c# - Passing a struct by reference from managed to unmanaged code? -

my struct looks in c code:

typedef struct _request {     char d [_d_max+1];     char m [_m_max+1];     char t [_t_max+1];     char clref [_cl_ref_max+1];     char load [_load_max];     ulong loadlen; } _request, *lp_request; 

in c# this:

[structlayout(layoutkind.sequential, charset=charset.ansi)] public struct _request_struct {     [marshalas(unmanagedtype.byvaltstr, sizeconst = 32)]     public string d;      [marshalas(unmanagedtype.byvaltstr, sizeconst = 32)]     public string m;      [marshalas(unmanagedtype.byvaltstr, sizeconst = 32)]     public string t;      [marshalas(unmanagedtype.byvaltstr, sizeconst = 20)]     public string clref;      [marshalas(unmanagedtype.byvaltstr, sizeconst = 1200)]     public string load;      public uint32 loadlen; } 

the method calling in c:

int  aj_stdcall  simptran (const char* svc_name,                     const lp_request query,                     lp_response      response) {     //init stuff     //validate input parameters     if ( query == null )         return(err_query_null);      if (query->loadlen == 0)         return (err_query_null); } 

in c#:

[dllimport(linuxlib, callingconvention = callingconvention.stdcall)] public static extern int simptran(string serid, ref _request_struct request, out intptr response); 

and calling this:

p_request_struct = new _request_struct(); // fill p_request_struct  // response p_response_struct = new _response_struct();  // initialize unmanaged memory hold return struct intptr ptrret = marshal.allochglobal(marshal.sizeof(p_response_struct)); marshal.structuretoptr(p_response_struct, ptrret, false);  iret = simptran(serid, ref p_request_struct, out ptrret); 

i getting null query! in c code query==null true , returns null. populating request struct?

do have allocate memory well?