error assignment to expression with array type

Question

I have a function named unpack, and compiling the C++ code leads to a couple of errors. The code is below:

static void unpack(void *v, ekf_t *ekf, int n, int m) {
    /* skip over n, m in data structure */
    char *cptr = (char *) v;
    cptr += 2 * sizeof(int);

    double *dptr = (double *) cptr;
    ekf->x = dptr;
    dptr += n;
    ekf->P = dptr;
    dptr += n * n;
    ekf->Q = dptr;
    dptr += n * n;
    ekf->R = dptr;
    dptr += m * m;
    ekf->G = dptr;
    dptr += n * m;
    ekf->F = dptr;
    dptr += n * n;
    ekf->H = dptr;
    dptr += m * n;
    ekf->Ht = dptr;
    dptr += n * m;
    ekf->Ft = dptr;
    dptr += n * n;
    ekf->Pp = dptr;
    dptr += n * n;
    ekf->fx = dptr;
    dptr += n;
    ekf->hx = dptr;
    dptr += m;
    ekf->tmp0 = dptr;
    dptr += n * n;
    ekf->tmp1 = dptr;
    dptr += n * m;
    ekf->tmp2 = dptr;
    dptr += m * n;
    ekf->tmp3 = dptr;
    dptr += m * m;
    ekf->tmp4 = dptr;
    dptr += m * m;
    ekf->tmp5 = dptr;
}

And I get these errors propagated over every line of the code:

.../src/ekf.c:203:12: error: assignment to expression with array type
     ekf->x = dptr;
            ^
.../src/ekf.c:205:12: error: assignment to expression with array type
     ekf->P = dptr;

It should be noted that the code unpack was grabbed from a C library. I have tried using an extern, but this does not seem to work too well. Now, when I make the original repository (https://github.com/simondlevy/TinyEKF), it seems to compile without a problem. The compile command for TinyEKF, which contains and uses this code, is:

gcc -Wall -I. -I../../src -o gps_ekf gps_ekf.c ../../src/tiny_ekf.c -lm

This is found within /extras/c/ within the repository and certainly uses the unpack function.

Just compile it as C, not C++ - you can still link it with your C++ code of course. — Paul R
– Paul R, Commented Mar 20, 2019 at 7:55
Ensure x and P type of the fields in ekf_t are pointer to double double * — wchmb
– wchmb, Commented Mar 20, 2019 at 7:59
All those members are arrays, and you can't assign to arrays. Pick up a good beginners' book. — molbdnilo
– molbdnilo, Commented Mar 20, 2019 at 8:00

Blaze · Accepted Answer · 2019-03-20 07:57:23Z

1

According to this, ekf.x is of type double x[]. You can't just assign to an array like ekf->x = dptr;, as the error says. If you want to copy data from dptr to ekf->x, you can use memcpy:

memcpy(&ekf->x, dptr, sizeof(ekf->x));

The same goes for all the other assignments, most of those fields are double arrays.

answered Mar 20, 2019 at 7:57

Blaze

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5 Comments

SDG Over a year ago

Here is what I don't understand. If it works for the examples given in the repository, I am just recreating the entire same thing within my program. Shouldn't it work?

SDG Over a year ago

I believe that they would be arrays in the original code too, being assigned to a double. Which is why I'm confused.

Blaze Over a year ago

Which example file is this happening in? It would be interesting to have a look.

SDG Over a year ago

if you look into /extras/c, there is a GPS example that uses this and doesn't spit out an error.

SDG Over a year ago

it compiles, now to check if it functions in the manner that we want it to.

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