Niall’s Weblog

I have been making very good progress with my BitTorrent implementation recently. However, I have felt the need to make it compile out of the box under systems other than OpenBSD. The first system I have ported to is Linux, specifically Ubuntu 7.10. In doing the port, I found some interesting differences between OpenBSD and Linux.

mmap() differences

Unworkable (not officially released yet, but blog readers can have a sneak peek) uses the mmap() system call rather heavily. For each piece in a torrent, I create at least one corresponding mmap’d segment - more if the piece spans multiple files. OpenBSD’s mmap() accepts arbitrary offsets, which is very straight forward. Linux, however, requires that the offset be aligned to page size. That is, offset must be a multiple of the page size.

On a POSIX operating system, the page size can be queried easily using the sysconf libc function, passing the _SC_PAGESIZE value. Once I have the page size, I find the nearest multiple to the offset we really want, and mmap that. Then I do some pointer arithmetic to get to the ‘real’ offset from there, and do all my reads and writes with a ’synthetic’ pointer into the mmap as the base address. The alignment requirement makes things a little more complicated, but its not too tough to handle.

dirname() / basename()

I use dirname() to ensure that directories exist before I try to write files underneath them - something analogous to mkdir -p. Under OpenBSD, it is safe to call dirname() directly on your path string, and use the returned value. On Linux however, dirname() and basename() modify the string you give it, meaning that you can’t re-use it. This caused me a bit of confusion until I realised what was going on. Under Linux, its necessary to create a copy of your string before passing it to dirname(), so that your original is not mangled.

snprintf()

The snprintf() function is very useful for conveniently building strings in C. I use it especially in Unworkable’s HTTP/1.0 client implementation, to build the query string. Under OpenBSD, its fine to snprintf a variable into itself, e.g.

snprintf(x, XLEN, "%s = %d", x, y);

Under Linux, this doesn’t work - in fact, glibc’s snprintf() fails completely silently in this case, from what I could tell. It was necessary for me to snprintf() into a temporary buffer, and copy that around, rather than just using the snrpintf() on its own, like you can under OpenBSD.

Well, thats it for now. I’m going to write more about the porting process, in terms of adding missing functions and dealing with Make incompatibilities, next.

Tags: BitTorrent, C, Linux, OpenBSD, Porting, UNIX

This entry was posted on Thursday, October 25th, 2007 at 10:11 pm and is filed under Technical. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.