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    Copyright
    Praise for The Linux Programming Interface
    Dedication
    Preface
    Ch. 1. History and Standards
    Ch. 2. Fundamental Concepts
    Ch. 3. System Programming Concepts
    Ch. 4. File I/O: The Universal I/O Model
    Ch. 5. File I/O: Further Details
    Ch. 6. Processes
    Ch. 7. Memory Allocation
    Ch. 8. Users and Groups
    Ch. 9. Process Credentials
    Ch. 10. Time
    Ch. 11. System Limits and Options
    Ch. 12. System and Process Information
    Ch. 13. File I/O Buffering
    Ch. 14. File Systems
    Ch. 15. File Attributes
    Ch. 16. Extended Attributes
    Ch. 17. Access Control Lists
    Ch. 18. Directories and Links
    Ch. 19. Monitoring File Events
    Ch. 20. Signals: Fundamental Concepts
    Concepts and Overview
    Signal Types and Default Actions
    Changing Signal Dispositions: signal()
    Introduction to Signal Handlers
    Sending Signals: kill()
    Checking for the Existence of a Process
    Other Ways of Sending Signals: raise() and killpg()
    Displaying Signal Descriptions
    Signal Sets
    The Signal Mask (Blocking Signal Delivery)
    Pending Signals
    Signals Are Not Queued
    Changing Signal Dispositions: sigaction()
    Waiting for a Signal: pause()
    Summary
    Exercises
    Ch. 21. Signals: Signal Handlers
    Ch. 22. Signals: Advanced Features
    Ch. 23. Timers and Sleeping
    Ch. 24. Process Creation
    Ch. 25. Process Termination
    Ch. 26. Monitoring Child Processes
    Ch. 27. Program Execution
    Ch. 28. Process Creation and Program Execution in More Detail
    Ch. 29. Threads: Introduction
    Ch. 30. Threads: Thread Synchronization
    Ch. 31. Threads: Thread Safety and Per-Thread Storage
    Ch. 32. Threads: Thread Cancellation
    Ch. 33. Threads: Further Details
    Ch. 34. Process Groups, Sessions, and Job Control
    Ch. 35. Process Priorities and Scheduling
    Ch. 36. Process Resources
    Ch. 37. Daemons
    Ch. 38. Writing Secure Privileged Programs
    Ch. 39. Capabilities
    Ch. 40. Login Accounting
    Ch. 41. Fundamentals of Shared Libraries
    Ch. 42. Advanced Features of Shared Libraries
    Ch. 43. Interprocess Communication Overview
    Ch. 44. Pipes and FIFOs
    Ch. 45. Introduction to System V IPC
    Ch. 46. System V Message Queues
    Ch. 47. System V Semaphores
    Ch. 48. System V Shared Memory
    Ch. 49. Memory Mappings
    Ch. 50. Virtual Memory Operations
    Ch. 51. Introduction to POSIX IPC
    Ch. 52. POSIX Message Queues
    Ch. 53. POSIX Semaphores
    Ch. 54. POSIX Shared Memory
    Ch. 55. File Locking
    Ch. 56. Sockets: Introduction
    Ch. 57. Sockets: UNIX Domain
    Ch. 58. Sockets: Fundamentals of TCP/IP Networks
    Ch. 59. Sockets: Internet Domains
    Ch. 60. Sockets: Server Design
    Ch. 61. Sockets: Advanced Topics
    Ch. 62. Terminals
    Ch. 63. Alternative I/O Models
    Ch. 64. Pseudoterminals
    Appx. A. Tracing System Calls
    Appx. B. Parsing Command-Line Options
    Appx. C. Casting the NULL Pointer
    Appx. D. Kernel Configuration
    Appx. E. Further Sources of Information
    Appx. F. Solutions to Selected Exercises
    Bibliography
    Appx. G. Updates
    Colophon
    Index
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    20.1. Concepts and Overview

    A signal is a notification to a process that an event has occurred. Signals are sometimes described as software interrupts. Signals are analogous to hardware interrupts in that they interrupt the normal flow of execution of a program; in most cases, it is not possible to predict exactly when a signal will arrive.

    One process can (if it has suitable permissions) send a signal to another process. In this use, signals can be employed as a synchronization technique, or even as a primitive form of interprocess communication (IPC). It is also possible for a process to send a signal to itself. However, the usual source of many signals sent to a process is the kernel. Among the types of events that cause the kernel to generate a signal for a process are the following:


      

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