View Source: sigaction(2) - Waikato Linux Users Group

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!!NAME
sigaction, sigprocmask, sigpending, sigsuspend - [POSIX] signal handling functions.

!!SYNOPSIS
__#include <signal.h>__

__int sigaction(int __ ''signum'' __, const struct sigaction *__ ''act'' __,__ __struct sigaction *__ ''oldact'' __);__

__int sigprocmask(int __ ''how'' __, const sigset_t *__ ''set'' __, __ __sigset_t *__ ''oldset'' __);__

__int sigpending(sigset_t *__ ''set'' __);__

__int sigsuspend(const sigset_t *__ ''mask'' __);__

!!DESCRIPTION
The __sigaction__ system call is used to change the action taken by a process on receipt of a specific signal.

''signum'' specifies the signal and can be any valid signal except __SIGKILL__ and __SIGSTOP__ .

If ''act'' is non-null, the new action for signal ''signum'' is installed from ''act'' . If ''oldact'' is non-null, the previous action is saved in ''oldact'' .

The __sigaction__ structure is defined as something like



 struct sigaction {
     void (*sa_handler)(int);
     void (*sa_sigaction)(int, siginfo_t *, void *);
     sigset_t sa_mask;
     int sa_flags;
     void (*sa_restorer)(void);
 }




On some architectures a union is involved - do not assign to both ''sa_handler'' and ''sa_sigaction'' .

The ''sa_restorer'' element is obsolete and should not be used. [POSIX] does not specify a ''sa_restorer'' element.

''sa_handler'' specifies the action to be associated with ''signum'' and may be __SIG_DFL__ for the default action,  __SIG_IGN__ to ignore this signal, or a pointer to a signal handling function.

''sa_mask'' gives a mask of signals which should be blocked during execution of the signal handler.  In addition, the signal which triggered the handler will be blocked, unless the __SA_NODEFER__ or __SA_NOMASK__ flags are used.

''sa_flags'' specifies a set of flags which modify the behaviour of the signal handling process. It is formed by the bitwise OR of zero or more of the following:

;__SA_NOCLDSTOP__ : If ''signum'' is [SIGCHLD] ,  do not receive notification when child processes stop (i.e., when child processes receive one of [SIGSTOP] ,  [SIGTSTP] ,  [SIGTTIN] or [SIGTTOU] ).
;__SA_ONESHOT__  or  __SA_RESETHAND__ : Restore the signal action to the default state once the signal handler has been called.  (This is the default behavior of the signal(2) system call.)
;__SA_RESTART__ : Provide behaviour compatible with BSD signal semantics by making certain system calls restartable across signals.
;__SA_NOMASK__  or  __SA_NODEFER__ : Do not prevent the signal from being received from within its own signal handler.
;__SA_SIGINFO__ : The signal handler takes 3 arguments, not one.  In this case, ''sa_sigaction'' should be set instead of ''sa_handler'' . (The sa_sigaction field was added in Linux 2.1.86.)


The ''siginfo_t'' parameter to ''sa_sigaction'' is a struct with the following elements



 siginfo_t {
        int     si_signo;       /* Signal number */
        int     si_errno;       /* An errno value */
        int     si_code;        /* Signal code */
        pid_t   si_pid; /* Sending process ID */
        uid_t   si_uid; /* Real user ID of sending process */
        int     si_status;      /* Exit value or signal */
        clock_t si_utime;       /* User time consumed */
        clock_t si_stime;       /* System time consumed */
        sigval_t        si_value;       /* Signal value */
        int     si_int; /* POSIX.1b signal */
        void *  si_ptr; /* POSIX.1b signal */
        void *  si_addr;        /* Memory location which caused fault */
        int     si_band;        /* Band event */
        int     si_fd;  /* File descriptor */
 }




''si_signo'' ,  ''si_errno''  and  ''si_code'' are defined for all signals. The rest of the struct may be a union, so that one should only read the fields that are meaningful for the given signal. kill(2), POSIX.1b signals and SIGCHLD fill in ''si_pid''  and  ''si_uid'' . SIGCHLD also fills in ''si_status'' ,  ''si_utime''  and  ''si_stime'' . ''si_int''  and  ''si_ptr'' are specified by the sender of the POSIX.1b signal. SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in ''si_addr'' with the address of the fault. SIGPOLL fills in ''si_band''  and  ''si_fd'' .

''si_code'' indicates why this signal was sent.  It is a value, not a bitmask.  The values which are possible for any signal are listed in this table:

||^__si_code__
|Value|Signal origin
|SI_USER|kill, sigsend or raise
|SI_KERNEL|The kernel
|SI_QUEUE|sigqueue
|SI_TIMER|timer expired
|SI_MESGQ|mesq state changed
|SI_ASYNCIO|AIO completed
|SI_SIGIO|queued SIGIO


||^SIGILL
|ILL_ILLOPC|illegal opcode
|ILL_ILLOPN|illegal operand
|ILL_ILLADR|illegal addressing mode
|ILL_ILLTRP|illegal trap
|ILL_PRVOPC|privileged opcode
|ILL_PRVREG|privileged register
|ILL_COPROC|coprocessor error
|ILL_BADSTK|internal stack error




||^SIGFPE
|FPE_INTDIV|integer divide by zero
|FPE_INTOVF|integer overflow
|FPE_FLTDIV|floating point divide by zero
|FPE_FLTOVF|floating point overflow
|FPE_FLTUND|floating point underflow
|FPE_FLTRES|floating point inexact result
|FPE_FLTINV|floating point invalid operation
|FPE_FLTSUB|subscript out of range




||^SIGSEGV
|SEGV_MAPERR|address not mapped to object
|SEGV_ACCERR|invalid permissions for mapped object




||^SIGBUS
|BUS_ADRALN|invalid address alignment
|BUS_ADRERR|non-existant physical address
|BUS_OBJERR|object specific hardware error




||^SIGTRAP
|TRAP_BRKPT|process breakpoint
|TRAP_TRACE|process trace trap




||^SIGCHLD
|CLD_EXITED|child has exited
|CLD_KILLED|child was killed
|CLD_DUMPED|child terminated abnormally
|CLD_TRAPPED|traced child has trapped
|CLD_STOPPED|child has stopped
|CLD_CONTINUED|stopped child has continued




||^SIGPOLL
|POLL_IN|data input available
|POLL_OUT|output buffers available
|POLL_MSG|input message available
|POLL_ERR|i/o error
|POLL_PRI|high priority input available
|POLL_HUP|device disconnected



The __sigprocmask__ call is used to change the list of currently blocked signals. The  behaviour of the call is dependent on the value of ''how'' , as follows.

;__SIG_BLOCK__ : The set of blocked signals is the union of the current set and the ''set'' argument.
;__SIG_UNBLOCK__ : The signals in ''set'' are removed from the current set of blocked signals.  It is legal to attempt to unblock a signal which is not blocked.
;__SIG_SETMASK__ : The set of blocked signals is set to the argument ''set'' .


If ''oldset'' is non-null, the previous value of the signal mask is stored in ''oldset'' .

The __sigpending__ call allows the examination of pending signals (ones which have been raised while blocked).  The signal mask of pending signals is stored in ''set'' .

The __sigsuspend__ call temporarily replaces the signal mask for the process with that given by  ''mask'' and then suspends the process until a signal is received.



!!RETURN VALUE
The functions __sigaction__ , __sigprocmask__ , __sigpending__ and __sigsuspend__ return 0 on success and -1 on error. (In the case of __sigsuspend__ there will be no success, and only the error return with __EINTR__ is possible.)



!!ERRORS

;[EINVAL]: An invalid signal was specified.  This will also be generated if an attempt is made to change the action for [SIGKILL]  or  [SIGSTOP] ,  which cannot be caught.
;[EFAULT]: ''act'' ,  ''oldact'' ,  ''set'' or  ''oldset'' point to memory which is not a valid part of the process address space.
;[EINTR]: System call was interrupted.



!!NOTES
It is not possible to block  [SIGKILL]  or  [SIGSTOP] with the sigprocmask call.  Attempts to do so will be silently ignored.

According to [POSIX], the behaviour of a process is undefined after it ignores a [SIGFPE], [SIGILL], or [SIGSEGV] signal that was not generated by the ''kill()'' or the ''raise()'' functions. Integer division by zero has undefined result. On some architectures it will generate a [SIGFPE] signal. (Also dividing the most negative integer by -1 may generate [SIGFPE].) Ignoring this signal might lead to an endless loop.

[POSIX] (B.3.3.1.3) disallows setting the action for [SIGCHLD] to SIG_IGN. The BSD and SYSV behaviours differ, causing [BSD] software that sets the action for SIGCHLD to SIG_IGN to fail on Linux.

The POSIX spec only defines __SA_NOCLDSTOP__ . Use of other ''sa_flags'' is non-portable.

The __SA_RESETHAND__ flag is compatible with the SVr4 flag of the same name.

The __SA_NODEFER__ flag is compatible with the SVr4 flag of the same name under kernels 1.3.9 and newer.  On older kernels the Linux implementation  allowed the receipt of any signal, not just the one we are installing (effectively overriding any ''sa_mask'' settings).

The __SA_RESETHAND__  and  __SA_NODEFER__ names for SVr4 compatibility are present only in library versions 3.0.9 and greater.

The __SA_SIGINFO__ flag is specified by POSIX.1b.  Support for it was added in Linux 2.2.

__sigaction__ can be called with a null second argument to query the current signal handler. It can also be used to check whether a given signal is valid for the current machine by calling it with null second and third arguments.

See sigsetops(3) for details on manipulating signal sets.

!!CONFORMING TO
POSIX, SVr4.  SVr4 does not document the [EINTR] condition.



!!UNDOCUMENTED
Before the introduction of __SA_SIGINFO__ it was also possible to get some additional information, namely by using a sa_handler with second argument of type ''struct sigcontext'' . See the relevant kernel sources for details. This use is obsolete now.



!!SEE ALSO
kill(1), kill(2), killpg(2), pause(2), raise(3), siginterrupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)
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Last edited on Friday, July 2, 2004 3:37:55 pm by JohnMcPherson
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