Read from serial port in a kernel module

Question

For my assignment I need to read some strings coming from a serial port. This has to be done in a kernel module, so I can't use stdio library. I'm trying in this way:

#include <linux/module.h>
#include <linux/unistd.h>
#include <asm/io.h>
#include <asm/fcntl.h>
#define SERIAL_PORT "/dev/ttyACM0"

void myfun(void){
  int fd = open(SERIAL_PORT,O_RDONLY | O_NOCTTY);
  ..reading...

}

but it gives me "implicit declaration of function open"

Mike · Accepted Answer · 2012-10-31 12:40:11Z

2

You want to use the filp_open() function, it is pretty much a helper to open a file in kernelspace. You can find the man on it here

The file pointer from filp_open() is of type struct file and don't forget to close it with filp_close() when you're done:

#include <linux/fs.h>
//other includes...

//other code...    

struct file *filp = filp_open("/dev/ttyS0");
//do serial stuff...
filp_close(filp);

edited Oct 31, 2012 at 12:40

answered Oct 31, 2012 at 11:56

Mike

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5 Comments

Mike Over a year ago

@user1610075 - Did you include the right header file? (see edit above)

user1610075 Over a year ago

Ok now it works...anyway in the man it says something like it is not recommendend...moreover I found a reading example here howtoforge.com/… and seems pretty tricky to me, are you sure it is a good way to read from kernel space?

Mike Over a year ago

@user1610075 - it's not saying filp_open is not recommended, it's saying opening a file from the kernel is not recommended. But if you have to, this, or a mechanism based off of this, is the correct way (to the best of my knowledge).

user1610075 Over a year ago

Ah ok thanks! Listen I would need a non-blocking version of read, but I can't find any documentation...could you help me please?

Mike Over a year ago

That's kind of off topic, you should probably open a new question. But briefly, you could open with the O_NONBLOCK flag, or set this in fcntl, or use select() with NULL for timeouts

dave · Accepted Answer · 2012-10-31 11:47:52Z

1

Finding your way around the kernel source can be pretty frightening since it's so large. Here's my favorite command: find . -iname "*.[chs]" -print0 | xargs -0 grep -i "<search term>.

A quick description:

find (obvious)
dot is the root directory of the kernel
iname is find the name ignoring case
.c .h and .s files contain code - look in them
print0 prints them out null terminated as they are found
xargs takes an input and uses it as an argument to another command (-0 is to use null terminated)
grep - search for string (ignoring case).

So for this search: "int open(" and you'll get some hits with tty in the name (those will be for consoles) - have a look at the code and see if they are what you want.

answered Oct 31, 2012 at 11:47

dave

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2 Comments

Nelson Benítez León Over a year ago

You can do the same with grep solely: grep -i -R --include="*.[chs]" "<search term>" .

dave Over a year ago

I like my version better still because I can use it for things other than grep (perl expressions to replace found terms is a good example). But thanks for the tip (it can also be done with just find and --exec ... but xargs is faster)

cdarke · Accepted Answer · 2012-10-31 12:56:45Z

-2

Include the header file for open, usually #include <unistd.h>

Edit:

Not sure about kernel use, but do a man 2 open.

On my version of linux you might need all following:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

depending on the macros you are using.

edited Oct 31, 2012 at 12:56

answered Oct 31, 2012 at 11:27

cdarke

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2 Comments

user1610075 Over a year ago

gives me "no existent"...maybe because I'm in a kernel module?

Mike Over a year ago

@user1610075 - Exactly. unistd.h is a part of glibc, that's not included in the kernel. The kernel has its own version of this linux/unistd.h

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