1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- Copyright (C) 1988-2021 Free Software Foundation, Inc. 4 5Permission is granted to copy, distribute and/or modify this document 6under the terms of the GNU Free Documentation License, Version 1.3 or 7any later version published by the Free Software Foundation; with the 8Invariant Sections being "Free Software" and "Free Software Needs 9Free Documentation", with the Front-Cover Texts being "A GNU Manual," 10and with the Back-Cover Texts as in (a) below. 11 12(a) The FSF's Back-Cover Text is: "You are free to copy and modify 13this GNU Manual. 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If one thread stops for a 78breakpoint, or for some other reason, and another thread is blocked in a 79system call, then the system call may return prematurely. This is a 80consequence of the interaction between multiple threads and the signals 81that <small>GDB</small> uses to implement breakpoints and other events that 82stop execution. 83</p> 84<p>To handle this problem, your program should check the return value of 85each system call and react appropriately. This is good programming 86style anyways. 87</p> 88<p>For example, do not write code like this: 89</p> 90<div class="smallexample"> 91<pre class="smallexample"> sleep (10); 92</pre></div> 93 94<p>The call to <code>sleep</code> will return early if a different thread stops 95at a breakpoint or for some other reason. 96</p> 97<p>Instead, write this: 98</p> 99<div class="smallexample"> 100<pre class="smallexample"> int unslept = 10; 101 while (unslept > 0) 102 unslept = sleep (unslept); 103</pre></div> 104 105<p>A system call is allowed to return early, so the system is still 106conforming to its specification. But <small>GDB</small> does cause your 107multi-threaded program to behave differently than it would without 108<small>GDB</small>. 109</p> 110<p>Also, <small>GDB</small> uses internal breakpoints in the thread library to 111monitor certain events such as thread creation and thread destruction. 112When such an event happens, a system call in another thread may return 113prematurely, even though your program does not appear to stop. 114</p> 115 116 117 118</body> 119</html> 120