size_t - NtDoc

#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
#ifdef _WIN64

typedef unsigned __int64    size_t;

#endif
#endif
#endif
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#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
#ifdef _WIN64
// ...
#else

typedef _W64 unsigned int   size_t;

#endif
#endif
#endif
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#ifndef _NTSTRSAFE_H_INCLUDED_
// typedefs
#ifdef  _WIN64

typedef unsigned __int64    size_t;

#endif
#endif
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#ifndef _NTSTRSAFE_H_INCLUDED_
// typedefs
#ifdef  _WIN64
// ...
#else

typedef _W64 unsigned int   size_t;

#endif
#endif
View code on GitHub