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 sysfs part of 伪文件系统 page.

Sysfs usage

Linux kernel provides a documentation 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

 len=snprintf(buf, sizeof(buf), "/sys/class/pwm/pwmchip0/pwm%d/duty_cycle", pwm_channel);
 fd = open(buf, O_RDWR);

• if fd is correctly opened, write/read value in the file: pay attention to the "text" format

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

• read: store data to buffer

 read(fd, buf, sizeof(buf));

• write: write data from buffer

 len = snprintf(buf, sizeof(buf), "%d", 900000);
 write(fd, buf, len);

• close file descriptor

 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

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