Difference between revisions of "How to access information in sysfs"

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1 Article purpose[edit]

This article provides some information about the sysfs pseudo filesystem usage from the user space.

2 Sysfs (/sys) pseudo filesystem[edit]

Sysfs provides a mean to export kernel data structures, their attributes, and the linkages between them to the user space.

Please refer to sysfs part of pseudo filesystem page.

3 Sysfs usage[edit]

Linux kernel provides a documentation[1] about the rules for sysfs usage.

Some examples are also described below with two different approaches for using sysfs entries from the user space:

  • Linux application in C language
  • bash script.

3.1 Example from Linux application[edit]

The below example is a typical sequence for using sysfs entry (here a PWM component):

  • open a file descriptor of the sysfs entry file
10  len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
11  fd = open(buf, O_RDWR);
  • if fd is correctly opened, write/read value in the file: pay attention to the "text" format
12  if (fd < 0)
13  {
14      perror("pwm/duty_cycle");
15      return fd;
16  }
  • read: store data to buffer
18  read(fd, buf, sizeof(buf));
  • write: write data from buffer
20  len = snprintf(buf, sizeof(buf), "%d", 900000);
21  write(fd, buf, len);
  • close file descriptor
30  close(fd);

3.2 Example for shell command / bash script[edit]

Operations on sysfs entries can be done by using command lines (i.e. echo for writing, cat for reading).

In this way, it is possible to use a bash script to execute a configuration sequence, similarly to what a user would do by typing multiple shell commands.

An example is provided in How_to_use_PWM_with_sysfs_interface.

4 References[edit]

  1. ↑ Documentation/admin-guide/sysfs-rules.rst 


<noinclude>

{{ArticleMainWriter | Jean-PhilippeR}}

{{ ArticleApprovedVersion | Jean-PhilippeR | Jean-ChristopheT,BenjaminG | No previous approved version  | BrunoB - 16Jan'19 - 10265 | 17Jan'19 }}

[[Category:How to trace and debug]]
[[Category:OpenSTLinux filesystem]]</noinclude>

== Article purpose ==
This article provides some information about the sysfs pseudo filesystem usage from the user space.

== Sysfs (/sys) pseudo filesystem ==
Sysfs provides a mean to export kernel data structures, their attributes, and the linkages between them to the user space.

Please refer to [[Pseudo filesystem#sysfs (/sys) - System filesystem|sysfs part]] of [[Pseudo filesystem|pseudo filesystem]] page.

== Sysfs usage ==
Linux kernel provides a documentation<ref>{{CodeSource | Linux kernel | Documentation/admin-guide/sysfs-rules.rst}}</ref> about the rules for sysfs usage.

Some examples are also described below with two different approaches for using sysfs entries from the user space:
* Linux application in C language
* bash script.

=== Example from Linux application ===
The below example is a typical sequence for using sysfs entry (here a PWM component):

* open a file descriptor of the sysfs entry file <pre class="brush:c;gutter:true;first-line: 10;">
<syntaxhighlight lang="c" line start="10">
len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
 fd = open(buf, O_RDWR);</pre></syntaxhighlight>

*if fd is correctly opened, write/read value in the file: pay attention to the "text" format<pre class="brush:c;gutter:true;first-line: 12;"><syntaxhighlight lang="c" line start="12">

 if (fd < 0)
 {
     perror("pwm/duty_cycle");
     return fd;
 }</pre></syntaxhighlight>

:*read: store data to buffer<pre class="brush:c;gutter:true;first-line: 18;"><syntaxhighlight lang="c" line start="18">

 read(fd, buf, sizeof(buf));</pre></syntaxhighlight>

:*write: write data from buffer<pre class="brush:c;gutter:true;first-line: 20;"><syntaxhighlight lang="c" line start="20">

 len = snprintf(buf, sizeof(buf), "%d", 900000);
 write(fd, buf, len);</pre></syntaxhighlight>

* close file descriptor<pre class="brush:c;gutter:true;first-line: 30;"><syntaxhighlight lang="c" line start="30">

 close(fd);</pre></syntaxhighlight>


=== Example for shell command / bash script ===
Operations on sysfs entries can be done by using command lines (i.e. ''echo'' for writing, ''cat'' for reading).

In this way, it is possible to use a bash script to execute a configuration sequence, similarly to what a user would do by typing multiple shell commands.

An example is provided in [[PWM_overview#How_to_use_PWM_with_sysfs_interface|How_to_use_PWM_with_sysfs_interface]].

== References ==<references />

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[[Category:How to trace and debug]]
[[Category:OpenSTLinux filesystem]]
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== Article purpose ==
  
== Article purpose ==
 
This article provides some information about the sysfs pseudo filesystem usage from the user space.
  
This article provides some information about the sysfs pseudo filesystem usage from the user space.
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* open a file descriptor of the sysfs entry file
  
* open a file descriptor of the sysfs entry file
−
<pre class="brush:c;gutter:true;first-line: 10;">
 +
<syntaxhighlight lang="c" line start="10">
 
  len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
  
  len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
 
  fd = open(buf, O_RDWR);
  
  fd = open(buf, O_RDWR);
−
</pre>
 +
</syntaxhighlight>
 
*if fd is correctly opened, write/read value in the file: pay attention to the "text" format
  
*if fd is correctly opened, write/read value in the file: pay attention to the "text" format
−
<pre class="brush:c;gutter:true;first-line: 12;">
 +
<syntaxhighlight lang="c" line start="12">
 
  if (fd < 0)
  
  if (fd < 0)
 
  {
  
  {
Line 38: Line 29:
 
     return fd;
  
     return fd;
 
  }
  
  }
−
</pre>
 +
</syntaxhighlight>
 
:*read: store data to buffer
  
:*read: store data to buffer
−
<pre class="brush:c;gutter:true;first-line: 18;">
 +
<syntaxhighlight lang="c" line start="18">
 
  read(fd, buf, sizeof(buf));
  
  read(fd, buf, sizeof(buf));
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</pre>
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</syntaxhighlight>
 
:*write: write data from buffer
  
:*write: write data from buffer
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<pre class="brush:c;gutter:true;first-line: 20;">
 +
<syntaxhighlight lang="c" line start="20">
 
  len = snprintf(buf, sizeof(buf), "%d", 900000);
  
  len = snprintf(buf, sizeof(buf), "%d", 900000);
 
  write(fd, buf, len);
  
  write(fd, buf, len);
−
</pre>
 +
</syntaxhighlight>
 
* close file descriptor
  
* close file descriptor
−
<pre class="brush:c;gutter:true;first-line: 30;">
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<syntaxhighlight lang="c" line start="30">
 
  close(fd);
  
  close(fd);
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</pre>
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</syntaxhighlight>
   
 
=== Example for shell command / bash script ===
  
=== Example for shell command / bash script ===
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== References ==
  
== References ==
 
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{{PublicationRequestId | 10265 | 2019-01-16 |BrunoB}}
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