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Minix/CD-ROM-2.0/MINIX/MANUALS/MAN2/SIGACTIO.2
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Minix/CD-ROM-2.0/MINIX/MANUALS/MAN2/SIGACTIO.2
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.TH SIGACTION 2
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.SH NAME
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sigaction, signal \- manage signal state and handlers
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.SH SYNOPSIS
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.ft B
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#include <signal.h>
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.in +5
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.ti -5
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int sigaction(int \fIsig\fP, const struct sigaction *\fIact\fP, struct sigaction *\fIoact\fP)
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.in -5
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.br
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void (*signal(int \fIsig\fP, void (*\fIhandler\fP)(int)))(int);
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.ft P
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.SH DESCRIPTION
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.de SP
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.if t .sp 0.4
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.if n .sp
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..
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.B Sigaction()
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is used to examine, set, or modify the attributes of a signal. The argument
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.I sig
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is the signal in question. The
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.I act
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argument points to a structure containing the new attributes of the signal,
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the structure pointed to by
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.I oact
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will receive the old attributes that were in effect before the call.
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.PP
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The
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.I act
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and
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.I oact
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arguments may be
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.B NULL
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to indicate that either no new attributes are to be set, or that the old
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attributes are not of interest.
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.PP
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The structure containing the signal attributes is defined in <signal.h> and
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looks like this:
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.PP
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.RS
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.nf
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.ft B
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.ta +4n +12n
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struct sigaction {
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void (*sa_handler)(int sig);
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sigset_t sa_mask;
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int sa_flags;
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};
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.ft R
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.fi
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.RE
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.PP
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The
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.B sa_handler
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field contains the address of a signal handler, a function that is called
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when the process is signalled, or one of these special constants:
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.PP
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.TP 12
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.B SIG_DFL
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Default signal handling is to be performed. This usually means that the
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process is killed, but some signals may be ignored by default.
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.TP
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.B SIG_IGN
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Ignore the signal.
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.PP
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The
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.B sa_mask
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field indicates a set of signals that must be blocked when the signal is
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being handled. Whether the signal
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.I sig
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itself is blocked when being handled is not controlled by this mask. The
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mask is of a "signal set" type that is to be manipulated by the
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.BR sigset (3)
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functions.
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.PP
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How the signal is handled precisely is specified by bits in
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.BR sa_flags .
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If none of the flags is set then the handler is called when the signal
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arrives. The signal is blocked during the call to the handler, and
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unblocked when the handler returns. A system call that is interrupted
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returns
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.B \-1
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with
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.B errno
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set to
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.BR EINTR .
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The following bit flags can be set to modify this behaviour:
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.PP
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.TP 15
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.B SA_RESETHAND
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Reset the signal handler to
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.B SIG_DFL
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when the signal is caught.
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.TP
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.B SA_NODEFER
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Do not block the signal on entry to the handler.
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.TP
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.B SA_COMPAT
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Handle the signal in a way that is compatible with the the old
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.B signal()
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call.
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.PP
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The old
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.B signal()
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signal system call sets a signal handler for a given signal and returns the
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old signal handler. No signals are blocked, the flags are
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.BR "SA_RESETHAND | SA_NODEFER | SA_COMPAT" .
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New code should not use
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.BR signal() .
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Note that
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.B signal()
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and all of the
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.B SA_*
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flags are Minix extensions.
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.PP
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Signal handlers are reset to
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.B SIG_DFL
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on an
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.BR execve (2).
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Signals that are ignored stay ignored.
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.SS Signals
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Minix knows about the following signals:
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.PP
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.nf
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.ta +11n +7n +8n
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signal num notes description
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.SP
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SIGHUP 1 k Hangup
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SIGINT 2 k Interrupt (usually DEL or CTRL\-C)
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SIGQUIT 3 kc Quit (usually CTRL\-\e)
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SIGILL 4 kc Illegal instruction
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SIGTRAP 5 xkc Trace trap
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SIGABRT 6 kc Abort program
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SIGFPE 8 k Floating point exception
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SIGKILL 9 k Kill
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SIGUSR1 10 k User defined signal #1
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SIGSEGV 11 kc Segmentation fault
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SIGUSR2 12 k User defined signal #2
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SIGPIPE 13 k Write to a pipe with no reader
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SIGALRM 14 k Alarm clock
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SIGTERM 15 k Terminate (default for kill(1))
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SIGCHLD 17 pvi Child process terminated
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SIGCONT 18 p Continue if stopped
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SIGSTOP 19 ps Stop signal
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SIGTSTP 20 ps Interactive stop signal
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SIGTTIN 21 ps Background read
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SIGTTOU 22 ps Background write
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SIGWINCH 23 xvi Window size change
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.ft R
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.fi
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.PP
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The letters in the notes column indicate:
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.PP
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.TP 5
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.B k
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The process is killed if the signal is not caught.
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.TP
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.B c
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The signal causes a core dump.
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.TP
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.B i
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The signal is ignored if not caught.
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.TP
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.B v
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Only Minix-vmd implements this signal.
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.TP
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.B x
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Minix extension, not defined by \s-2POSIX\s+2.
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.TP
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.B p
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These signals are not implemented, but \s-2POSIX\s+2 requires that they are
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defined.
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.TP
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.B s
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The process should be stopped, but is killed instead.
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.PP
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The
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.B SIGKILL
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signal cannot be caught or ignored. The
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.B SIGILL
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and
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.B SIGTRAP
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signals cannot be automatically reset. The system silently enforces these
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restrictions. This may or may not be reflected by the attributes of these
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signals and the signal masks.
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.SS Types
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\s-2POSIX\s+2 prescribes that <sys/types.h> has the following definition:
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.PP
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.RS
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.B "typedef int (*sighandler_t)(int)"
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.RE
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.PP
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With this type the following declarations can be made:
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.PP
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.RS
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.ft B
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.nf
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sighandler_t sa_handler;
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sighandler_t signal(int \fIsig\fP, sighandler_t \fIhandler\fP);
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.fi
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.ft R
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.RE
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.PP
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This may help you to understand the earlier declarations better. The
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.B sighandler_t
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type is also very useful in old style C code that is compiled by a compiler
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for standard C.
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.SH "SEE ALSO"
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.BR kill (1),
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.BR kill (2),
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.BR pause (2),
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.BR sigprocmask (2),
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.BR sigsuspend (2),
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.BR sigpending (2),
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.BR sigset (3).
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.SH DIAGNOSTICS
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.B Sigaction()
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returns
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.B 0
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on success or
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.B \-1
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on error.
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.B Signal()
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returns the old handler on success or
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.B SIG_ERR
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on error. The error code may be:
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.PP
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.TP 10
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.B EINVAL
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Bad signal number.
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.TP
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.B EFAULT
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Bad
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.I act
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or
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.I oact
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addresses.
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.SH AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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