mingw/info/gdb/Signals.html

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  19 <h3 class="section">Signals</h3>
  20
  21    <p>A signal is an asynchronous event that can happen in a program.  The
  22 operating system defines the possible kinds of signals, and gives each
  23 kind a name and a number.  For example, in Unix <code>SIGINT</code> is the
  24 signal a program gets when you type an interrupt character (often <kbd>C-c</kbd>);
  25 <code>SIGSEGV</code> is the signal a program gets from referencing a place in
  26 memory far away from all the areas in use; <code>SIGALRM</code> occurs when
  27 the alarm clock timer goes off (which happens only if your program has
  28 requested an alarm).
  29
  30    <p>Some signals, including <code>SIGALRM</code>, are a normal part of the
  31 functioning of your program.  Others, such as <code>SIGSEGV</code>, indicate
  32 errors; these signals are <dfn>fatal</dfn> (they kill your program immediately) if the
  33 program has not specified in advance some other way to handle the signal.
  34 <code>SIGINT</code> does not indicate an error in your program, but it is normally
  35 fatal so it can carry out the purpose of the interrupt: to kill the program.
  36
  37    GDB has the ability to detect any occurrence of a signal in your
  38 program.  You can tell GDB in advance what to do for each kind of
  39 signal.
  40
  41    <p>Normally, GDB is set up to let the non-erroneous signals like
  42 <code>SIGALRM</code> be silently passed to your program
  43 (so as not to interfere with their role in the program's functioning)
  44 but to stop your program immediately whenever an error signal happens.
  45 You can change these settings with the <code>handle</code> command.
  46
  47      <dl>
  48 <dt><code>info signals</code>
  49      <dd><dt><code>info handle</code>
  50      <dd>Print a table of all the kinds of signals and how GDB has been told to
  51 handle each one.  You can use this to see the signal numbers of all
  52 the defined types of signals.
  53
  54      <p><code>info handle</code> is an alias for <code>info signals</code>.
  55
  56      <br><dt><code>handle </code><var>signal</var><code> </code><var>keywords</var><code>...</code>
  57      <dd>Change the way GDB handles signal <var>signal</var>.  <var>signal</var>
  58 can be the number of a signal or its name (with or without the
  59 <code>SIG</code> at the beginning); a list of signal numbers of the form
  60 <code></code><var>low</var><code>-</code><var>high</var><code></code>; or the word <code>all</code>, meaning all the
  61 known signals.  The <var>keywords</var> say what change to make.
  62 </dl>
  63
  64    <p>The keywords allowed by the <code>handle</code> command can be abbreviated.
  65 Their full names are:
  66
  67      <dl>
  68 <dt><code>nostop</code>
  69      <dd>GDB should not stop your program when this signal happens.  It may
  70 still print a message telling you that the signal has come in.
  71
  72      <br><dt><code>stop</code>
  73      <dd>GDB should stop your program when this signal happens.  This implies
  74 the <code>print</code> keyword as well.
  75
  76      <br><dt><code>print</code>
  77      <dd>GDB should print a message when this signal happens.
  78
  79      <br><dt><code>noprint</code>
  80      <dd>GDB should not mention the occurrence of the signal at all.  This
  81 implies the <code>nostop</code> keyword as well.
  82
  83      <br><dt><code>pass</code>
  84      <dd><dt><code>noignore</code>
  85      <dd>GDB should allow your program to see this signal; your program
  86 can handle the signal, or else it may terminate if the signal is fatal
  87 and not handled.  <code>pass</code> and <code>noignore</code> are synonyms.
  88
  89      <br><dt><code>nopass</code>
  90      <dd><dt><code>ignore</code>
  91      <dd>GDB should not allow your program to see this signal.
  92 <code>nopass</code> and <code>ignore</code> are synonyms.
  93 </dl>
  94
  95    <p>When a signal stops your program, the signal is not visible to the
  96 program until you
  97 continue.  Your program sees the signal then, if <code>pass</code> is in
  98 effect for the signal in question <em>at that time</em>.  In other words,
  99 after GDB reports a signal, you can use the <code>handle</code>
 100 command with <code>pass</code> or <code>nopass</code> to control whether your
 101 program sees that signal when you continue.
 102
 103    <p>The default is set to <code>nostop</code>, <code>noprint</code>, <code>pass</code> for
 104 non-erroneous signals such as <code>SIGALRM</code>, <code>SIGWINCH</code> and
 105 <code>SIGCHLD</code>, and to <code>stop</code>, <code>print</code>, <code>pass</code> for the
 106 erroneous signals.
 107
 108    <p>You can also use the <code>signal</code> command to prevent your program from
 109 seeing a signal, or cause it to see a signal it normally would not see,
 110 or to give it any signal at any time.  For example, if your program stopped
 111 due to some sort of memory reference error, you might store correct
 112 values into the erroneous variables and continue, hoping to see more
 113 execution; but your program would probably terminate immediately as
 114 a result of the fatal signal once it saw the signal.  To prevent this,
 115 you can continue with <code>signal 0</code>.  See <a href="Signaling.html#Signaling">Giving your program a signal</a>.
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