在C中作为函数参数的指针为NULL

Question

Pardon any ignorance - I'm relatively new to C. I have the below function:

int get_fingerprint_device(struct fp_dev *device) {
    struct fp_dscv_dev **devices;
    int rtn;

    if (!(devices = fp_discover_devs())) {
        rtn = -1;
    }
    else {
        if (!(device = fp_dev_open(*devices))) {
            rtn = -2;
        }
        else {
            if (device) {
                printf("Device OK\n");
            }

            rtn = 1;
        }

        fp_dscv_devs_free(devices);
    }

    return rtn;
}

And I have a main() as follows:

int main(void)
{
    // Vars
    struct fp_dev *device;
    struct fp_print_data **print_data;
    struct fp_img **img;
    int err;

    // Init libfprint
    fp_init();

    // Get the first fingerprint device
    if ((err = get_fingerprint_device(device)) < 0) { // Errorz
        if (err == -1) {
            error("No devices found");
        }
        else if (err == -2) {
            error("Couldn't open the device");
        }

        return 1;
    }

    if (device == NULL) {
        printf("No\n");
        return 1;
    }
    else {
        printf("Yes\n");
        return 0;
    }

    fp_enroll_finger_img(device, print_data, img);

    // Deinit libfprint
    fp_exit();

    return 0;
}

The result when run is that it first prints "Device OK" and then prints "No".

Why is it that device is a valid pointer in get_fingerprint_device() but not when back in main()? My understanding is that I should be using a double pointer here such that the inner pointer value can change and be propagated back to main()... however the implementation has so far eluded me.

Answer 1

This is because device is passed into get_fingerprint_device by value. In other words, the function gets its own copy of device pointer. Then it assigns something to it, but the change is not visible outside that function, so once it returns, the caller (in this case main ) does not see any change. If you want this to work, you have to pass pointer to pointer into get_fingerprint_device function (ie struct fp_dev **device ).

Answer 2

You're a little confused about passing pointers in C - currently you're just passing "wild" (unallocated) pointers. You should change:

struct fp_dev *device;

to:

struct fp_dev device;

and change:

if ((err = get_fingerprint_device(device)) < 0)

to:

if ((err = get_fingerprint_device(&device)) < 0)