How do you catch a signal without terminating the process ?

[Kevin L. McBride]

In article <1991Jun7.145102.24125 at thunder.mcrcim.mcgill.edu> mouse at thunder.mcrcim.mcgill.edu (der Mouse) writes:
>In article <1991Jun5.215644.20581 at csl.dl.nec.com>, manohar at csl.dl.nec.com (mun o her) writes:
>
>> Server1 = Original Server ( something similar to inetd)
>> Server2 = Effective current Server.
>
>> Assume that we are writing a program to implement a concurrent
>> server.  Assume that Server1 spawns a Server2 when the client wants
>> the connection and Server1 goes back to the accept state.  When the
>> Server2 terminates as a result of the termination of client.  The
>> Server1 must be notified of its death so that it can execute wait( to
>> get rid of zombie of Server2).  If Signal Handler is used to catch
>> SIGCHLD the accept (Server1) is interrupted
>
>This is normal, or at least not abnormal, depending on your system.
>
>> and the PROBLEM is that the 'whole program' dies.
>
>This is a bug in the server.
>
>> The intention is to resume accept state of Server1.  The question is
>> How do you PREVENT Server1 from [dying] and resume accept state.
>
>Fix the bug.
>
>There is nothing about SIGCHLD that should kill off the server.  I
>assume you have set up a handler for SIGCHLD; the handler should wait
>for the dead child.  (The details of this differ from system to system,
>and you don't say what your system is, so I can't elaborate.)
>
>Most likely the code assumes that any error return from accept() should
>be fatal.  This is the bug; if accept() returns with errno indicating
>EINTR, you should simply go back and retry the accept().

Another likely possibility (that bit me in the ass recently), is that
the signal handler is doing a signal() (or something similar) to
re-instate itself as the signal handler BEFORE doing the wait() on the
child corpse.

This will, of course, cause the immediate re-issuance of the interrupt
and your process will die from a memory fault or segment violation due
to a stack blasting infinite recursion.

Remember, never re-enable an interrupt until the condition that caused
the interrupt has been cleared.

After 14 years one would think I'd have learned, but we all @#$% up
occasionally :-)

--
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