RtlLengthSecurityDescriptor - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#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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This function is documented in Windows Driver Kit.