Date: Thu, 04 Sep 2003 11:20:00 -0700 From: Tim Kientzle <kientzle@acm.org> To: Paul Richards <paul@inty.com> Cc: current@freebsd.org Subject: Re: Text file busy Message-ID: <3F578250.7020100@acm.org> In-Reply-To: <1062686653.67807.77.camel@localhost> References: <1062686653.67807.77.camel@localhost>
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Paul Richards wrote: > Overwriting a file that's currently executing results in a "Text file > busy" error. I guess there are folks around who don't know this: When you execute a program, the program is not simply copied into memory. Instead, the kernel keeps the file open and pages the executable in as necessary. This is called "demand-paging of executables" and it's an old performance optimization that improves VM operation (executable code never needs to be copied out to swap; it can just be dumped and paged back in later) and quickens application startup (only the immediately-required parts of the application are read into memory immediately). I'm not certain, but I suspect it first appeared in Unix in the mid-1970s. In essence, the file _is_ the executable contents of memory. Overwriting it is almost always a bad idea; if the system has to swap in another part of that executable, the program is almost certain to crash. > This was something that was fixed way back on FreeBSD but it seems to be > a problem again. Depends on how you're installing the binary. It has always been safe to do either of the following: * Rename the current executable and then install the new one. * Unlink the current executable and then install the new one. Many tools that claim to "overwrite" really do the latter, which causes a certain amount of understandable confusion. (I'm pretty sure "install" does unlink/copy by default and will do rename/copy if you specify -b.) True overwriting of in-use executable files (e.g., "cat new > old") is dangerous and should be prohibited. Tim P.S. I wonder if demand-paging of executables is still a win for program startup on modern systems with dynamically-linked executables? Large reads are a lot more efficient, and it seems that dynamic linking might cause more startup thrashing. Hmmm...
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