4.1 Debugging Request Structure ...................... 35 4.2 Debugging Reply Structure ....................... 35 4.3 Debugging Methods ............................ 36 4.4 Debugging Event Message Structure ................... 38 4.5 Debugger Commands ........................... 54 6.1 Loader Low-Level Library API ...................... 76 6.2 Implemented IA-32 Relocation Types .................. 83 6.3 Implemented PowerPC Relocation Types ................ 83 List of Figures 2.1 IPC Message Processing ......................... 20 4.1 Debugging-Session Management Example ................ 43 4.2 Debugging State Transitions ....................... 49 5.1 Thread-Local Storage Data Structures ................. 70 6.1 Debugging-Session Management Example ................ 73 6.2 Task Address-Space Layout with Loader Present ............ 73 6.3 Task Address-Space Layout with Loader and Linker .........
Load more

Recommended publications
  • Architectural Support for Dynamic Linking Varun Agrawal, Abhiroop Dabral, Tapti Palit, Yongming Shen, Michael Ferdman COMPAS Stony Brook University
    Architectural Support for Dynamic Linking Varun Agrawal, Abhiroop Dabral, Tapti Palit, Yongming Shen, Michael Ferdman COMPAS Stony Brook University Abstract 1 Introduction All software in use today relies on libraries, including All computer programs in use today rely on software standard libraries (e.g., C, C++) and application-specific libraries. Libraries can be linked dynamically, deferring libraries (e.g., libxml, libpng). Most libraries are loaded in much of the linking process to the launch of the program. memory and dynamically linked when programs are Dynamically linked libraries offer numerous benefits over launched, resolving symbol addresses across the applica- static linking: they allow code reuse between applications, tions and libraries. Dynamic linking has many benefits: It conserve memory (because only one copy of a library is kept allows code to be reused between applications, conserves in memory for all applications that share it), allow libraries memory (because only one copy of a library is kept in mem- to be patched and updated without modifying programs, ory for all the applications that share it), and allows enable effective address space layout randomization for libraries to be patched and updated without modifying pro- security [21], and many others. As a result, dynamic linking grams, among numerous other benefits. However, these is the predominant choice in all systems today [7]. benefits come at the cost of performance. For every call To facilitate dynamic linking of libraries, compiled pro- made to a function in a dynamically linked library, a trampo- grams include function call trampolines in their binaries. line is used to read the function address from a lookup table When a program is launched, the dynamic linker maps the and branch to the function, incurring memory load and libraries into the process address space and resolves external branch operations.
    [Show full text]
  • CS429: Computer Organization and Architecture Linking I & II
    CS429: Computer Organization and Architecture Linking I & II Dr. Bill Young Department of Computer Sciences University of Texas at Austin Last updated: April 5, 2018 at 09:23 CS429 Slideset 23: 1 Linking I A Simplistic Translation Scheme m.c ASCII source file Problems: Efficiency: small change Compiler requires complete re-compilation. m.s Modularity: hard to share common functions (e.g., printf). Assembler Solution: Static linker (or Binary executable object file linker). p (memory image on disk) CS429 Slideset 23: 2 Linking I Better Scheme Using a Linker Linking is the process of m.c a.c ASCII source files combining various pieces of code and data into a Compiler Compiler single file that can be loaded (copied) into m.s a.s memory and executed. Linking could happen at: Assembler Assembler compile time; Separately compiled m.o a.o load time; relocatable object files run time. Linker (ld) Must somehow tell a Executable object file module about symbols p (code and data for all functions defined in m.c and a.c) from other modules. CS429 Slideset 23: 3 Linking I Linking A linker takes representations of separate program modules and combines them into a single executable. This involves two primary steps: 1 Symbol resolution: associate each symbol reference throughout the set of modules with a single symbol definition. 2 Relocation: associate a memory location with each symbol definition, and modify each reference to point to that location. CS429 Slideset 23: 4 Linking I Translating the Example Program A compiler driver coordinates all steps in the translation and linking process.
    [Show full text]
  • Protecting Against Unexpected System Calls
    Protecting Against Unexpected System Calls C. M. Linn, M. Rajagopalan, S. Baker, C. Collberg, S. K. Debray, J. H. Hartman Department of Computer Science University of Arizona Tucson, AZ 85721 {linnc,mohan,bakers,collberg,debray,jhh}@cs.arizona.edu Abstract writing the return address on the stack with the address of the attack code). This then causes the various actions This paper proposes a comprehensive set of techniques relating to the attack to be carried out. which limit the scope of remote code injection attacks. These techniques prevent any injected code from mak- In order to do any real damage, e.g., create a root ing system calls and thus restrict the capabilities of an shell, change permissions on a file, or access proscribed attacker. In defending against the traditional ways of data, the attack code needs to execute one or more sys- harming a system these techniques significantly raise the tem calls. Because of this, and the well-defined system bar for compromising the host system forcing the attack call interface between application code and the underly- code to take extraordinary steps that may be impractical ing operating system kernel, many researchers have fo- in the context of a remote code injection attack. There cused on the system call interface as a convenient point are two main aspects to our approach. The first is to for detecting and disrupting such attacks (see, for exam- embed semantic information into executables identify- ple, [5, 13, 17, 19, 29, 32, 35, 38]; Section 7 gives a more ing the locations of legitimate system call instructions; extensive discussion).
    [Show full text]
  • Cray XT Programming Environment's Implementation of Dynamic Shared Libraries
    Cray XT Programming Environment’s Implementation of Dynamic Shared Libraries Geir Johansen, Cray Inc. Barb Mauzy, Cray Inc. ABSTRACT: The Cray XT Programming Environment will support the implementation of dynamic shared libraries in future Cray XT CLE releases. The Cray XT implementation will provide the flexibility to allow specific library versions to be chosen at both link and run times. The implementation of dynamic shared libraries allows dynamically linked executables built with software other than the Cray Programming Environment to be run on the Cray XT. KEYWORDS: Cray XT, Programming Environment CUG 2009 Proceedings 1 of 10 information in physical memory that can be shared among 1.0 Introduction multiple processes. This feature potentially reduces the memory allocated for a program on a compute node. This The goal of this paper is provide application analysts advantage becomes more important as the number of a description of Cray XT Programming Environment‟s available cores increases. implementation of dynamic shared libraries. Topics Another advantage of dynamically linked executables covered include the building of dynamic shared library is that the system libraries can be upgraded and take effect programs, configuring the version of dynamic shared without the need to relink applications. The ability to libraries to be used at execution time, and the execution of install library changes without having to rebuild a program a dynamically linked program not built with the Cray XT is a significant advantage in quickly addressing problems, Programming Environment. While a brief description of such as a security issue with a library code. The the Cray XT system implementation of dynamic shared experiences of system administrators have shown that libraries will be given, the Cray XT system architecture codes using dynamic shared libraries are more resilient to and system administration issues of dynamic shared system upgrades than codes using static libraries [1].
    [Show full text]
  • Dynamic Linking Considered Harmful
    DYNAMIC LINKING CONSIDERED HARMFUL 1 WHY WE NEED LINKING ¡ Want to access code/data defined somewhere else (another file in our project, a library, etc) ¡ In compiler-speak, “we want symbols with external linkage” § I only really care about functions here ¡ Need a mechanism by which we can reference symbols whose location we don’t know ¡ A linker solves this problem. Takes symbols annotated by the compiler (unresolved symbols) and patches them 2 DYNAMIC LINKING ¡ We want to: ¡ use code defined somewhere else, but we don’t want to have to recompile/link when it’s updated ¡ be able to link only those symbols used as runtime (deferred/lazy linking) ¡ be more efficient with resources (may get to this later) 3 CAVEATS ¡ Applies to UNIX, particularly Linux, x86 architecture, ELF Relevant files: -glibcX.X/elf/rtld.c -linux-X.X.X/fs/exec.c, binfmt_elf.c -/usr/include/linux/elf.h ¡ (I think) Windows linking operates similarly 4 THE BIRTH OF A PROCESS 5 THE COMPILER ¡ Compiles your code into a relocatable object file (in the ELF format, which we’ll get to see more of later) ¡ One of the chunks in the .o is a symbol table ¡ This table contains the names of symbols referenced and defined in the file ¡ Unresolved symbols will have relocation entries (in a relocation table) 6 THE LINKER ¡ Patches up the unresolved symbols it can. If we’re linking statically, it has to fix all of them. Otherwise, at runtime ¡ Relocation stage. Will not go into detail here. § Basically, prepares program segments and symbol references for load time 7 THE SHELL fork(), exec() 8 THE KERNEL (LOADER) ¡ Loaders are typically kernel modules.
    [Show full text]
  • Linker Script Guide Emprog
    Linker Script Guide Emprog Emprog ThunderBench™ Linker Script Guide Version 1.2 ©May 2013 Page | 1 Linker Script Guide Emprog 1 - Linker Script guide 2 -Arm Specific Options Page | 2 Linker Script Guide Emprog 1 The Linker Scripts Every link is controlled by a linker script. This script is written in the linker command language. The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. Most linker scripts do nothing more than this. However, when necessary, the linker script can also direct the linker to perform many other operations, using the commands described below. The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the ‘--verbose’ command line option to display the default linker script. Certain command line options, such as ‘-r’ or ‘-N’, will affect the default linker script. You may supply your own linker script by using the ‘-T’ command line option. When you do this, your linker script will replace the default linker script. You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked. 1.1 Basic Linker Script Concepts We need to define some basic concepts and vocabulary in order to describe the linker script language. The linker combines input files into a single output file. The output file and each input file are in a special data format known as an object file format.
    [Show full text]
  • Sta C Linking
    Carnegie Mellon Linking 15-213: Introduc;on to Computer Systems 13th Lecture, Oct. 13, 2015 Instructors: Randal E. Bryant and David R. O’Hallaron Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 1 Carnegie Mellon Today ¢ Linking ¢ Case study: Library interposioning Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 2 Carnegie Mellon Example C Program int sum(int *a, int n);! int sum(int *a, int n)! ! {! int array[2] = {1, 2};! int i, s = 0;! ! ! int main()! for (i = 0; i < n; i++) {! {! s += a[i];! int val = sum(array, 2);! }! return val;! return s;! }! }! main.c ! sum.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 3 Carnegie Mellon Sta7c Linking ¢ Programs are translated and linked using a compiler driver: § linux> gcc -Og -o prog main.c sum.c § linux> ./prog main.c sum.c Source files Translators Translators (cpp, cc1, as) (cpp, cc1, as) main.o sum.o Separately compiled relocatable object files Linker (ld) Fully linked executable object file prog (contains code and data for all func;ons defined in main.c and sum.c) Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 4 Carnegie Mellon Why Linkers? ¢ Reason 1: Modularity § Program can be wriNen as a collec;on of smaller source files, rather than one monolithic mass. § Can build libraries of common func;ons (more on this later) § e.g., Math library, standard C library Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspec;ve, Third Edi;on 5 Carnegie Mellon Why Linkers? (cont) ¢ Reason 2: Efficiency § Time: Separate compilaon § Change one source file, compile, and then relink.
    [Show full text]
  • Introduction to Computer Systems 15-213/18-243, Spring 2009
    Carnegie Mellon Linking 15-213: Introduction to Computer Systems 13th Lecture, February 23rd, 2016 Instructors: Franz Franchetti & Seth Copen Goldstein, Ralf Brown, and Brian Railing Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 1 Carnegie Mellon Today Linking Case study: Library interpositioning Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 2 Carnegie Mellon Example C Program int sum(int *a, int n); int sum(int *a, int n) { int array[2] = {1, 2}; int i, s = 0; int main() for (i = 0; i < n; i++) { { s += a[i]; int val = sum(array, 2); } return val; return s; } } main.c sum.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 3 Carnegie Mellon Static Linking Programs are translated and linked using a compiler driver: . linux> gcc -Og -o prog main.c sum.c . linux> ./prog main.c sum.c Source files Translators Translators (cpp, cc1, as) (cpp, cc1, as) main.o sum.o Separately compiled relocatable object files Linker (ld) Fully linked executable object file prog (contains code and data for all functions defined in main.c and sum.c) Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 4 Carnegie Mellon 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 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 5 Carnegie Mellon Why Linkers? (cont) Reason 2: Efficiency .
    [Show full text]
  • Linkers and Loaders Do?
    Linkers & Loaders by John R. Levine Table of Contents 1 Table of Contents Chapter 0: Front Matter ........................................................ 1 Dedication .............................................................................................. 1 Introduction ............................................................................................ 1 Who is this book for? ......................................................................... 2 Chapter summaries ............................................................................. 3 The project ......................................................................................... 4 Acknowledgements ............................................................................ 5 Contact us ........................................................................................... 6 Chapter 1: Linking and Loading ........................................... 7 What do linkers and loaders do? ............................................................ 7 Address binding: a historical perspective .............................................. 7 Linking vs. loading .............................................................................. 10 Tw o-pass linking .............................................................................. 12 Object code libraries ........................................................................ 15 Relocation and code modification .................................................... 17 Compiler Drivers .................................................................................
    [Show full text]
  • Let Your Mach-O fly
    Let your Mach-O fly Vincenzo Iozzo [email protected] January 15, 2009 Abstract Mac OS X is starting to spread among users, so new exploitation techniques have to be discovered. Even if a lot of interesting ways of exploitation on OSX were presented in the past, the lack of anti-forensics techniques is clear. The talk is focused on an in-memory injection technique. Specifically, how it is possible to inject into a victim's machine any kind of binaries ranging from your own piece of code to real applications like Safari. This is accomplished without leaving traces on the hard disk and without creating a new process, since the whole exploitation is performed in memory. If an attacker is able to execute code in the target machine, it is possible to run this attack instead of a classic shellcode and to use it as a trampoline for higher-lever payloads. Other similar payloads like meterpreter or meterpretux[11] exist but none of them is able to run on Mac OS X. Besides, many of these techniques require to run specific crafted binaries, that way precompiled applications are left out from the possible range of payloads. 1 Introduction Mac OS X is constantly attracting new users. Given that fact, in the last few years researchers focused more and more on this topic highlighting a number of security concerns, mainly caused by the lack of modern security counter mea- sures. Nonetheless there is a lack of advanced techniques and tools that are already present and used on other OSes instead.
    [Show full text]
  • Mach-O Programming Topics
    Mach-O Programming Topics Tools > Compiling & Debugging 2006-11-28 subsidiaries in the United States and other Apple Inc. countries. © 2003, 2006 Apple Computer, Inc. Java and all Java-based trademarks are All rights reserved. trademarks or registered trademarks of Sun Microsystems, Inc. in the U.S. and other No part of this publication may be countries. reproduced, stored in a retrieval system, or transmitted, in any form or by any means, PowerPC and and the PowerPC logo are mechanical, electronic, photocopying, trademarks of International Business recording, or otherwise, without prior Machines Corporation, used under license written permission of Apple Inc., with the therefrom. following exceptions: Any person is hereby UNIX is a registered trademark of The Open authorized to store documentation on a Group single computer for personal use only and Simultaneously published in the United to print copies of documentation for States and Canada. personal use provided that the documentation contains Apple’s copyright Even though Apple has reviewed this document, APPLE MAKES NO WARRANTY OR notice. REPRESENTATION, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THIS The Apple logo is a trademark of Apple Inc. DOCUMENT, ITS QUALITY, ACCURACY, MERCHANTABILITY, OR FITNESS FOR A Use of the “keyboard” Apple logo PARTICULAR PURPOSE. AS A RESULT, THIS (Option-Shift-K) for commercial purposes DOCUMENT IS PROVIDED “AS IS,” AND YOU, THE READER, ARE ASSUMING THE without the prior written consent of Apple ENTIRE RISK AS TO ITS QUALITY AND may constitute trademark infringement and ACCURACY. unfair competition in violation of federal IN NO EVENT WILL APPLE BE LIABLE FOR and state laws.
    [Show full text]
  • “Hello, World”* (*But Were Afraid to Ask.)
    Everything you ever wanted to know about “hello, world”* (*but were afraid to ask.) Brooks Davis SRI International <[email protected]> 1 Abstract compile with a modern compiler in C11 mode. In practice we need to specify a return type (or at least The first example in the classic book “The C Pro- void for returning nothing) and arguments. In the- gramming Language” by Kernighan and Ritchie[1] ory the main() function is defined to return an inte- is in fact a remarkably complete test of the C pro- ger and take two arguments for the command line gramming language. This paper provides a guided argument list; however, compilers tend to accept tour of a slightly more complex program, where this alternative form for compatibility sake. printf() is called with multiple arguments. Along #include <stdio.h> the way from the initial processes’s call to exec() void to the final _exit(), we’ll tour the program loading main (void) code in the kernel, the basics of system-call imple- { mentation, the implementation of the memory allo- printf ("hello, world\n"); cator, and of course printf(). We’ll also touch on } localization, a little on threading support, and a brief One of the more interesting features of the overview of the dynamic linker. printf() function is that it takes a variable number of arguments. Varidac functions such as printf() 2 Introduction are both interesting and error prone, so it’s useful to understand how they work. As such, we’ll use the The first example in the classic K&R C book is a following enhanced version of helloworld with a simple program that prints the text “hello, world” string argument and an integer argument.
    [Show full text]