ssize_t dev_read(struct file *filp,char *buf,size_t count,loff_t *offset)
{
int len = count >= strlen(chr_arr.array) ? strlen(chr_arr.array) : count;
*offset += len;
if (*offset >= strlen(chr_arr.array))
return 0;
if (copy_to_user(buf,chr_arr.array,len))
return -EFAULT;
return len;
}
I want to read a value from kernel and use it in a user application, so i am using procfs api to read from the kernel and use it in a user space.
The above is the read function to read from the kernel and store it in a user buffer(buf). But If i want to read the output from user application then where will be value read from kernel stored in a user space ?? could someone help me in this ??
If the value is exposed in procfs, your user application just needs to open the procfs node as a file and read it like any other file. The fancy stuff's all done in the kernel.
If you're trying to write a kernel component that exposes something to procfs, then you'll need something similar to the code that you quoted to handle read() calls to the procfs node.
Related
I am developing a Linux kernel module which read my embedded board button code and use it in order to turn on/off the board led.
When I push button, the module is sending correctly the button code, which is 260, to user-space.
Now, from user-space, I am using the write function in order to re-send this code kernel module :
write(fd, buf_wr, strlen(buf_wr));
Where fd is the file descriptor of my module, and buf_wr is the buffer which will be written to the file.
In kernel space, I am using the write_pid function in this way :
static ssize_t write_pid(struct file *pfile, const char __user *buffer,
size_t length, loff_t *offset)
{
char cod_buf[12];
printk("WE ARE IN WRITE_PID FUNCTION\n");
copy_from_user(cod_buf, buffer, length);
sscanf(cod_buf, "%i", &lcode);
printk("lcode = %i\n", lcode);
return 0;
}
I defined lcode in my kernel module as a global variable :
int lcode = 0;
and I can see that it is receiving the correct button code which is 260.
Now, I created a thread in the module, I want this thread to run some instructions basing on the button code :
int write_in_thread(void *data) {
printk("Under write_in_thread, lcode = %i\n", lcode);
switch (lcode) {
case 260:
//instructions....
//instructions....
//instructions....
break;
default :
//instructions....
break;
}
return 0;
}
The problem here, I can see that, Under write_in_thread, lcode = 0. It's not 260. So the case statement is running the default one which I don't need here.
How can I fix the lcode variable in order to keep it's value in the thread function?
Thank you!
I have been writing a kernel space device driver that can be read from and written to from user space. The open, read, release operations all work perfectly. The problem I am having is with the user-space code that should access the device driver and and write something to it.
The user-space program writes to two files: 1) to a .txt file (and prints a to the console to let the user know it was completed), and 2) to the device driver (and also prints a text to let the user know it was also completed).
Below is the user-space code in full:
int main() {
FILE *fp;
fp = fopen("./test.txt","w");
fputs("Test\n", fp);
fclose(fp);
printf("Printed to txt\n"); //Prints normally.
fp = fopen("/dev/testchar", "w");
fputs("Test\n", fp);
fclose(fp);
printf("Printed to dev\n"); //Never gets to this point
return 0;
}
When I compile and run the code the program spits out
Printed to txt
and just hangs until ctrl+c is called. It never gets beyond the second fputs().
While monitoring kern.log I see endless calls to write to the device driver.
Here I have extracted relevant code from the device driver:
static char msg[256] = {0};
static struct file_operations fops =
{
.write = dev_write
};
static ssize_t dev_write(struct file *file, const char *buf, size_t len, loff_t *ppos)
{
sprintf(msg, "Input:%s, Chars:%lu\n", buf, len);
printk(KERN_NOTICE "%s\n", msg);
return 0;
}
uname -r: 4.10.0-38-generic
gcc -v: gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)
My question is: why is the program getting stuck in an infinite loop when writing to the device, and how do I fix it?
Thanks in advance. Any help will be greatly appreciated.
I think the kernel write operation is supposed to return the number of bytes written. You return 0. So the write system call returns to userspace with 0. However, since your userspace code is using stdio, then your userspace code tries the write again, assuming the system call simply didn't write out all the data. If you return the length of the input, then stdio will know all the data was written. Alternatively you can use the write system call directly rather than fputs. Your kernel code will still be incorrect, but your program will terminate.
You can test this using strace and see all the system calls.
I've been having some headache with infinite reads from cat (cat doesn’t close because it doesn’t receive end of function from my read function. How can I implement an end of read so that reading the file with cat will only produce 1 output per command in the terminal?
function. This is the kernel read() function I've written:
static ssize_t dev_read(struct file *file, char *buf, size_t count, loff_t *ppos)
{
char tmp_buf[MAX_BUF_SIZE]; //defined as 100
int bLen=0;
sprintf(tmp_buf, "Some message");
bLen = strlen(tmp_buf);
if(copy_to_user(buf,tmp_buf, bLen)){
return -EFAULT;
}
return bLen;
}
I'm answering because I found this early on in my search.
Cat continually reads until it gets an empty response. Once it finished getting some data it goes back and asks "have anything else?" To which your module says yes and sends it the data again. You need to break the chain and have it send an empty response. The best way to do this would be to place
if(*ppos > 0){
return 0;
}
at the beginning of the function and add the length of the data you are sending back to *ppos before exiting.
This is my program:
#include <stdio.h>
int main() {
FILE *logh;
logh = fopen("/home/user1/data.txt", "a+");
if (logh == NULL)
{
printf("error creating file \n");
return -1;
}
// write some data to the log handle and check if it gets written..
int result = fprintf(logh, "this is some test data \n");
if (result > 0)
printf("write successful \n");
else
printf("couldn't write the data to filesystem \n");
while (1) {
};
fclose(logh);
return 0;
}
When i run this program, i see that the file is getting created but it does not contain any data. what i understand i that there is data caching in memory before the data is actually written to the filesystem to avoid multiple IOs to increase performance. and I also know that i can call fsync/fdatasync inside the program to force a sync. but can i force the sync from outside without having to change the program?
I tried running sync command from Linux shell but it does not make the data to appear on the file. :(
Please help if anybody knows any alternative to do the same.
One useful information: I was researching some more on this and finally found this, to remove internal buffering altogether, the FILE mode can be set to _IONBF using int setvbuf(FILE *stream, char *buf, int mode, size_t size)
The IO functions usingFILE pointers cache the data to be written in an internal buffer within the program's memory until they decide to perform a system call to 'really' write it (which is for normal files usually when the size of the data cached reaches BUFSIZ).
Until then, there is no way to force writing from outside the progam.
The problem is that your program does not close the file because of your while statement. Remove these lines:
while (1) {
};
If the intent is to wait forever, then close the file with fclose before executing the while statement.
I'd like to share a variable between kernel and user space and I've found that it's possible with procfs.
The kernel module must act in certain way if given value is set. The user space program is responsible for changing this value, but the kernel module must read it when necessary.
I know that I must create the /proc file in the kernel module.
My question is, how to read the file from the kernel module?
Source : linux.die.net/lkmpg/x769.html
/**
* This function is called with the /proc file is written
*
*/
int procfile_write(struct file *file, const char *buffer, unsigned long count,
void *data)
{
/* get buffer size */
procfs_buffer_size = count;
if (procfs_buffer_size > PROCFS_MAX_SIZE ) {
procfs_buffer_size = PROCFS_MAX_SIZE;
}
/* write data to the buffer */
if ( copy_from_user(procfs_buffer, buffer, procfs_buffer_size) ) {
return -EFAULT;
}
return procfs_buffer_size;
}
To clarify, in Your module whenever user writes to Your file in procfs, this example shows how to handle such write.
In kernel >= 3.10 proc_write is moved to structure file_operations where declaration of write is different, so in newest your solution won't work.
You can implement typical file_operations.write(struct file *, const char __user *, size_t, loff_t *) and reference this to:
struct proc_dir_entry your_proc_dir_entry{
.proc_fops = &your_fops,
}