KeyboardInterrupt eats my error and then won't be caught
philip at semanchuk.com
Thu Jun 18 23:19:09 EDT 2009
I need help understanding how Python deals with Ctrl-C.
A user has reported a bug in my posix_ipc module. When a Python app is
waiting to acquire an IPC semaphore and the user hits Ctrl-C, my code
should return a custom error indicating that the semaphore wait was
interrupted by a signal.
However, the caller never sees the error I set. Instead they get a
KeyboardInterrupt that refuses to be caught by try/except. Here's a
sample program that demonstrates the problem when run from the command
sem = posix_ipc.Semaphore(None, posix_ipc.O_CREX)
sem.acquire() # User hits Ctrl + C while this is waiting
print "********* I caught it!"
I expected that code to raise a posix_ipc.Error with the text, "The
wait was interrupted by a signal" which would then be trapped by the
except statement which would print the "I caught it!" message.
Instead a KeyboardInterrupt error is propagated up to the interpreter
and the process is killed as if the try/except wasn't there at all.
I have verified that the C function sem_wait() returns -1 (failure),
that errno is set to EINTR and that my detects that properly. So far,
so good. PyErr_Occurred() returns NULL at that point. So my code calls
PyErr_SetString() to set a custom error for the caller and returns
NULL. It's apparently at some point after that that the
KeyboardInterrupt error is being set.
If I substitute my sysv_ipc module for posix_ipc (very similar to
posix_ipc but uses Sys V semaphores instead of POSIX), I get the same
I see this w/Python 2.5 under OS X and also w/Python 2.5 under Ubuntu
If anyone wants to look at my C code, the relevant case statement is
on line 555 of posix_ipc_module.c.
Any suggestions would be appreciated.
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