| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Free Software" and "Free Software Needs |
| Free Documentation", with the Front-Cover Texts being "A GNU Manual," |
| and with the Back-Cover Texts as in (a) below. |
| |
| (a) The FSF's Back-Cover Text is: "You are free to copy and modify |
| this GNU Manual. Buying copies from GNU Press supports the FSF in |
| developing GNU and promoting software freedom." --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Debugging with GDB: Interrupted System Calls</title> |
| |
| <meta name="description" content="Debugging with GDB: Interrupted System Calls"> |
| <meta name="keywords" content="Debugging with GDB: Interrupted System Calls"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Thread-Stops.html#Thread-Stops" rel="up" title="Thread Stops"> |
| <link href="Observer-Mode.html#Observer-Mode" rel="next" title="Observer Mode"> |
| <link href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints" rel="prev" title="Thread-Specific Breakpoints"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Interrupted-System-Calls"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Observer-Mode.html#Observer-Mode" accesskey="n" rel="next">Observer Mode</a>, Previous: <a href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints" accesskey="p" rel="prev">Thread-Specific Breakpoints</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Interrupted-System-Calls-1"></a> |
| <h4 class="subsection">5.5.5 Interrupted System Calls</h4> |
| |
| <a name="index-thread-breakpoints-and-system-calls"></a> |
| <a name="index-system-calls-and-thread-breakpoints"></a> |
| <a name="index-premature-return-from-system-calls"></a> |
| <p>There is an unfortunate side effect when using <small>GDB</small> to debug |
| multi-threaded programs. If one thread stops for a |
| breakpoint, or for some other reason, and another thread is blocked in a |
| system call, then the system call may return prematurely. This is a |
| consequence of the interaction between multiple threads and the signals |
| that <small>GDB</small> uses to implement breakpoints and other events that |
| stop execution. |
| </p> |
| <p>To handle this problem, your program should check the return value of |
| each system call and react appropriately. This is good programming |
| style anyways. |
| </p> |
| <p>For example, do not write code like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> sleep (10); |
| </pre></div> |
| |
| <p>The call to <code>sleep</code> will return early if a different thread stops |
| at a breakpoint or for some other reason. |
| </p> |
| <p>Instead, write this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> int unslept = 10; |
| while (unslept > 0) |
| unslept = sleep (unslept); |
| </pre></div> |
| |
| <p>A system call is allowed to return early, so the system is still |
| conforming to its specification. But <small>GDB</small> does cause your |
| multi-threaded program to behave differently than it would without |
| <small>GDB</small>. |
| </p> |
| <p>Also, <small>GDB</small> uses internal breakpoints in the thread library to |
| monitor certain events such as thread creation and thread destruction. |
| When such an event happens, a system call in another thread may return |
| prematurely, even though your program does not appear to stop. |
| </p> |
| |
| |
| |
| </body> |
| </html> |
