I have written a program to demonstrate a problem I have encountered when using BSD style asynchronous input using the O_ASYNC flag in conjunction with a real time interval timer sending regular SIGALRM signals to the program. The SIGIO handler obeys all safe practices, using only an atomic update to a volatile sig_atomic_t variable.
The program is below. The idea is that when the user enters a 'q', the program is supposed to stop. It does not. If you uncomment the printf statement in teh main program's loop you will see that the loop is exited, but the return statement is never reached because the SIGALRMS seem to keep the program alive. Be prepared to kill it with a Ctrl-C.
If I use the AIO style asynchronous I/O there is no problem. It is related to the O_ASYNC flag. (In fact if you simply call fcntl() and tell the kernel to send signals to the process, without setting the flag, the same problem will occur! )
Can anyone explain what is wrong?
Last edited by Perderabo; 04-10-2008 at 11:30 PM..
Reason: Adding code tags... it's just 13 little keystrokes to readability.
AIX 4.3.3
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Discussion started by: Lauris_k
3 Replies
LEARN ABOUT MOJAVE
ualarm
UALARM(3) BSD Library Functions Manual UALARM(3)NAME
ualarm -- schedule signal after specified time
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
useconds_t
ualarm(useconds_t useconds, useconds_t interval);
DESCRIPTION
This is a simplified interface to setitimer(2).
The ualarm() function waits a count of useconds before asserting the terminating signal SIGALRM. System activity or time used in processing
the call may cause a slight delay.
If the interval argument is non-zero, the SIGALRM signal will be sent to the process every interval microseconds after the timer expires
(e.g., after useconds number of microseconds have passed).
Due to a setitimer(2) restriction, the maximum number of useconds and interval is limited to 100,000,000,000,000 (in case this value fits in
the unsigned integer).
RETURN VALUES
When the signal has successfully been caught, ualarm() returns the amount of time left on the clock.
NOTES
A microsecond is 0.000001 seconds.
SEE ALSO getitimer(2), setitimer(2), sigpause(2), sigvec(2), alarm(3), signal(3), sleep(3), usleep(3)HISTORY
The ualarm() function appeared in 4.3BSD.
BSD April 19, 1994 BSD