copytouser

Copy_to_user is a kernel helper used to transfer data from kernel space to user space in a safe and controlled way. In the Linux kernel, the function is commonly referred to as copy_to_user and is defined in include/linux/uaccess.h. Its primary purpose is to copy up to a specified number of bytes from a kernel-space source to a user-space destination while performing validation of the user address and handling faults that could arise during the copy.

The function copies data from a kernel pointer to a user-space pointer and returns the number of bytes that could not be copied. On successful completion, it returns zero. This behavior allows system calls and other kernel routines to detect partial copies and take appropriate action. Copying to user space is a sensitive operation because user addresses may be invalid or inaccessible, and copy_to_user provides the necessary safeguards to prevent kernel crashes.

There are several variants and related helpers in the Linux kernel. copy_from_user performs the opposite operation, copying data from user space into kernel space. In atomic contexts where sleeping is not allowed, specialized forms such as __copy_to_user_in_atomic or similar arch-specific variants may be used to avoid potentially blocking operations. The basic copy_to_user interface is designed to be architecture-independent, with implementation details handled by lower-level architecture code.

Usage guidelines emphasize validating inputs and checking the return value. When implementing system calls or interface functions that return data to user processes, developers typically use copy_to_user rather than direct memory access to ensure safe interaction with user memory and to gracefully handle partial failures.