查看“How to access information in sysfs”的源代码

{{DISPLAYTITLE:如何在sysfs中访问信息}}
== 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
<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);
</syntaxhighlight>
*if fd is correctly opened, write/read value in the file: pay attention to the "text" format
<syntaxhighlight lang="c" line start="12">
 if (fd < 0)
 {
     perror("pwm/duty_cycle");
     return fd;
 }
</syntaxhighlight>
:*read: store data to buffer
<syntaxhighlight lang="c" line start="18">
 read(fd, buf, sizeof(buf));
</syntaxhighlight>
:*write: write data from buffer
<syntaxhighlight lang="c" line start="20">
 len = snprintf(buf, sizeof(buf), "%d", 900000);
 write(fd, buf, len);
</syntaxhighlight>
* close file descriptor
<syntaxhighlight lang="c" line start="30">
 close(fd);
</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 />

<noinclude>
[[Category:How to trace and debug]]
[[Category:OpenSTLinux filesystem]]
{{PublicationRequestId | 10265 | 2019-01-16 |BrunoB}}
</noinclude>

[[Category:How_to ]][[Category:How_to_trace_and_debug ]]
[[Category:sysfs]]