[asio] Problem Assignin a File Descriptor to posix:basic_stream_descriptor
Hi All, I want to use Boost ASIO as an event manager in a server which, among other things, asynchronously writes into a file. I use open() to create my file and then try to assign it to a posix::stream_descriptor object. However, the following simple code throws an "Operation not permitted" exception. int main () { boost::asio::io_service ioService; int fd = open("./asio_sample.txt", O_WRONLY | O_CREAT | O_ASYNC | O_NONBLOCK, S_IRUSR | S_IWUSR | S_IRGRP); assert(fd != -1); boost::asio::posix::stream_descriptor descriptor(ioService); descriptor.assign(fd); return 0; } I traced down the epoll_reactor::register_descriptor() where the function fails to add the descriptor to the poll list. Any idea what I am doing wrong? I would appreciate any comment. Cheers, Nader
On Wed, 2010-03-17 at 14:42 -0700, Nader Salehi wrote:
I traced down the epoll_reactor::register_descriptor() where the function fails to add the descriptor to the poll list.
Any idea what I am doing wrong? I would appreciate any comment.
Filesystem fds do not support epoll, as they lack the poll function internally in the linux kernel. otoh, select does supports filesystem fds. If you do an strace of your sample program, you will see that the call to epoll_ctl fails with the error number EPERM. Yes, the value chosen is quite unfortunate, I always thought it would have been better to return ENOTSUP or EOPNOTSUPP... anyhow, that means that epoll is rejecting the fd you supplied, in this case because it was a filesystem fd and in Linux, this type of fd lacks a poll function pointer in its file_operations structure. I remember looking through the epoll sources in an older version of the kernel, and finding that this was the only reason that EPERM is returned as an error. You may have to find another method of managing IO from and to the file, perhaps you can use another standard or asio provided stream type? Thanks Nick
I see. At this time the path of least resistance would be to use libevent which allows me --albiet awkwardly via setenv()-- to choose the polling method. Thanks for the reply. Nader On 3/17/2010 21:19 Nick Jones writes:
On Wed, 2010-03-17 at 14:42 -0700, Nader Salehi wrote:
I traced down the epoll_reactor::register_descriptor() where the function fails to add the descriptor to the poll list.
Any idea what I am doing wrong? I would appreciate any comment.
Filesystem fds do not support epoll, as they lack the poll function internally in the linux kernel. otoh, select does supports filesystem fds.
If you do an strace of your sample program, you will see that the call to epoll_ctl fails with the error number EPERM. Yes, the value chosen is quite unfortunate, I always thought it would have been better to return ENOTSUP or EOPNOTSUPP... anyhow, that means that epoll is rejecting the fd you supplied, in this case because it was a filesystem fd and in Linux, this type of fd lacks a poll function pointer in its file_operations structure. I remember looking through the epoll sources in an older version of the kernel, and finding that this was the only reason that EPERM is returned as an error.
You may have to find another method of managing IO from and to the file, perhaps you can use another standard or asio provided stream type?
Thanks
Nick
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Correction! There's actually a better solution than what I said in my previous reply; You could explicitly disable 'epoll' in asio and switch to 'select' by defining BOOST_ASIO_DISABLE_EPOLL. Cheers, Nader On 3/18/2010 10:22 Nader Salehi writes:
I see. At this time the path of least resistance would be to use libevent which allows me --albiet awkwardly via setenv()-- to choose the polling method.
Thanks for the reply.
Nader
On 3/17/2010 21:19 Nick Jones writes:
On Wed, 2010-03-17 at 14:42 -0700, Nader Salehi wrote:
I traced down the epoll_reactor::register_descriptor() where the function fails to add the descriptor to the poll list.
Any idea what I am doing wrong? I would appreciate any comment.
Filesystem fds do not support epoll, as they lack the poll function internally in the linux kernel. otoh, select does supports filesystem fds.
If you do an strace of your sample program, you will see that the call to epoll_ctl fails with the error number EPERM. Yes, the value chosen is quite unfortunate, I always thought it would have been better to return ENOTSUP or EOPNOTSUPP... anyhow, that means that epoll is rejecting the fd you supplied, in this case because it was a filesystem fd and in Linux, this type of fd lacks a poll function pointer in its file_operations structure. I remember looking through the epoll sources in an older version of the kernel, and finding that this was the only reason that EPERM is returned as an error.
You may have to find another method of managing IO from and to the file, perhaps you can use another standard or asio provided stream type?
Thanks
Nick
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