| <html lang="en"> |
| <head> |
| <title>Interrupted Primitives - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Signal-Handling.html#Signal-Handling" title="Signal Handling"> |
| <link rel="prev" href="Defining-Handlers.html#Defining-Handlers" title="Defining Handlers"> |
| <link rel="next" href="Generating-Signals.html#Generating-Signals" title="Generating Signals"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 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 Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Interrupted-Primitives"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Generating-Signals.html#Generating-Signals">Generating Signals</a>, |
| Previous: <a rel="previous" accesskey="p" href="Defining-Handlers.html#Defining-Handlers">Defining Handlers</a>, |
| Up: <a rel="up" accesskey="u" href="Signal-Handling.html#Signal-Handling">Signal Handling</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">24.5 Primitives Interrupted by Signals</h3> |
| |
| <p>A signal can arrive and be handled while an I/O primitive such as |
| <code>open</code> or <code>read</code> is waiting for an I/O device. If the signal |
| handler returns, the system faces the question: what should happen next? |
| |
| <p>POSIX specifies one approach: make the primitive fail right away. The |
| error code for this kind of failure is <code>EINTR</code>. This is flexible, |
| but usually inconvenient. Typically, POSIX applications that use signal |
| handlers must check for <code>EINTR</code> after each library function that |
| can return it, in order to try the call again. Often programmers forget |
| to check, which is a common source of error. |
| |
| <p>The GNU library provides a convenient way to retry a call after a |
| temporary failure, with the macro <code>TEMP_FAILURE_RETRY</code>: |
| |
| <!-- unistd.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: <b>TEMP_FAILURE_RETRY</b> (<var>expression</var>)<var><a name="index-TEMP_005fFAILURE_005fRETRY-2940"></a></var><br> |
| <blockquote><p>This macro evaluates <var>expression</var> once, and examines its value as |
| type <code>long int</code>. If the value equals <code>-1</code>, that indicates a |
| failure and <code>errno</code> should be set to show what kind of failure. |
| If it fails and reports error code <code>EINTR</code>, |
| <code>TEMP_FAILURE_RETRY</code> evaluates it again, and over and over until |
| the result is not a temporary failure. |
| |
| <p>The value returned by <code>TEMP_FAILURE_RETRY</code> is whatever value |
| <var>expression</var> produced. |
| </p></blockquote></div> |
| |
| <p>BSD avoids <code>EINTR</code> entirely and provides a more convenient |
| approach: to restart the interrupted primitive, instead of making it |
| fail. If you choose this approach, you need not be concerned with |
| <code>EINTR</code>. |
| |
| <p>You can choose either approach with the GNU library. If you use |
| <code>sigaction</code> to establish a signal handler, you can specify how that |
| handler should behave. If you specify the <code>SA_RESTART</code> flag, |
| return from that handler will resume a primitive; otherwise, return from |
| that handler will cause <code>EINTR</code>. See <a href="Flags-for-Sigaction.html#Flags-for-Sigaction">Flags for Sigaction</a>. |
| |
| <p>Another way to specify the choice is with the <code>siginterrupt</code> |
| function. See <a href="BSD-Handler.html#BSD-Handler">BSD Handler</a>. |
| |
| <!-- !!! not true now about _BSD_SOURCE --> |
| <p>When you don't specify with <code>sigaction</code> or <code>siginterrupt</code> what |
| a particular handler should do, it uses a default choice. The default |
| choice in the GNU library depends on the feature test macros you have |
| defined. If you define <code>_BSD_SOURCE</code> or <code>_GNU_SOURCE</code> before |
| calling <code>signal</code>, the default is to resume primitives; otherwise, |
| the default is to make them fail with <code>EINTR</code>. (The library |
| contains alternate versions of the <code>signal</code> function, and the |
| feature test macros determine which one you really call.) See <a href="Feature-Test-Macros.html#Feature-Test-Macros">Feature Test Macros</a>. |
| <a name="index-EINTR_002c-and-restarting-interrupted-primitives-2941"></a><a name="index-restarting-interrupted-primitives-2942"></a><a name="index-interrupting-primitives-2943"></a><a name="index-primitives_002c-interrupting-2944"></a><!-- !!! want to have @cindex system calls @i{see} primitives [no page #] --> |
| |
| <p>The description of each primitive affected by this issue |
| lists <code>EINTR</code> among the error codes it can return. |
| |
| <p>There is one situation where resumption never happens no matter which |
| choice you make: when a data-transfer function such as <code>read</code> or |
| <code>write</code> is interrupted by a signal after transferring part of the |
| data. In this case, the function returns the number of bytes already |
| transferred, indicating partial success. |
| |
| <p>This might at first appear to cause unreliable behavior on |
| record-oriented devices (including datagram sockets; see <a href="Datagrams.html#Datagrams">Datagrams</a>), |
| where splitting one <code>read</code> or <code>write</code> into two would read or |
| write two records. Actually, there is no problem, because interruption |
| after a partial transfer cannot happen on such devices; they always |
| transfer an entire record in one burst, with no waiting once data |
| transfer has started. |
| |
| </body></html> |
| |
