Question

I’ve struggled with at p/invoke problem for some time now. Keep getting “Attempted to read or write protected memory.” all the time, and I suspect my marshaling is a bit off.

Here is the c signature:

long ft_show(                             /* show file-attributes        */
    const struct ft_admission *admis,     /* I: transfer admission       */
    const struct ft_shwpar    *par,       /* IO: parameter-list          */
    struct ft_fileinfo        *info,      /* O: requested information    */
    struct ft_err             *errorinfo, /* O: error information        */
    void                      *options    /* I: options                  */
    );

and here are the related structures (in c)

struct ft_admission
{
    char    *remsys;
    char    *remadmis;
    char    *remaccount;
    char    *rempasswd;
};

struct ft_shwpar
{
    int                shwparvers;
    char               *fn;
    char               *mgmtpasswd;
    char               *fud;
    int                fudlen;
};

struct ft_fileinfo
{
    int             ftshowivers;
    char            fn[INFO_FN_LEN];
    enum ft_ftype   filetype;
    enum ft_charset charset;
    enum ft_rform   recordform;
    long            recsize;
    enum ft_available availability;
    int             access;
    char            account[ACC_LEN];
    long            size;
    long            maxsize;
    char            legalqual[LQ_LEN];
    char            cre_user[USER_LEN];
    long            cre_date;
    char            mod_user[USER_LEN];
    long            mod_date;
    char            rea_user[USER_LEN];
    long            rea_date;
    char            atm_user[USER_LEN];
    long            atm_date;
    long long       fsize;
    long long       fmaxsize;
};

struct ft_err
{
    long    main;
    long    detail;
    long    additional;
};

struct ft_options
{                                                                       
    int ftoptsvers;
    int ftapivers;
};

I’ve tried to create c# structs and calls like this:

[DllImport("ftapi.dll", EntryPoint = "ft_showdir")]
private static extern long ft_showdir(ft_admission admis, ref ft_shwpar par, ft_fileinfo[] buf, int bufsize, ref ft_err errorinfo, ft_options options);

    public struct ft_admission
    {
        public String remsys;
        public String remadmis;
        public String remaccount;
        public String rempasswd;
    };
    public struct ft_shwpar
    {
        public int shwparvers;
        public String fn;
        public String mgmtpasswd;
        public IntPtr fud;
        public int fudlen;
    };
    [StructLayout(LayoutKind.Sequential, Size = 1464, CharSet = CharSet.Ansi), Serializable]
    public struct ft_fileinfo
    {
        public int ftshowivers;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 257)]
        public String fn;
        public ft_ftype filetype;
        public ft_charset charset;
        public ft_rform recordform;
        public long recsize;
        public ft_available availability;
        public int access;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 65)]
        public String account;
        public long size;
        public long maxsize;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 81)]
        public String legalqual;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 68)]
        public String cre_user;
        public long cre_date;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 68)]
        public String mod_user;
        public long mod_date;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 68)]
        public String rea_user;
        public long rea_date;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 68)]
        public String atm_user;
        public long atm_date;
        public long fsize;
        public long fmaxsize;
    };
    public struct ft_err
    {
        public long main;
        public long detail;
        public long additional;
    }
    public struct ft_options
    {
        public int ftoptsvers;
        public int ftapivers;
    };

I’ve already used the ft_err and ft_options structs successfully in another call, so I think those are marshaled correctly.

I think the problem is the ft_fileinfo structure. I want to use StringBuilder in there, but there is a bug in the framework preventing correct marshaling of StringBuilder inside structs. I’ve tried the workarounds described here: http://support.microsoft.com/kb/327109
but no success.

Here’s the code snippet provided in the documentation of the api I try to access (some code has been removed as it’s only used for printing info to the screen):

/* sample3.c     File management requests                    */
/*************************************************************/
static const char Sccsid[] = "@(#)sample3.c   2.33 2009/03/25";
/*************************************************************

  Program call: sample3 <directory>

 *************************************************************/
/*   Include Files                                           */
/*************************************************************/
#include <stdio.h>       /* printf()                         */
#include <stdlib.h>      /* exit()                           */
#include <string.h>      /* strcat(), strcpy()               */
#include <ftapi.h>

/*************************************************************/
/*                Local Constants and Macros                 */
/*************************************************************/
#define BUFSIZE 200      /* number of ft_fileinfo structures */
                         /* in output buffer                 */

/*************************************************************/
/*               Local Functions Declarations                */
/*************************************************************/
static void error_print(struct ft_err *, char *);
static void printinfo(struct ft_fileinfo *info); 

int main (int argc, char *argv[])
{
    int i;                     /* Some local counter         */
    int count;                 /* Number of files            */
    char remotesys[201];       /* Address of remote systems  */
    char remoteadm[68];        /* Transfer admission         */
    char fud[STAT_FUD_LEN];    /* Further details            */
    struct ft_err errorinfo;   /* Error information          */
    struct ft_admission admis; /* Admission parameters       */
    struct ft_shwpar par;      /* Show directory parameters  */
    struct ft_options opt;     /* Options                    */
    struct ft_fileinfo buf[BUFSIZE]; /* Output buffer        */

    /* Check program arguments                               */
    if (argc != 2)
    {
        printf("Call: %s <directory>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    /* Initialize ft_transpar structure                      */
    memset(&par, 0, sizeof(par));
    /* Set version of the parameter list                     */
    par.shwparvers  = FT_SPARV2;
    /* Set the name of the remote directory                  */
    par.fn = argv[1];
    /* Set buffer and buffer length for the further details  */
    par.fud = fud;               
    par.fudlen = sizeof(fud);      
    /* Set version of the ft_fileinfo structure in the       */
    /* output buffer. The version in the first structure     */
    /* is valid for all structtures in the output buffer.    */
    buf[0].ftshowivers = FT_SHOWIV2;

    /* Get the name/address of the remote system and the     */
    /* transfer admission                                    */
    printf("Enter name of remote system: ");
    scanf("%s", remotesys);
    printf("Input transfer admission to remote system: ");
    scanf("%s", remoteadm);
    admis.remsys     = remotesys;
    admis.remadmis   = remoteadm;
    admis.remaccount = NULL;
    admis.rempasswd  = NULL;

    /* Prepare the options structure                         */
    opt.ftoptsvers = FT_OPTSV1;
    opt.ftapivers = FT_APIV2; 

    /* Read the contents of the remote directory             */
    count = ft_showdir(&admis, &par, buf, BUFSIZE, &errorinfo,
                       &opt);
    if (count == -1) 
    {   
        /* Error                                             */
        error_print(&errorinfo, par.fud); 
    }

    /* Display result of request                             */
    printf("There are %d entries in remote directory %s\n",
           count, argv[1]);

    /* If the output range was not large enough for all      */
    /* the information, only the data in the buffer is       */
    /* displayed                                             */
    if (count > BUFSIZE)
        count = BUFSIZE;
    for (i = 0; i < count; i++)
         printinfo(&buf[i]);
    return(0);
}

Can someone help me achieve the same in C#? And even explain why it have to be that way, so we all can learn something from this 🙂

Thanks!

Answer 1

Ok, I figured it out.

When looking at the c declaration of the function we can see that all of the parameters are sent in by reference or via pointers. So I just altered the pinvoke declaration so that all parameters where ref.

[DllImport("ftapi.dll", EntryPoint = "ft_showdir", CallingConvention = CallingConvention.Cdecl)]
private static extern Int32 ft_showdir(ref ft_admission admis, ref ft_shwpar par, ref ft_fileinfo[] buf, int bufsize, ref ft_err errorinfo, ref ft_options options);