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Ref-docs/syscalls/syscalls_99.html
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Ref-docs/syscalls/syscalls_99.html
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<html><!-- This HTML file has been created by texi2html 1.29
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from syscalls.texi on 4 June 1994 -->
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<TITLE>Syscall specifications of Linux - sigaction</TITLE>
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<P>Go to the <A HREF="syscalls_98.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_98.html">previous</A>, <A HREF="syscalls_100.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_100.html">next</A> section.<P>
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<H2><A NAME="SEC99" HREF="syscalls_toc.html#SEC99" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_toc.html#SEC99">sigaction and signal</A></H2>
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<P>
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<H3>SYNOPSIS</H3>
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<P>
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<CODE>int sigaction(int <VAR>signum</VAR>, const struct sigaction *<VAR>new</VAR>,
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struct sigaction *<VAR>old</VAR>);</CODE>
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<P>
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<CODE>(void *) (int) signal(int <VAR>signum</VAR>, (void *<VAR>handler</VAR>)(int));</CODE>
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<P>
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<H3>PARAMETERS</H3>
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<P>
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<VAR>signum</VAR>: [in] a signal number.
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<P>
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<VAR>new</VAR>: [in] the action to take for this signal.
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<P>
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<VAR>old</VAR>: [out] the previous action that was associated to the signal.
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<P>
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<VAR>handler</VAR>: [in] points to the new signal handler.
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<P>
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<H3>DESCRIPTION</H3>
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<P>
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<CODE>sigaction</CODE> is used to specify the action to take in case the
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signal <VAR>signum</VAR> is raised. <VAR>signum</VAR> can take one of the
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following values:
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<P>
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<DL COMPACT>
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<DT><CODE>SIGHUP</CODE>
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<DD>(terminates) the controling terminal has been disconnected.
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<P>
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<DT><CODE>SIGINT</CODE>
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<DD>(terminates) the interrupt key has been pressed (Ctrl-C usually).
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<P>
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<DT><CODE>SIGQUIT</CODE>
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<DD>(terminates and dumps core) the quit key has been pressed (Ctrl-\ usually).
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<P>
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<DT><CODE>SIGILL</CODE>
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<DD>(terminates and dumps core) the task has executed an illegal instruction
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(or is ill?).
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<P>
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<DT><CODE>SIGTRAP</CODE>
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<DD>(terminates and dumps core) caught an hardware fault.
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<P>
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<DT><CODE>SIGABRT</CODE>
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<DD>(terminates and dumps core) usually generated by the <CODE>abort</CODE> function.
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<P>
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<DT><CODE>SIGIOT</CODE>
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<DD>(terminates and dumps core) caught an hardware fault.
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<P>
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<DT><CODE>SIGUNUSED</CODE>
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<DD>(terminates) unused.
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<P>
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<DT><CODE>SIGFPE</CODE>
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<DD>(terminates and dumps core) the task has generated a floating-point exception.
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<P>
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<DT><CODE>SIGKILL</CODE>
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<DD>(terminates) kills the task. This signal can't be caught or ignored.
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<P>
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<DT><CODE>SIGUSR1</CODE>
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<DD>(terminates) user defined signal.
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<P>
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<DT><CODE>SIGSEGV</CODE>
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<DD>(terminates and dumps core) the task has made an invalid memory reference.
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<P>
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<DT><CODE>SIGUSR2</CODE>
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<DD>(terminates) user defined signal.
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<P>
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<DT><CODE>SIGPIPE</CODE>
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<DD>(terminates) the task tried to write to a pipe/socket but the reader is dead.
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<P>
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<DT><CODE>SIGALRM</CODE>
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<DD>(terminates) an alarm has expired.
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<P>
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<DT><CODE>SIGTERM</CODE>
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<DD>(terminates) termination signal.
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<P>
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<DT><CODE>SIGSTKFLT</CODE>
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<DD>??
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<P>
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<DT><CODE>SIGCHLD</CODE>
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<DD>(ignore) a child has died.
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<P>
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<DT><CODE>SIGCONT</CODE>
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<DD>(ignored or resumes) send to a stopped task to make it resume its execution.
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<P>
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<DT><CODE>SIGSTOP</CODE>
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<DD>(stops) send to a task to make it stop its execution. This signal can't
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be caught or ignored.
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<P>
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<DT><CODE>SIGTSTP</CODE>
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<DD>(stops) interactive stop signal generated by the terminal (usually Ctrl-Z).
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<P>
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<DT><CODE>SIGTTIN</CODE>
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<DD>(stops) the process is in background and tried to read from its
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controling terminal.
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<P>
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<DT><CODE>SIGTTOU</CODE>
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<DD>(stops) the process is in background and tried to write to its controling
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terminal.
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<P>
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<DT><CODE>SIGIO</CODE>
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<DD>(terminates) an asynchronous I/O event has occured.
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<P>
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<DT><CODE>SIGPOLL</CODE>
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<DD>(terminates) an event has occurend on a pooling device.
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<P>
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<DT><CODE>SIGURG</CODE>
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<DD>(terminates) an urgent condition has occured. (Usually the reception of out-of-band
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data on a network connection.)
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<P>
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<DT><CODE>SIGXCPU</CODE>
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<DD>(terminates) the task has exceeded its CPU time limit.
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<P>
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<DT><CODE>SIGXFSZ</CODE>
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<DD>(terminates) the taks exceeds its size limit.
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<P>
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<DT><CODE>SIGVTALRM</CODE>
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<DD>(terminates) a virtual interval timer has expired.
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<P>
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<DT><CODE>SIGPROF</CODE>
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<DD>(terminates) a profiling interval timer has expired.
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<P>
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<DT><CODE>SIGWINCH</CODE>
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<DD>(ignored) the controling terminal size has changed.
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<P>
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<DT><CODE>SIGPWR</CODE>
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<DD>(terminates) power failure: run for cover!
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<P>
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<DT><CODE>SIGBUS</CODE>
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<DD>(terminates) a hardware fault has occured. If we look in the kernel
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sources, we see: Arggh. Bad user source code wants this.
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</DL>
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<P>
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There is also a signal <CODE>SIGLOST</CODE> in the sources, however it is
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commented out.
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<P>
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The action to take is specified by a <CODE>sigcaction</CODE> structure that
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has the following layout:
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<P>
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<PRE>
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struct sigaction {
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__sighandler_t sa_handler; /* the handler (or a special value) */
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sigset_t sa_mask; /* signals to block on entry */
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int sa_flags; /* some flags */
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void (*sa_restorer)(void); /* not used */
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};
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</PRE>
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<P>
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<CODE>sa_handler</CODE> may be set to the address of the handler to start to
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handle the signal or it may be set to one of the following special
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values:
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<P>
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<DL COMPACT>
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<DT><CODE>SIG_ING</CODE>
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<DD>means to ignore the signal.
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<P>
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<DT><CODE>SIG_DFL</CODE>
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<DD>means to use the default handler.
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</DL>
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<P>
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<VAR>sa_mask</VAR> specifies signals to be added to the signal mask of the
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process before calling the signal handler. The signal mask is restored
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to its initial value upon return from the handler.
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<P>
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<VAR>sa_flags</VAR> specifies some options for the handling of the signal:
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<P>
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<DL COMPACT>
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<DT><CODE>SA_NOCLDSTOP</CODE>
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<DD>disable the generation of the <CODE>SIGCHLD</CODE> signal when a child stops.
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<VAR>signum</VAR> must be <CODE>SIGCHLD</CODE>.
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<P>
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<DT><CODE>SA_RESTART</CODE>
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<DD>ask for automatic restart of system calls interrupted by this signal.
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<P>
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<DT><CODE>SA_STACK</CODE>
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<DD>not implemented.
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<P>
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<DT><CODE>SA_INTERRUPT</CODE>
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<DD>ignored.
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<P>
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<DT><CODE>SA_NOMASK</CODE>
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<DD>do not mask anything (not even the current signal). (Looks like
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<CODE>SA_NODEFER</CODE> in [Stevens].)
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<P>
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<DT><CODE>SA_ONESHOT</CODE>
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<DD>the handler will be used only once. (Looks like
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<CODE>SA_RESETHAND</CODE> in [Stevens].)
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</DL>
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<P>
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The previous value of the <CODE>sigaction</CODE> structure for the signal is
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stored to the area pointed to by <VAR>old</VAR>.
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<P>
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<CODE>signal</CODE> is some kind of proto-<CODE>sigaction</CODE>. It sets the
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handler of <VAR>signum</VAR> to <VAR>handler</VAR>. It is equivalent to
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<P>
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<PRE>
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struct sigaction new;
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new.sa_handler=handler;
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new.sa_mask=0;
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new.sa_flags=SA_NOMASK | SA_ONESHOT;
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new.sa_restorer=NULL;
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sigaction(signum, &new, NULL);
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</PRE>
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<P>
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It could be implemented as a <CODE>sigaction</CODE> wrapper.
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<P>
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<H3>RETURN VALUE</H3>
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<P>
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On success, <CODE>sigaction</CODE> returns zero and <CODE>signal</CODE> returns the
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pointer to the signal handler. On error, both return -1. The possible
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values of <CODE>errno</CODE> are:
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<P>
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<UL>
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<LI><CODE>EINVAL</CODE>: tried to set the handler of <CODE>SIGKILL</CODE> or
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<CODE>SIGSTOP</CODE>.
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<LI><CODE>EFAULT</CODE>.
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</UL>
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<P>
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<P>Go to the <A HREF="syscalls_98.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_98.html">previous</A>, <A HREF="syscalls_100.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_100.html">next</A> section.<P>
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