<p>A bit of context.
My OS is Fedora and my laptop is a MacBook Air.
The default key mappings for screen brightness don't work out of the box, so i have to implement them myself.
This can be done by writing an "acceptable" value to /sys/class/backlight/intel_backlight/brightness.
So I was writing a script to do exactly this, but when i try to write to the file, i keep getting the error : write <filename> invalid argument.</p>
<p>How Could one write to a sysfile using golang?
doing this... is it a bad ideia? any sugestions?</p>
<p>I was able to do it using python.
Could someone explain me the different behaviour between the two languages?
Thanks </p>
<hr/>**评论：**<br/><br/>hobbified: <pre><p>As <a href="/u/tv64738" rel="nofollow">/u/tv64738</a> says, the handlers for the pseudo-files in /sys, /proc, etc. can respond with anything they like to a write op, and they will usually error EINVAL if the data isn't in the format they expect. Looking at the code in backlight.c, the possible causes are either:</p>
<ol>
<li>The string you sent doesn't parse (it should be an optional <code>+</code> followed by 1 or more decimal digits, followed by optional newline; or <code>0</code> followed by one or more octal digits, followed by optional newline; or <code>0x</code> followed by one or more hex digits, followed by optional newline), or</li>
<li>It parses, but represents an invalid value (in this case, greater than the backlight's <code>max_brightness</code>).</li>
</ol></pre>seomisS: <pre><p>I just woke up what you and <a href="/u/tv64738" rel="nofollow">/u/tv64738</a> wrote... You guys are spot on, if i replace it with a string with a valid number it works!</p>
<p>Thank you so much for pointing me to the right direction!!</p></pre>cfsalguero: <pre><p>Can you share the code? a gist maybe?</p></pre>seomisS: <pre><p>Yes, no problem.</p>
<pre><code>data:=[]byte(string(n))
f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 644)
if err != nil {
return err
}
n, err := f.Write(data)
if err == nil && n < len(data) {
err = io.ErrShortWrite
}
if err1 := f.Close(); err == nil {
err = err1
}
return err
}
</code></pre>
<p>Any feedback is appreciated</p></pre>denise-bryson: <pre><p>Your error is most likely caused by this <code>data:=[]byte(string(n))</code>. If you need to convert numbers into strings, use <a href="https://golang.org/pkg/strconv/" rel="nofollow">strconv</a> (<a href="https://golang.org/pkg/fmt/#Sprintf" rel="nofollow">fmt.Sprintf</a> is also handy in some situations).</p>
<p>The conversion <code>string(140)</code> doesn't do what you think it does. Search for <code>Conversions to and from a string type</code> in the <a href="https://golang.org/ref/spec#Conversions" rel="nofollow">spec</a>.</p></pre>seomisS: <pre><p>It seems to me that the problem is related to the Apple Hardware. I just runned the code on another computer and it worked.
Anyone has any idea of what is happening?</p></pre>jerf: <pre><p>I don't think you can os.CREATE that file. I don't know what the right flags are because I don't have a Mac, but play with those flags a bit.</p></pre>seomisS: <pre><p>Thanks for the answer.
You are right, i shouldn't have a os.Create, just O_WRONLY since the file doesn't exist i don't want to create it.
Even so, in this case, i don't think it makes a difference because the file exists so first option prevail.
Still, i did what you suggested. Tried multiple combinations of O_RDWR O_APPEND O_SYNC and O_TRUNC without success</p></pre>etherealflaim: <pre><p>Your error looks like it's coming from write, not from open, but you don't add context from your error messages before returning them. I'd suggest doing that and posting a playground link or a gist with the full code you're using in Go and the working Python script.</p></pre>seomisS: <pre><p>You are right, my error is coming from write. I should have mention that.
printing the error gives: write <filename> invalid argument.
Sample code in playground:
<a href="https://play.golang.org/p/1Pw-iswC3N" rel="nofollow">https://play.golang.org/p/1Pw-iswC3N</a></p>
<p>Python code:</p>
<pre><code> fh = open("/sys/class/backlight/intel_backlight/brightness", "w")
fh.write(str(brightness))
fh.close()
</code></pre></pre>epiris: <pre><p>Everything in sysfs is exported in kernel space, the /sys/class directory is going to be a symlink to /sys/devices which are all owned by root. The devices that support writes will be 0644 in all cases I can think of. So I would add the script to your /etc/sudoers for your user and alias it in your bash/zsh/whatever if it's more convenient. The only flag you want to provide is os.O_WRONLY and nothing else. Have fun.</p></pre>nirs: <pre><p>Whats wrong with:</p>
<pre><code>echo "data" > /sys/class/backlight/intel_backlight/brightness
</code></pre>
<p>You can write this in go or any other language, but why?</p></pre>epiris: <pre><p>It is probably done conditionally, perhaps it reads values first and does smoothing or something which wouldn't be very fun in sh. Regardless "why not" is a question of similar merit though.</p></pre>seomisS: <pre><p>you are absolutely right. Still, my bash script skill's are non-existent and i didn't want to call the command from go.
But... maybe this time bash script makes more sense </p></pre>tv64738: <pre><p>Most of those pseudofiles give you an <code>EINVAL</code> ("invalid argument") error if the data you give them isn't in the format they expect. Be careful about final newlines etc.</p></pre>seomisS: <pre><p>You really helped me, it was exactly this. Thank you so much for taking the time of reading the post and help me.</p></pre>
