#ifndef _NTRTL_H

/**
 * The RtlLengthSecurityDescriptor routine returns the size of a given security descriptor.
 *
 * \param SecurityDescriptor A pointer to a \ref SECURITY_DESCRIPTOR structure whose length the function retrieves.
 * \return Returns the length, in bytes, of the SECURITY_DESCRIPTOR structure.
 * \see https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtllengthsecuritydescriptor
 */
NTSYSAPI
ULONG
NTAPI
RtlLengthSecurityDescriptor(
    _In_ PSECURITY_DESCRIPTOR SecurityDescriptor
    );

#endif
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// wdm.h

NTSYSAPI ULONG RtlLengthSecurityDescriptor(
  [in] PSECURITY_DESCRIPTOR SecurityDescriptor
);
View the official Windows Driver Kit DDI reference