How do I extract an array of `struct*` from a pointer to that array (in JNA)?

Question

I need to call a function with a signature like this:

size_t findDevices(devStruct_t **devs[]);

Some example C code uses the function like this:

devStruct_t **arrOfPointers;
size_t size;
size = findDevices(&arrOfPointers);
for(size_t i = 0; i < size; i++) {
    printf("devId:%d\n", arrOfPointers[i]->id);
}

How would I replicate the above code in Java using JNA?

I think the JNA function signature should look like this:

NativeLong findDevices(PointerByReference devs);

I can run this without crashing:

NativeLong size;
PointerByReference stdevs = new PointerByReference();
size = libstlink.stlink_probe_usb(stdevs);

But, I don't know how to access the structures.

Answer 1

function definition, as you can see in source code is

size_t stlink_probe_usb(stlink_t **stdevs[]);

That type is defined into this source code

struct _stlink {
    struct _stlink_backend *backend;
    void *backend_data;

    // Room for the command header
    unsigned char c_buf[C_BUF_LEN];
    // Data transferred from or to device
    unsigned char q_buf[Q_BUF_LEN];
    int q_len;

    // transport layer verboseness: 0 for no debug info, 10 for lots
    int verbose;
    uint32_t core_id;
    uint32_t chip_id;
    int core_stat;

    char serial[16];
    int serial_size;

    enum stlink_flash_type flash_type;
    stm32_addr_t flash_base;
    size_t flash_size;
    size_t flash_pgsz;

    /* sram settings */
    stm32_addr_t sram_base;
    size_t sram_size;

    // bootloader
    stm32_addr_t sys_base;
    size_t sys_size;

    struct stlink_version_ version;
};

Answer 2

You have the address of the start of the array in your "returned" pointer ( stdevs.getValue() ). Use that to extract the array of pointers, then initialize your structures from that.

Pointer[] ptrs = stdevs.getValue().getPointerArray(0, size.intValue());
DevStruct[] array = new DevStruct[size.intValue()];
for (int i=0;i < array.length;i++) {
    array[i] = new DevStruct(ptrs[i]);
}

public class DevStruct extends Structure {
    public DevStruct(Pointer p) {
        super(p);
        read();
    }
}