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On the device I'm working on, I can write to /sys/class/leds to turn a led light on/off. Can I keep the file open in my code and only close when the process exits, or should I only open the file when I need to write to it?

Artjom B.
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mr49
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3 Answers3

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Yes. There seems to be no harm in doing so.

I tried the following in first shell 

#tailf /sys/class/leds/mmc0\:\:/brightness

And in second shell, tried the same thing.

#tailf /sys/class/leds/mmc0\:\:/brightness

This experiment is just to show that, at no point opening(tailf) from another process(second shell) was affected by a previous open(tailf) in the first shell.

Drad
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  • Just keep one file open for a longer time (`tail -f`). – CL. Mar 08 '16 at 12:45
  • @subin I beg to differ from the point that sysfs entry will be cached. sysfs entries are in ram like a ramfs/tmpfs. There is no disk cache like in the case of actual disks. – Drad Mar 09 '16 at 20:12
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If you look at tools/thermal/tmon/sysfs.c in Linux kernel source, all the functions use an open->read/write->close. Especially when using standard C library functions like fread/fwrite, the buffering can cause problems. Or you can use fflush() after each write. I would still prefer opening and closing it for each write as sysfs entries are not disk based and can change in runtime. Especially if they correspond to hotpluggable devices.

subin
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It is always best practice to open and close the file descriptor (fopen) every time. If you will always keep your sysfs open; then it will hold that particular amount of memory till the end.

vinod maverick
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