Calling _msize() via PInvoke from C# -

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i'm writing c# library calling app pass in large amount of contiguous, unmanaged memory. calling app can either .net or visual c++ (it go through intermediate c++/cli library before calling library if c++). useful validate there sufficient memory, decided call _msize() function. unfortunately, _msize seems give me wrong size back. went , modified allocation routine in sample app , call _msize. here code snipet:

public unsafe class mymemory {     /// <returns></returns>     [dllimport("msvcrt.dll", setlasterror = true)]     public static extern int _msize(intptr handle);      public static intptr myalloc(int size)     {         intptr retval = marshal.allochglobal(size);          ...          int memsize = mymemory._msize(retval);         if (memsize < size)         {         ...         }         return retval;     }

when pass in size 199229440, memsize of 199178885. i've seen similar results different numbers. less 0.01% off, totally understand if over, fact is under, meaning _msize thinks allocated memory less asked for. have clue why is? , recommendations on should instead appreciated well.

p/invoke localsize function instead.

_msize determining size of block allocated malloc (and friends). allochglobal wrapper around globalalloc or localalloc (depending on reference believe; think 2 equivalent), , want localsize function determine size of block returned. far can tell, marshal doesn't contain wrapper localsize, can call using p/invoke.

so seems it's sheer luck _msize returning useful @ all. perhaps malloc uses globalalloc (or localalloc), either or when asked large blocks, , requests bit of space bookkeeping; in case _msize trying compensate that.


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