The SYSTEM_CALL_COUNT_INFORMATION structure is a fixed-size header for the variable amount of data that a successful call to ZwQuerySystemInformation or NtQuerySystemInformation produces in its output buffer when given the information class SystemCallCountInformation (0x06).

This information class is nowadays not supported.

Documentation Status

The SYSTEM_CALL_COUNT_INFORMATION structure is not documented.

Microsoft does publish the practical equivalent of a C-language definition as type information in public symbol files, though not for the kernel, where the structure is prepared, nor even for low-level user-mode DLLs that interpret the structure, but for various higher-level user-mode DLLs such as URLMON.DLL and only then starting with version 6.2.

Two earlier disclosures of type information are known, though not in symbol files but in statically linked libraries: GDISRVL.LIB from the Device Driver Kit (DDK) for Windows NT 3.51; and SHELL32.LIB from the DDK for Windows NT 4.0.


The SYSTEM_CALL_COUNT_INFORMATION is 0x08 bytes in both 32-bit and 64-bit Windows in all versions starting from 3.51 but 0x04 bytes in version 3.50.

Offset Definition Versions
ULONG Length;
3.50 and higher
ULONG NumberOfTables;
3.51 and higher

The fixed-size header is followed by an array of dwords, one per table, and then by the tables. Each of the dwords is in turn the number of 32-bit counts in the corresponding table. Version 3.50 has only the only one table. That there can be more starts with version 3.51 and the KeAddSystemServiceTable function.