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POmake file appear readonly to a library function
 

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  1. POmake file appear readonly to a library function
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    <p>I'm using a library that can read or write some metadata that I need from files.</p> <p>I only need to read and I know that my files are on slow storage.</p> <p>I noticed that while reading the values I need is very fast, closing the files is slow. Basically, if I <code>open(); read(); close()</code> each file my program is quite slow, but if I only <code>open(); read()</code> during operation and <code>close()</code> right before exit (i.e. I let my run-time close the files during finalization of the leaked objects) the program runs fast, but stops for several seconds just before exit - when the files are being closed. </p> <p>On the other hand, if I remove write permission from the files this doesn't happen: it seems that files open readonly are closed much faster than the files open read-write. It could be that the library writes metadata on close "just to be sure" if the file is writable, or some other reason, but anyway - it's impossible to specify for the library that I wan't to open the file only for reading: it tries to be intelligent and decides based on file permissions.</p> <p>At the moment to speed things up I'm changing file permissions just before opening it and restoring permissions after the metadata is read. It works, but it feels wrong. </p> <p>Is there a way in linux / unix to make files appear readonly to the current process temporarily? I can't use setuid(2) because the program is ran by unpriviledged user (or am I wrong about this?).</p> <p><strong>details</strong></p> <p>The library is <a href="https://github.com/robinst/taglib-ruby" rel="nofollow">taglib-ruby</a> which is in turn a wrapper around <a href="http://taglib.github.io/" rel="nofollow">taglib</a>'s C++ API.</p> <p>The program may list the tracks on the device and needs to know their length in seconds (the reason I use taglib), but the same media will be written to (new tracks uploaded, some tracks deleted) hence I can't just force users to mount the device readonly.</p> <p>I realize that a better solution would be patching taglib to allow to specify that I just want to read the metadata, then patch taglib-ruby to wrap the new API. But I'm not up to it, plus I have a workaround already, thus not much insetive to go down that route. I'm just wondering if there is a better workaround.</p>
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    1. COIs it the library that calls the `open()` function, or is it you opening the file and passing a `FILE *` as argument to functions from the library?
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    2. COremount the directory using "-bind,ro" switch. And ask taglib to operate on read-only filesystem. It should stop library from opening file in write mode.( not sure if -bind switch is supported on all distributions )
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    3. COSorry, I don't understand. You access the file and then the `atime` is updated on a `close()`. This causes a `write()` to the file system. Also: [relatime](http://kerneltrap.org/node/14148). At the very least, you can rule out `atime` by a mount with **noatime**. Mounting with **ro** will have the same effect as per [Icarus3](http://stackoverflow.com/users/1919896/icarus3). I guess the library maybe re-writing something; is the modified time updated? At least that could provide helpful information.
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