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StringLengthFromMode function (usermode_accessors.h)

The StringLengthFromMode function calculates the length of a null-terminated string, based on the specified processor mode.

Syntax

SIZE_T StringLengthFromMode(
  const CHAR      *String,
  KPROCESSOR_MODE Mode
);

Parameters

String

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

Mode

[in] The processor mode that determines how the memory access is performed. Mode can be one of the following values.

Value Meaning
KernelMode String points to kernel-mode memory. The function performs a direct string length calculation. See Remarks for more details.
UserMode String points to user-mode memory. The function raises an exception if String doesn't point to user-mode memory; otherwise it performs a safe string length calculation at the specified address. See Remarks for more details.

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 in memory. The behavior depends on the processor mode specified:

  • When Mode is UserMode, the function ensures that the string address is a valid user-mode address and performs the operation safely for user-mode memory access.
  • When Mode is KernelMode, the function operates directly on kernel-mode addresses.

When the mode is UserMode, the function includes validation to prevent kernel crashes when accessing potentially invalid user-mode pointers. When the mode is KernelMode, it operates on known-safe kernel addresses for optimal performance.

The function raises a structured exception if the operation fails, such as when the string address is not accessible or is invalid for the specified mode.

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

Requirement Value
Minimum supported client See Remarks
Header usermode_accessors.h
Library umaccess.lib
IRQL Less than or equal to APC_LEVEL

See also

StringLengthFromUser

WideStringLengthFromMode