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Hi, I am having one doubt, need your help. Can anyone tell me how can we use the
poll() or select() system call with asynchronous mechanism. Let’s take the following example: Let’s say our process is using the
asynchronous mechanism by having the following code: signal(SIGIO, &input_handler); fcntl(STDIN_FILENO, F_SETOWN, getpid()); oflags = fcntl(STDIN_FILENO, F_GETFL); fcntl(STDIN_FILENO, F_SETFL, oflags | FASYNC); In above code, as we can see, we are
registering the signal handler for SIGIO with kernel. In second line it is setting the file
owner to the process owner. In fourth line we are setting the asynchronous flag
on. After this the STDIN (Standard input file,
fd = 0) will deliver the SIGIO signal to our process, whenever some input comes
on input file (when we type something on keyboard). At this instance of time,
control comes to our signal handler, which has been defined in our program and
we can take action according to SIGIO signal. Now my question is: If we do the same
thing for more than one open file, I mean if we enable the asynchronous flag,
as shown above, for more than one open file (let’s say, standard input
file, standard output file and any other regular file as well), how the signal
handler will come to know which file has actually initiated or delivered the
SIGIO signal. As far as I can think and have also read
in Rubini’s book, we can make use of poll() or select() system calls in
signal handler to know which file or set of files has asynchronously delivered
the SIGIO signal, but the problem is if we have two or more than two files
(which has been set for asynchronous notification) which delivers SIGIO at same
time, even poll and select can not help, because poll() and select () system
call returns integer which specifies the number of files which are ready for a
particular event (read or write or anything else we queried for). poll() and
select() system calls do not tell, which particular file or file descriptor has
an event to perform. I hope I have not confused you on this.
Well if anyone of you knows a solution to it, please reply back. Regards, Gaurav |
