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SubjectNeed to know the size of data send from Kernel to USer Ap

I am new to kernel programming so can anyone point me to the right
forum/reply for the question below.

I would like to implement a character device and using a node of type
character read and write data. However I am not sure of the length of
data from kernel which a user app will receive, so I would like to code
safely. If the data exceeds the application gets killed.
Any idea or API to perform the task of knowing the data length when
passing data to and fro from kernel to user level would be appreciated.

Kernel code:

copy_to_user(buff, kernel_msg, strlen(kernel_msg));

User App:
ret=read(fd,buff,strlen(buf)); //How can I know
the size of buff, which would be sent/filled from kernel???


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