<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="Author" content="Riccardo Facchetti"> <meta name="GENERATOR" content="Mozilla/4.74 [en] (X11; U; Linux 2.4.3 i586) [Netscape]"> </head> <body text="#000000" bgcolor="#FFFFFF" link="#0000EF" vlink="#51188E" alink="#FF0000"> <h2> APCUPSD threads</h2> <p><br>APCUPSD now is using threads (processes) and IPC shared memory for all its operations. On blocking operations, we make use of timed select() to ensure short thread execution. There is one chunk of shared memory that is an UPSINFO structure and this is the way all threads communicate one to the other. The area must be locked before modifying it and can be unlocked only at the end of an handler. The area can be leaved unlocked only if the routine involved don't want to modify myUPS. <p>Graph of operations: <p> +- net thread ---+ <br> | \ <br>apcupsd ------+- serial thread -[ UPSINFO shared structure] <br> | / <br> +- client thread -+ <br> <p>If a thread must write to shared structure we _have_ to lock it. Imagine serial and net that need to write: <br> <ul> <li> do_serial wait for unlock then read shared data and unlock</li> <li> do_net wait for unlock then read shared data and unlock</li> <li> do_serial wait for unlock then write shared data and unlock</li> <li> do_net wait for unlock then write shared data and unlock but this data is just been updated by serial so the shared data now contain thrashed data.</li> </ul> <p><br>We have to lock the data until the thread is ended: <br> <ul> <li> do_serial wait for unlock then read shared data (don't unlock)</li> <li> do_net wait for unlock</li> <li> do_serial write shared data and unlock</li> <li> do_net waiting for unlock can read shared data</li> <li> do_net write shared data and unlock</li> </ul> <p><br>This is called serialization. For read-only threads we just wait for unlock then read shared data and unlock and that's all. <br> </body> </html>
