StringLengthFromUser function (usermode_accessors.h)

The StringLengthFromUser function safely calculates the length of a null-terminated string in user-mode memory.

Syntax

SIZE_T StringLengthFromUser(
  const CHAR *String
);

Parameters

String

[in] A pointer to a null-terminated string in user-mode memory.

Return value

The function returns the length of the string in characters, not including the terminating null character.

Remarks

This function provides safe access to calculate the length of a null-terminated string located in user-mode memory. It is designed for use by kernel-mode code that needs to determine the length of strings passed from user-mode applications.

The function ensures that the string address is a valid user-mode address before performing the length calculation. It raises a structured exception if the operation fails, such as when the string address is not a valid user-mode address or is inaccessible.

Unlike standard string functions, this function includes validation to prevent kernel crashes when accessing potentially invalid user-mode pointers. This makes it essential for secure kernel programming when processing user-mode string data.

This function works on all versions of Windows, not just the latest. You need to consume the latest WDK to get the function declaration from the usermode_accessors.h header. You also need the library (umaccess.lib) from the latest WDK. However, the resulting driver will run fine on older versions of Windows.

Requirements

See also

StringLengthFromMode

WideStringLengthFromUser