get_user; put_user; copy_to_user; copy_from_user

用户空间和内核空间传递数据：get_user；put_user;copy_to_user;copy_from_user
在从事Linux Kernel开发的过程中，user_app和kernel之间传递数据时不能使用memcpy()函数，必须使用copy_to/from_kernel或者是put/get_user。原因是kernel和user_app内存不能直接互访。

1. put_user
Name

put_user --    Write a simple value into user space.
Synopsis

put_user ( x, ptr);

Arguments

x
Value to copy to user space.

ptr
Destination address, in user space.

Context

User context only. This function may sleep.

Description

This macro copies a single simple value from kernel space to user space. It supports simple types like char and int, but not larger data types like structures or arrays.

ptr must have pointer-to-simple-variable type, and x must be assignable to the result of dereferencing ptr .

Returns zero on success, or -EFAULT on error.

2. get_user
Name

get_user --    Get a simple variable from user space.
Synopsis

get_user ( x, ptr);

Arguments

x
Variable to store result.

ptr
Source address, in user space.

Context

User context only. This function may sleep.

Description

This macro copies a single simple variable from user space to kernel space. It supports simple types like char and int, but not larger data types like structures or arrays.

ptr must have pointer-to-simple-variable type, and the result of dereferencing ptr must be assignable to x without a cast.

Returns zero on success, or -EFAULT on error. On error, the variable x is set to zero.





1. copy_to_user
Name

copy_to_user --   Copy a block of data into user space.
Synopsis

unsigned long copy_to_user (void __user * to, const void * from, unsigned long n);

Arguments

to
Destination address, in user space.

from
Source address, in kernel space.

n
Number of bytes to copy.

Context

User context only. This function may sleep.

Description

Copy data from kernel space to user space.

Returns number of bytes that could not be copied. On success, this will be zero.

2. copy_from_user
Name

copy_from_user --   Copy a block of data from user space.
Synopsis

unsigned long copy_from_user (void * to, const void __user * from, unsigned long n);

Arguments

to
Destination address, in kernel space.

from
Source address, in user space.

n
Number of bytes to copy.

Context

User context only. This function may sleep.

Description

Copy data from user space to kernel space.

Returns number of bytes that could not be copied. On success, this will be zero.

If some data could not be copied, this function will pad the copied data to the requested size using zero bytes.