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{{DISPLAYTITLE:如何在sysfs中访问信息}}
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== Article purpose ==
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This article provides some information about the sysfs pseudo filesystem usage from the user space.
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== Sysfs (/sys) pseudo filesystem ==
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Sysfs provides a mean to export kernel data structures, their attributes, and the linkages between them to the user space.
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Please refer to [[Pseudo filesystem#sysfs (/sys) - System filesystem|sysfs part]] of [[Pseudo filesystem|pseudo filesystem]] page.
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== Sysfs usage ==
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Linux kernel provides a documentation<ref>{{CodeSource | Linux kernel | Documentation/admin-guide/sysfs-rules.rst}}</ref> about the rules for sysfs usage.
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Some examples are also described below with two different approaches for using sysfs entries from the user space:
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* Linux application in C language
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* bash script.
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=== Example from Linux application ===
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The below example is a typical sequence for using sysfs entry (here a PWM component):
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* open a file descriptor of the sysfs entry file
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<syntaxhighlight lang="c" line start="10">
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len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
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fd = open(buf, O_RDWR);
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</syntaxhighlight>
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*if fd is correctly opened, write/read value in the file: pay attention to the "text" format
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<syntaxhighlight lang="c" line start="12">
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if (fd < 0)
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{
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    perror("pwm/duty_cycle");
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    return fd;
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}
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</syntaxhighlight>
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:*read: store data to buffer
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<syntaxhighlight lang="c" line start="18">
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read(fd, buf, sizeof(buf));
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</syntaxhighlight>
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:*write: write data from buffer
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<syntaxhighlight lang="c" line start="20">
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len = snprintf(buf, sizeof(buf), "%d", 900000);
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write(fd, buf, len);
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</syntaxhighlight>
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* close file descriptor
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<syntaxhighlight lang="c" line start="30">
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close(fd);
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</syntaxhighlight>
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=== Example for shell command / bash script ===
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Operations on sysfs entries can be done by using command lines (i.e. ''echo'' for writing, ''cat'' for reading).
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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.
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An example is provided in [[PWM_overview#How_to_use_PWM_with_sysfs_interface|How_to_use_PWM_with_sysfs_interface]].
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== References ==
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<references />
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<noinclude>
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[[Category:How to trace and debug]]
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[[Category:OpenSTLinux filesystem]]
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{{PublicationRequestId | 10265 | 2019-01-16 |BrunoB}}
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</noinclude>
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[[Category:How_to ]][[Category:How_to_trace_and_debug ]]
 
[[Category:How_to ]][[Category:How_to_trace_and_debug ]]
 
[[Category:sysfs]]
 
[[Category:sysfs]]

2020年5月6日 (三) 10:06的最新版本

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 sysfs part of 伪文件系统 page.

Sysfs usage

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.

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
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);

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 How_to_use_PWM_with_sysfs_interface.

References

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

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