Linux编程接口详解：包括epoll、inotify等Linux特性的系统编程手册

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"The Linux Programming Interface" 是一本深入探讨Linux和UNIX系统编程的手册，作者是Michael Kerris。这本书不仅涵盖了标准POSIX接口，还详细介绍了Linux特有的编程特性，旨在帮助程序员在各种UNIX实现上编写代码，同时也充分考虑了Linux的流行度和独特功能。在书中，作者特别强调了Linux扩展的系统编程接口，这些扩展对于性能优化或访问标准接口中未提供的功能至关重要。以下是书中提及的一些Linux特有编程特性： 1. **epoll**: 这是一种用于文件I/O事件通知的机制，它改进了传统的select和poll函数，能更高效地处理大量并发连接。 2. **inotify**: 提供了对文件和目录变化的监控功能，让程序可以实时响应文件系统的变动。 3. **capabilities**: 这是一种权限管理机制，允许进程拥有超级用户（root）的部分权力，而无需完全获得root权限，提高了安全性。 4. **extended attributes**: 扩展属性允许在文件和目录上附加额外的信息，增强了文件系统的灵活性。 5. **i-node flags**: i-node标志提供了针对文件的内核级别标志，可用来控制文件的特定行为。 6. **clone() system call**: 这个系统调用用于创建与父进程相似但不完全相同的子进程，是实现轻量级线程（也称为线程）的关键。 7. **/proc文件系统**: 这是一个虚拟文件系统，提供了一种方式来获取和修改运行时的内核状态和进程信息。 8. **Linux特定的文件I/O、信号、定时器、线程、共享库、进程间通信和套接字实现细节**: 书中详细阐述了这些主题在Linux下的具体操作和注意事项。这本书适合从初学者到经验丰富的Linux程序员，无论读者的水平如何，都能从中学习到有价值的知识。作者Michael Kerris不仅详尽地描述了接口和例子，还积极参与到Linux社区中，帮助修复错误并改进man页面，使Linux编程变得更加容易。《The Linux Programming Interface》是Linux系统编程领域的一本权威参考书，对于理解Linux底层API的细节和微妙之处至关重要，是每个Linux开发者必备的参考资料。

xiv Contents in Detail

11 SYSTEM LIMITS AND OPTIONS 211

11.1 System Limits............................................................................................................. 212

11.2 Retrieving System Limits (and Options) at Run Time ........................................................ 215

11.3 Retrieving File-Related Limits (and Options) at Run Time.................................................. 217

11.4 Indeterminate Limits ................................................................................................... 219

11.5 System Options......................................................................................................... 219

11.6 Summary.................................................................................................................. 221

11.7 Exercises .................................................................................................................. 222

12 SYSTEM AND PROCESS INFORMATION 223

12.1 The /proc File System................................................................................................. 223

12.1.1 Obtaining Information About a Process:

/proc/PID................................... 224

12.1.2 System Information Under

/proc.............................................................. 226

12.1.3 Accessing

/proc Files ............................................................................ 226

12.2 System Identification:

uname() .................................................................................... 229

12.3 Summary.................................................................................................................. 231

12.4 Exercises .................................................................................................................. 231

13 FILE I/O BUFFERING 233

13.1 Kernel Buffering of File I/O: The Buffer Cache .............................................................. 233

13.2 Buffering in the

stdio Library ....................................................................................... 237

13.3 Controlling Kernel Buffering of File I/O........................................................................ 239

13.4 Summary of I/O Buffering .......................................................................................... 243

13.5 Advising the Kernel About I/O Patterns........................................................................ 244

13.6 Bypassing the Buffer Cache: Direct I/O........................................................................ 246

13.7 Mixing Library Functions and System Calls for File I/O .................................................. 248

13.8 Summary.................................................................................................................. 249

13.9 Exercises .................................................................................................................. 250

14 FILE SYSTEMS 251

14.1 Device Special Files (Devices) ..................................................................................... 252

14.2 Disks and Partitions ................................................................................................... 253

14.3 File Systems.............................................................................................................. 254

14.4 I-nodes..................................................................................................................... 256

14.5 The Virtual File System (VFS) ....................................................................................... 259

14.6 Journaling File Systems............................................................................................... 260

14.7 Single Directory Hierarchy and Mount Points ................................................................ 261

14.8 Mounting and Unmounting File Systems ....................................................................... 262

14.8.1 Mounting a File System:

mount() ............................................................ 264

14.8.2 Unmounting a File System:

umount() and umount2() ................................ 269

14.9 Advanced Mount Features.......................................................................................... 271

14.9.1 Mounting a File System at Multiple Mount Points....................................... 271

14.9.2 Stacking Multiple Mounts on the Same Mount Point................................... 271

14.9.3 Mount Flags That Are Per-Mount Options ................................................. 272

14.9.4 Bind Mounts......................................................................................... 272

14.9.5 Recursive Bind Mounts........................................................................... 273

14.10 A Virtual Memory File System:

tmpfs............................................................................ 274

14.11 Obtaining Information About a File System:

statvfs()...................................................... 276

14.12 Summary.................................................................................................................. 277

14.13 Exercise ................................................................................................................... 278

Contents in Detail xv

15 FILE ATTRIBUTES 279

15.1 Retrieving File Information: stat() ................................................................................. 279

15.2 File Timestamps......................................................................................................... 285

15.2.1 Changing File Timestamps with

utime() and utimes()................................. 287

15.2.2 Changing File Timestamps with

utimensat() and futimens() ........................ 289

15.3 File Ownership ......................................................................................................... 291

15.3.1 Ownership of New Files ........................................................................ 291

15.3.2 Changing File Ownership:

chown(), fchown(), and lchown()....................... 291

15.4 File Permissions......................................................................................................... 294

15.4.1 Permissions on Regular Files................................................................... 294

15.4.2 Permissions on Directories...................................................................... 297

15.4.3 Permission-Checking Algorithm ............................................................... 297

15.4.4 Checking File Accessibility:

access()......................................................... 299

15.4.5 Set-User-ID, Set-Group-ID, and Sticky Bits ................................................. 300

15.4.6 The Process File Mode Creation Mask:

umask()........................................ 301

15.4.7 Changing File Permissions:

chmod() and fchmod()..................................... 303

15.5 I-node Flags (

ext2 Extended File Attributes) ................................................................... 304

15.6 Summary.................................................................................................................. 308

15.7 Exercises .................................................................................................................. 309

16 EXTENDED ATTRIBUTES 311

16.1 Overview ................................................................................................................. 311

16.2 Extended Attribute Implementation Details .................................................................... 313

16.3 System Calls for Manipulating Extended Attributes......................................................... 314

16.4 Summary.................................................................................................................. 318

16.5 Exercise ................................................................................................................... 318

17 ACCESS CONTROL LISTS 319

17.1 Overview ................................................................................................................. 320

17.2 ACL Permission-Checking Algorithm............................................................................. 321

17.3 Long and Short Text Forms for ACLs............................................................................. 323

17.4 The

ACL_MASK Entry and the ACL Group Class................................................................ 324

17.5 The

getfacl and setfacl Commands ............................................................................... 325

17.6 Default ACLs and File Creation ................................................................................... 327

17.7 ACL Implementation Limits .......................................................................................... 328

17.8 The ACL API ............................................................................................................. 329

17.9 Summary.................................................................................................................. 337

17.10 Exercise ................................................................................................................... 337

18 DIRECTORIES AND LINKS 339

18.1 Directories and (Hard) Links........................................................................................ 339

18.2 Symbolic (Soft) Links .................................................................................................. 342

18.3 Creating and Removing (Hard) Links:

link() and unlink()............................................... 344

18.4 Changing the Name of a File:

rename() ....................................................................... 348

18.5 Working with Symbolic Links:

symlink() and readlink().................................................. 349

18.6 Creating and Removing Directories:

mkdir() and rmdir() ............................................... 350

18.7 Removing a File or Directory:

remove()......................................................................... 352

18.8 Reading Directories:

opendir() and readdir() ................................................................ 352

18.9 File Tree Walking:

nftw() ........................................................................................... 358

18.10 The Current Working Directory of a Process ................................................................. 363

18.11 Operating Relative to a Directory File Descriptor ........................................................... 365

18.12 Changing the Root Directory of a Process:

chroot() ........................................................ 367

18.13 Resolving a Pathname:

realpath() ................................................................................ 369

xvi Contents in Detail

18.14 Parsing Pathname Strings: dirname() and basename() .................................................... 370

18.15 Summary.................................................................................................................. 372

18.16 Exercises .................................................................................................................. 373

19 MONITORING FILE EVENTS 375

19.1 Overview ................................................................................................................. 376

19.2 The

inotify API .......................................................................................................... 376

19.3

inotify Events ............................................................................................................ 378

19.4 Reading

inotify Events................................................................................................ 379

19.5 Queue Limits and

/proc Files....................................................................................... 385

19.6 An Older System for Monitoring File Events:

dnotify....................................................... 386

19.7 Summary.................................................................................................................. 386

19.8 Exercise ................................................................................................................... 386

20 SIGNALS: FUNDAMENTAL CONCEPTS 387

20.1 Concepts and Overview............................................................................................. 388

20.2 Signal Types and Default Actions ................................................................................ 390

20.3 Changing Signal Dispositions:

signal()......................................................................... 397

20.4 Introduction to Signal Handlers ................................................................................... 398

20.5 Sending Signals:

kill() ............................................................................................... 401

20.6 Checking for the Existence of a Process........................................................................ 403

20.7 Other Ways of Sending Signals:

raise() and killpg() ..................................................... 404

20.8 Displaying Signal Descriptions .................................................................................... 406

20.9 Signal Sets ............................................................................................................... 406

20.10 The Signal Mask (Blocking Signal Delivery) .................................................................. 410

20.11 Pending Signals ........................................................................................................ 411

20.12 Signals Are Not Queued............................................................................................ 412

20.13 Changing Signal Dispositions:

sigaction()..................................................................... 416

20.14 Waiting for a Signal:

pause()...................................................................................... 418

20.15 Summary.................................................................................................................. 418

20.16 Exercises .................................................................................................................. 419

21 SIGNALS: SIGNAL HANDLERS 421

21.1 Designing Signal Handlers ......................................................................................... 422

21.1.1 Signals Are Not Queued (Revisited) ........................................................ 422

21.1.2 Reentrant and Async-Signal-Safe Functions............................................... 422

21.1.3 Global Variables and the

sig_atomic_t Data Type..................................... 428

21.2 Other Methods of Terminating a Signal Handler ........................................................... 428

21.2.1 Performing a Nonlocal Goto from a Signal Handler.................................. 429

21.2.2 Terminating a Process Abnormally:

abort()............................................... 433

21.3 Handling a Signal on an Alternate Stack:

sigaltstack() ................................................... 434

21.4 The

SA_SIGINFO Flag................................................................................................... 437

21.5 Interruption and Restarting of System Calls ................................................................... 442

21.6 Summary.................................................................................................................. 445

21.7 Exercise ................................................................................................................... 446

22 SIGNALS: ADVANCED FEATURES 447

22.1 Core Dump Files ....................................................................................................... 448

22.2 Special Cases for Delivery, Disposition, and Handling ................................................... 450

22.3 Interruptible and Uninterruptible Process Sleep States..................................................... 451

22.4 Hardware-Generated Signals...................................................................................... 452

22.5 Synchronous and Asynchronous Signal Generation ....................................................... 452

Contents in Detail xvii

22.6 Timing and Order of Signal Delivery ........................................................................... 453

22.7 Implementation and Portability of

signal()..................................................................... 454

22.8 Realtime Signals........................................................................................................ 456

22.8.1 Sending Realtime Signals....................................................................... 458

22.8.2 Handling Realtime Signals ..................................................................... 460

22.9 Waiting for a Signal Using a Mask:

sigsuspend() .......................................................... 464

22.10 Synchronously Waiting for a Signal............................................................................. 468

22.11 Fetching Signals via a File Descriptor........................................................................... 471

22.12 Interprocess Communication with Signals ..................................................................... 474

22.13 Earlier Signal APIs (System V and BSD) ........................................................................ 475

22.14 Summary.................................................................................................................. 477

22.15 Exercises .................................................................................................................. 478

23 TIMERS AND SLEEPING 479

23.1 Interval Timers........................................................................................................... 479

23.2 Scheduling and Accuracy of Timers ............................................................................. 485

23.3 Setting Timeouts on Blocking Operations...................................................................... 486

23.4 Suspending Execution for a Fixed Interval (Sleeping) ..................................................... 487

23.4.1 Low-Resolution Sleeping:

sleep().............................................................. 487

23.4.2 High-Resolution Sleeping:

nanosleep()...................................................... 488

23.5 POSIX Clocks............................................................................................................ 491

23.5.1 Retrieving the Value of a Clock:

clock_gettime() ........................................ 491

23.5.2 Setting the Value of a Clock:

clock_settime() ............................................. 492

23.5.3 Obtaining the Clock ID of a Specific Process or Thread ............................. 493

23.5.4 Improved High-Resolution Sleeping:

clock_nanosleep() ............................... 493

23.6 POSIX Interval Timers................................................................................................. 495

23.6.1 Creating a Timer:

timer_create() ............................................................. 495

23.6.2 Arming and Disarming a Timer:

timer_settime() ........................................ 498

23.6.3 Retrieving the Current Value of a Timer:

timer_gettime()............................. 499

23.6.4 Deleting a Timer:

timer_delete() .............................................................. 499

23.6.5 Notification via a Signal........................................................................ 499

23.6.6 Timer Overruns..................................................................................... 503

23.6.7 Notification via a Thread....................................................................... 504

23.7 Timers That Notify via File Descriptors: the

timerfd API ................................................... 507

23.8 Summary.................................................................................................................. 511

23.9 Exercises .................................................................................................................. 512

24 PROCESS CREATION 513

24.1 Overview of fork(), exit(), wait(), and execve() .............................................................. 513

24.2 Creating a New Process:

fork()................................................................................... 515

24.2.1 File Sharing Between Parent and Child .................................................... 517

24.2.2 Memory Semantics of

fork() ................................................................... 520

24.3 The

vfork() System Call .............................................................................................. 522

24.4 Race Conditions After

fork() ....................................................................................... 525

24.5 Avoiding Race Conditions by Synchronizing with Signals............................................... 527

24.6 Summary.................................................................................................................. 529

24.7 Exercises .................................................................................................................. 530

25 PROCESS TERMINATION 531

25.1 Terminating a Process: _exit() and exit()....................................................................... 531

25.2 Details of Process Termination..................................................................................... 533

25.3 Exit Handlers ............................................................................................................ 533

25.4 Interactions Between

fork(), stdio Buffers, and _exit() ..................................................... 537

xviii Contents in Detail

25.5 Summary.................................................................................................................. 538

25.6 Exercise ................................................................................................................... 539

26 MONITORING CHILD PROCESSES 541

26.1 Waiting on a Child Process ........................................................................................ 541

26.1.1 The

wait() System Call........................................................................... 541

26.1.2 The

waitpid() System Call ...................................................................... 544

26.1.3 The Wait Status Value ........................................................................... 545

26.1.4 Process Termination from a Signal Handler .............................................. 549

26.1.5 The

waitid() System Call........................................................................ 550

26.1.6 The

wait3() and wait4() System Calls...................................................... 552

26.2 Orphans and Zombies ............................................................................................... 553

26.3 The

SIGCHLD Signal .................................................................................................... 555

26.3.1 Establishing a Handler for

SIGCHLD .......................................................... 555

26.3.2 Delivery of

SIGCHLD for Stopped Children ................................................. 559

26.3.3 Ignoring Dead Child Processes ............................................................... 559

26.4 Summary.................................................................................................................. 561

26.5 Exercises .................................................................................................................. 562

27 PROGRAM EXECUTION 563

27.1 Executing a New Program: execve() ............................................................................. 563

27.2 The

exec() Library Functions......................................................................................... 567

27.2.1 The

PATH Environment Variable ............................................................... 568

27.2.2 Specifying Program Arguments as a List................................................... 570

27.2.3 Passing the Caller’s Environment to the New Program ............................... 570

27.2.4 Executing a File Referred to by a Descriptor:

fexecve()................................ 571

27.3 Interpreter Scripts ...................................................................................................... 572

27.4 File Descriptors and

exec() .......................................................................................... 575

27.5 Signals and

exec() ..................................................................................................... 578

27.6 Executing a Shell Command:

system() .......................................................................... 579

27.7 Implementing

system() ................................................................................................ 582

27.8 Summary.................................................................................................................. 588

27.9 Exercises .................................................................................................................. 589

28 PROCESS CREATION AND PROGRAM EXECUTION IN

MORE DETAIL 591

28.1 Process Accounting.................................................................................................... 591

28.2 The

clone() System Call .............................................................................................. 598

28.2.1 The

clone() flags Argument ..................................................................... 603

28.2.2 Extensions to

waitpid() for Cloned Children ............................................. 609

28.3 Speed of Process Creation.......................................................................................... 610

28.4 Effect of

exec() and fork() on Process Attributes.............................................................. 612

28.5 Summary.................................................................................................................. 616

28.6 Exercise ................................................................................................................... 616

29 THREADS: INTRODUCTION 617

29.1 Overview ................................................................................................................. 617

29.2 Background Details of the Pthreads API ........................................................................ 620

29.3 Thread Creation........................................................................................................ 622

29.4 Thread Termination.................................................................................................... 623

29.5 Thread IDs................................................................................................................ 624

29.6 Joining with a Terminated Thread................................................................................ 625

29.7 Detaching a Thread................................................................................................... 627

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