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| author | Eric S. Raymond <esr@thyrsus.com> | 2009-01-08 16:35:02 +0000 |
|---|---|---|
| committer | Eric S. Raymond <esr@thyrsus.com> | 2009-01-08 16:35:02 +0000 |
| commit | e943f6eda5326607a998ee7ffd469f1ed3599211 (patch) | |
| tree | 9dabb441ff30f22c24599165319bd3b3baad9568 /www/hacking.html | |
| parent | 1a8208f2578bbcecb1cef11f59d24c75648b1275 (diff) | |
| download | gpsd-e943f6eda5326607a998ee7ffd469f1ed3599211.tar.gz | |
Add a discussion of the short-write-on-Bluetooth issue.
Diffstat (limited to 'www/hacking.html')
| -rw-r--r-- | www/hacking.html | 24 |
1 files changed, 23 insertions, 1 deletions
diff --git a/www/hacking.html b/www/hacking.html index 937a62a9..063739f8 100644 --- a/www/hacking.html +++ b/www/hacking.html @@ -469,7 +469,7 @@ to be embedding a PATH_MAX-sized buffer in the gpsd.h structure.</p> <h2 id="debugging">Debugging</h2> <p>For debugging purposes, it may be helpful to configure with ---disable-shared. This turns off all the shared-library crud, making +--disable-shared. This turns off all the shared-library crud, making it somewhat easier to use gdb.</p> <p>There is a script called <code>logextract</code> in the @@ -595,6 +595,28 @@ within a few minutes, it's probably not useful to that client. So if data is backing up to a client, drop that client. That's why we set the client socket to nonblocking.</p> +<p>For similar reasons, we don't try to recover from short writes to +the GPS, e.g. of DGPS corrections. They're packetized, so the device +will ignore a fragment, and there will generally be another correction +coming along shortly. Fixing this would require one of two +strategies:</p> + +<ol> +<li><p><b>Buffer any data not shipped by a short write for +retransmission.</b> Would require us to use malloc and just be begging +for memory leaks.</p></li> +<li><p><b>Block till select indicates the hardware or lower layer is +read for write.</b> Could introduce arbitrary delays for +time-sensitive data.</p></li> +</ol> + +<p>So far, as of early 2009, we've only seen short writes on Bluetooth +devices under Linux. It is not clear whether this is a problem +with the Linux Bluetooth driver (it could be failing to coalesce +and buffer adjacent writes properly) or with the underlying hardware +(Bluetooth devices tend to be cheaply made and dodgy in other respects +as well).</p> + <p>GPS input updates an internal data structure which has slots in it for all the data you can get from a GPS. Client commands mine that structure and ship reports up the socket to the client. DGPS data is |