Signal handler for SIGSEGV.

[vashwath]

Hi all,
In my current project I am using signals for error handling. Since I
cannot show full code, I have just shown important piece of code which
is relevant.

void sigsegenv()
{
printf("\n\n ********** F D S Message ***********\n\n");
printf("\n\n ********** S E G M E N T A T I O N F A U L T
inside F D S ***********\n\n");
exit(-1);
}


int main(void)
{
............
...........
signal ( SIGSEGV ,sigsegenv);
...........
...........
}

I think this might give some idea about what is happening.
My signal handler is not catching SIGSEGV always. Some time it is
catching sometimes not. Please let me know what am I doing wrong

 

[Walter Roberson]

In my current project I am using signals for error handling. Since I
cannot show full code, I have just shown important piece of code which
is relevant.

void sigsegenv()
{
printf("\n\n ********** F D S Message ***********\n\n");
printf("\n\n ********** S E G M E N T A T I O N F A U L T
inside F D S ***********\n\n");
exit(-1);
}

int main(void)
{
...........
..........
signal ( SIGSEGV ,sigsegenv);
..........
..........
}

You cannot use printf() inside a signal handler.

In standard C, other than calling exit, resetting the signal handler,
or a *very* small number of other things, all you can do is set
a volatile variable of sig_atomic_t .

I think this might give some idea about what is happening.
My signal handler is not catching SIGSEGV always. Some time it is
catching sometimes not. Please let me know what am I doing wrong

What evidence have you found that the handler is not catching SIGSEGV ?
If it is just that the printf() are not occuring, then you might
be encountering the restrictions on what you can do in a handler.

 

[Madhav]

You cannot use printf() inside a signal handler.

In standard C, other than calling exit, resetting the signal handler,
or a *very* small number of other things, all you can do is set
a volatile variable of sig_atomic_t .

Could you please tell me about restrictions on signal handlers or where
I can find more info about restrictions on signal handlers?

Thanks,
Madhav.

 

[Richard Heathfield]

Madhav said:

Could you please tell me about restrictions on signal handlers

He just did.

or where I can find more info about restrictions on signal handlers?

That's the whole thing.

 

[vashwath]

In standard C, other than calling exit, resetting the signal handler,

or a *very* small number of other things, all you can do is set
a volatile variable of sig_atomic_t .

Can I invoke longjmp in signal handler?
Actually, In our project we have to return status to the scheduler(Some
what like scheduler) which will execute our program. The scheduler is
in JAVA. The values I should return is 0 (if everything is fine) or
50(warning, If not a very serious error has occurred) or 99(If some
serious error has occurred like segmentation fault).I was thinking to
use longjmp inside signal handler and jump to main and then return
appropriate error status.

What evidence have you found that the handler is not catching SIGSEGV ?

Programming terminated due to segmentation fault.

 

[Richard Heathfield]

(e-mail address removed) said:

Can I invoke longjmp in signal handler?

Not with well-defined behaviour, no.

 

[vashwath]

(e-mail address removed) said:


Not with well-defined behaviour, no.

Is it possible to return certain value to the calling program from
signal handler?My aim is to exit gracefully and to return the
predefined values to the scheduler, which invokes my program.Note that
scheduler is coded using JAVA.
OK, let me put it in a better eay. This is what happens in our project

JAVA progam calls list of C programs. C programs should return the
status(0, 50 or 100) to JAVA. I somehow want to return the status to
JAVA program if there is a segmentation fault. I don't want to exit
from signal handler
..

 

[Richard Heathfield]

(e-mail address removed) said:

Is it possible to return certain value to the calling program from
signal handler?

No. Signal handlers have the form void handler(int); and thus do not return
a value. You can, however, use the signal handler to modify a file scope
object of type sig_atomic_t.

 

[Flash Gordon]

(e-mail address removed) said:


No. Signal handlers have the form void handler(int); and thus do not return
a value. You can, however, use the signal handler to modify a file scope
object of type sig_atomic_t.

I think he meant return a value to the program that invoked his C
program, not return a value in to his program. If I read him right, then
calling exit from within the signal handler might be the way to go,
although whether doing exit(99) is valid is system dependant.

 

[Eric Sosman]

You cannot use printf() inside a signal handler.

In standard C, other than calling exit, resetting the signal handler,
or a *very* small number of other things, all you can do is set
a volatile variable of sig_atomic_t .

Calling exit() in a signal handler is impermissible.
Perhaps you mean _Exit(), a function introduced in C99;
see 7.14.1.1/5. Handlers for signals that were generated
by raise() or abort() are not subject to these restrictions;
handlers for all other signals must respect them.

 

[SM Ryan]

(e-mail address removed) wrote:

# Can I invoke longjmp in signal handler?

Your program is in a fragile state when the signal handler
is called, and it might not be possible to recover and
continue. On some systems, you can run the program in a
separate process; if it fails, it can report failure and
then exit with recovery. The caller monitors the process
and captures the result or error status.

 

[Markus.Elfring]

Could you please tell me about restrictions on signal handlers or where

I can find more info about restrictions on signal handlers?

Please look at the list of async-signal-safe functions.
See section "2.4.3 Signal Actions" from the document " 2.4 Signal
Concepts".
http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_03
http://en.wikipedia.org/wiki/Signal_(computing)

Regards,
Markus

 

[Flash Gordon]

Please look at the list of async-signal-safe functions.
See section "2.4.3 Signal Actions" from the document " 2.4 Signal
Concepts".
http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_03
http://en.wikipedia.org/wiki/Signal_(computing)

The above links are Posix, not C. I can't remember the OP saying that
this was only for Posix systems, and if it was then this would be the
wrong group.

A better place to look would be the C standard, a draft of the latest
version being freely available if you search for n1124.pdf.

 

[Markus.Elfring]

A better place to look would be the C standard, a draft of the latest

version being freely available if you search for n1124.pdf.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/1997/N1124.pdf

This document (ISO/IEC 9899:TC2 Committee Draft - May 6, 2005
WG14/N1124) contains the following wording.
Page 20, section "5.2.3 Signals and interrupts":
"...
Functions shall be implemented such that they may be interrupted at any
time by a signal, or may be called by a signal handler, or both, with
no alteration to earlier, but still active, invocations' control flow
(after the interruption), function return values, or objects with
automatic storage duration.
...."

Page 167, section "7.1.4 Use of library functions":
"...
The functions in the standard library are not guaranteed to be
reentrant and may modify objects with static storage duration.
....
161) Thus, a signal handler cannot, in general, call standard library
functions.
...."

There are more descriptions about signal handling in the sections 7.14
and 7.26.6. But the term "async-signal-safe function" is not used in
the specification. I suggest to look at other information sources for
better understanding of the topic "async-signal safety".
- Multithreaded Programming Guide

http://docs.sun.com/app/docs/doc/816-5137/6mba5vpk6?a=view#compat-ix626
http://docs.sun.com/app/docs/doc/816-5137/6mba5vpk4?a=view#gen-ix561

- http://serpentine.com/blog/threads-faq/signals.html

- http://en.wikipedia.org/wiki/Reentrant
http://en.wikipedia.org/wiki/SIGSEGV

Regards,
Markus