将值从struct分配给变量失败

Question

Edit 3 describes the narrowed-down problem after debugging

Edit 4 contains the solution - it's all about type difference between C and C#

Today I came across a curious problem. In CI have the following struct:

typedef struct s_z88i2
{
    long Node;
    long DOF;
    long TypeFlag;
    double CValue;

}s_z88i2;

Furthermore I have a function (this is a simplified version):

DLLIMPORT int pass_i2(s_z88i2 inp, int total)
{
    long nkn=0,ifg=0,iflag1=0;
    double wert=0;
    int val;

    // Testplace 1
    nkn=inp.Node;
    ifg=inp.DOF;
    iflag1=inp.TypeFlag;
    wert=inp.CValue;
    // Testplace 2

    return 0;
}

The assigned values are used nowhere - I'm aware of that. When I reach // Testplace 1 the following statement is executed:

char tmpstr[256];
sprintf(tmpstr,"RB, Node#: %li, DOF#: %li, Type#: %li, Value: %f", inp.Node, inp.DOF, inp.TypeFlag, inp.CValue);

tmpstr then is passed to a messagebox. It shows - as one would expect - the values given in my struct I passed to the function in an nice and orderly way. Moving on through the function the values inside the struct get assigned to some variables. On reaching Testplace 2 the following is executed:

sprintf(tmpstr,"RB, Node#: %li, DOF#: %li, Type#: %li, Value: %f",nkn, ifg, iflag1, wert);

Again, tmpstr is passed to a messagebox. However, this doesn't show what one would expect. The values for Node and Type are still correct. For DOF and Value the displayed values are 0 which leads me to the conclusion that something is going terribly wrong during assigning the values. I somehow sometimes managed to get a way to long number for value whis was as incorrect as 0 . But I have not been able to reproduce that mistake during my last tests. Possible values for inp are eg {2,1,1,-451.387} , so the first 1 and -451.387 are forgotten.

Does anyone know what I'm doing wrong or how to fix this?

Many thanks in advance!

Edit:

Changed %i to %li but the result did not change. Thank to unwind!

I'm developing this dll with Dev-Cpp using MinGW (unfortunately) because I wasn't able to convince Visual Studio 2012 Pro to compile this properly. Although the documentation of the original source says it is plain ANSI-C. This bugs me a bit because I cannot debug this dll properly with Dev-Cpp. Hence the messageboxes.

Edit 2:

As Neil Townsend suggested, I switched to passing a reference. But this also did not cure the problem. When I access the values in my struct directly everything is fine. When I assign them to variables some get lost.

A short notice on how I'm calling the function. The dll is to be accessed from C#, so I'm meddeling with P/Invoke (as I get it).

[DllImport("z88rDLL", CallingConvention = CallingConvention.Cdecl)]
public static extern int pass_i2(ref s_z88i2 inp, int total);

is my definition in C#. I have lots of other functions imported and they all work fine. It is this function I encounter these Problems for the first time. I call the function via:

s_z88i2 tmpi2 = FilesZ88.z88i2F.ConstraintsList[i];
int res = SimulationsCom.pass_i2(ref tmpi2, FilesZ88.z88i2F.ConstraintsList.Count);

First I set the struct, then I call the function.

Why Oh Why has VS to be picky when it comes to compiling ANSI-C? It certainly would make things easier.

Edit 3:

I can narrow the problem down to sprintf , I think. Having convinced VS to build my dll I was able to step through it. It appears that the values are assigned very nicely indeed to the variables they belong in. If, however I want to print these variables via sprintf they turn out rather empty ( 0 ). Curiously, that the value is always 0 and not something else. I'm still interested in why sprintf behaves that way, but I consider my initial problem solved/panic defeated. So thanks everyone!

Edit 4:

As supercat points out below, I had a rethink about type-compatibility between C and C#. I was aware that an int in C# evaluates as a long in C. But after double-checking I found that in C my variables are really FR_INT4 (which I kept out of the original question for reasons of clarity => bad idea). Internally FR_INT4 is defined as: #define FR_INT4 long long , so as a super-long-long. A quick test showed that passing a long from C# gives the best compatibility. So the sprintf -issue can maybe be simplified to the question: "What is the format-identifier of a long long?".

It is %lli which would is quite simple, actually. So I can announce drumroll that my problem really is solved!

sprintf(tmpstr,"RB, Node#: %lli, DOF#: %lli, Typ#: %lli, Wert: %f\n", inp.Node, inp.DOF, inp.TypeFlag, inp.CValue);

returns every value I want. Thank you very much everyone!

Answer 1

Formatting a value of type long with the format specifier %i is not valid. You should use %li .

Answer 2

In C, it is a better approach to pass a reference or pointer to the struct rather than the struct. So:

DLLIMPORT int pass_i2(s_z88i2 *inp, int total) {
    long nkn=0,ifg=0,iflag1=0;
    double wert=0;
    int val;

    // Testplace 1
    nkn=inp->Node;
    ifg=inp->DOF;
    iflag1=inp->TypeFlag;
    wert=inp->CValue;
    // Testplace 2

   return 0;
}

You will need to correct the sprintf liness accordingly, inp.X becomes inp->X . To use this function either:

 // Option A - create it in a declaration, fill it, and send a pointer to that
 struct s_z88i2 thing;
 // fill out thing
 // eg. thing.Node = 2;
 pass_i2(&thing, TOTAL);

or:

 // Option B - create a pointer; create the memory for the struct, fill it, and send the pointer
 struct s_z88i2 *thing;
 thing = malloc(sizeof(struct s_z88i2));
 // fill out thing
 // eg thing->Node = 2;
 pass_i2(thing, TOTAL);

This way pass_i2 will operate on the struct you send it, and any changes it makes will be there on return from pass_i2 .

Answer 3

To clarify this as answered:

My struct actually is:

typedef struct s_z88i2
{
    long long Node;
    long long DOF;
    long long TypeFlag;
    double CValue;
}s_z88i2;

which requires long to be passed from C# (and not int as I previously thought). Through debugging I found out that the assignment of values behaves as it should, the problem was within sprintf . If I use %lli as format-identifier even this problem is solved.

sprintf(tmpstr,"RB, Node#: %lli, DOF#: %lli, Typ#: %lli, Wert: %f\n", inp.Node, inp.DOF, inp.TypeFlag, inp.CValue);

Is the statement I need to use. So thanks again everyone who contributed!