Camera SDK callback wrapper DLL for LabVIEW: bridge events with C

Your camera SDK ships as a plain DLL, and its manual shows one road to the pixels: install a frame hook, receive a property struct and a frame buffer per image. The Call Library Function Node cannot accept that hook, so the camera stays silent in LabVIEW. The fix is a small C wrapper DLL you can build in an afternoon. This guide gives you the design, the buffer math that keeps it from dropping frames, the LabVIEW wiring, and a synthetic test that proves the bridge before any hardware is involved.

Why the Call Library Function Node cannot take the hook

A compiled function lives at an address the operating system can call; a LabVIEW VI is data interpreted by the runtime, so there is no address to hand over. The Call Library Function Node calls DLL exports with data parameters and receives data back — it cannot offer LabVIEW code as the destination of the SDK's call. The rule that falls out: any SDK parameter typed as a function pointer needs compiled C behind it, and everything else in the same SDK (open, exposure, start, stop) goes through the node normally. One small bridge DLL, then, and only for the hook. The background is spelled out in why a LabVIEW VI cannot be the callback for a DLL function.

What the wrapper does

Give the bridge four flat exports — install, pop, drop counter, shutdown; the names below are this article's illustrative exports, rename them freely. install passes a static C function to the SDK's hook installer. That callback fires on the SDK's own thread with the property struct and the frame pointer, and it does exactly three things: copy the 28-byte struct, copy the frame bytes into the next slot of a ring buffer, return. Total time in the callback stays under a millisecond, and a counter records any frame that arrives while the ring is full. LabVIEW never appears in that thread; a reader loop on the LabVIEW side pops finished frames at its own pace.

Copying inside the callback is not optional: the SDK may reuse its frame buffer the moment your callback returns — the same trap that tears frames when a callback queues a buffer before copying it.

The wrapper in C

The skeleton fits on one screen. SDK identifiers are copied as the header prints them; everything named bridge_* is yours.

/* bridge.c — bridge_* exports are this article's illustrative names.
   TProcessedDataProperty, FrameCallBack and CCDUSB_InstallFrameHooker
   are the SDK's own identifiers, copied as its header prints them. */
#include <string.h>

typedef struct {
    int CameraID;
    int ExposureTime;
    int TimeStamp;
    int TriggerOccurred;
    int TriggerEventCount;
    int OverSaturated;
    int LightShieldAverageValue;
} TProcessedDataProperty;            /* 7 x 4-byte int = 28 bytes */

static TProcessedDataProperty g_prop[64];  /* size to your maximum */
static unsigned char         *g_data[64];  /* one buffer per slot  */
static long g_slots = 0, g_bytes = 0;
static long g_wr = 0, g_rd = 0, g_count = 0, g_dropped = 0;
/* one critical section (g_lock), created in bridge_install */

void _cdecl FrameCallBack(int c0, int c1, int c2,  /* header leaves these
                           unnamed; keep the three int types */
                          TProcessedDataProperty *Attributes,
                          unsigned char *FrameData)
{
    lock(&g_lock);                       /* your critical-section calls */
    if (g_count < g_slots) {
        long i = g_wr;
        g_prop[i] = *Attributes;         /* 28-byte struct copy */
        memcpy(g_data[i], FrameData, (size_t)g_bytes);
        g_wr = (g_wr + 1) % g_slots;  g_count++;
    } else {
        g_dropped++;                     /* ring full: keep oldest */
    }
    unlock(&g_lock);
}

long bridge_install(long frame_bytes, long slots)
{
    g_bytes = frame_bytes;  g_slots = slots;
    /* allocate the g_data buffers and the lock here, then hook: */
    CCDUSB_InstallFrameHooker(0, FrameCallBack);  /* check its return */
    return 0;
}

long bridge_pop(int *cam, int *exp, int *ts, int *trg, int *cnt,
                int *sat, int *shd, unsigned char *dst, long cap)
{
    long i;
    lock(&g_lock);
    if (g_count == 0) { unlock(&g_lock); return 0; }
    if (cap < g_bytes) { unlock(&g_lock); return -1; }
    i = g_rd;
    *cam = g_prop[i].CameraID;   *exp = g_prop[i].ExposureTime;
    *ts  = g_prop[i].TimeStamp;  *trg = g_prop[i].TriggerOccurred;
    *cnt = g_prop[i].TriggerEventCount;
    *sat = g_prop[i].OverSaturated;
    *shd = g_prop[i].LightShieldAverageValue;
    memcpy(dst, g_data[i], (size_t)g_bytes);
    g_rd = (g_rd + 1) % g_slots;  g_count--;
    unlock(&g_lock);
    return 1;
}

long bridge_dropped(void) { return g_dropped; }

long bridge_shutdown(void)
{
    /* stop acquisition in the SDK first; free g_data and the lock here */
    return 0;
}

Size the ring before wiring LabVIEW

Two numbers drive the size: how fast frames arrive and how long your reader loop can stall. Pick the longest stall you will tolerate — a slow disk write, a paused window — and buy twice that, so two bad moments in a row still fit.

If the hook fires per scan line at a few thousand lines per second, aggregate lines inside the wrapper first — sum N lines into one slot — because a 2 ms poll cannot chase individual lines. If a dropped frame scraps a part, let the ring grow; if a counted drop is tolerable, a smaller ring and an alarm on the counter work too.

The same math covers a board-level camera on a custom carrier — the bytes are the bytes.

Build the DLL

1. Create an empty DLL project in the C toolchain you already use, configured for your LabVIEW bitness: 32-bit LabVIEW loads 32-bit DLLs, 64-bit loads 64-bit. A mismatch fails at load time, the failure described for camera SDK DLLs that will not load on the wrong bitness.
2. Copy the SDK's struct and callback signature exactly as its header prints them — field order, types, and the cdecl. On a 32-bit build a convention mismatch corrupts the stack; the usual symptom is error 1097 or a crash on the first frame.
3. Export the four bridge functions with flat parameters only: scalars by pointer, one byte-array pointer with a capacity argument. Flat survives the node boundary without cluster gymnastics.
4. Build Release, place the DLL beside your VIs, and run the console test below before LabVIEW ever loads it.

Wire the LabVIEW side

5. One Call Library Function Node per export. For bridge_pop, declare seven scalars passed by pointer, then the byte array as a pointer with a size parameter, and set the node's calling convention to match the export.
6. Preallocate the destination array once outside the loop — its size never changes — for the same reason you would reuse image buffers instead of allocating per frame.
7. Reader loop: a While loop with a 2 ms wait that calls bridge_pop. A return of 1 means unbundle the seven fields — CameraID, ExposureTime, TimeStamp, TriggerOccurred, TriggerEventCount, OverSaturated, LightShieldAverageValue, named as the SDK prints them — and hand the payload to your display; 0 means idle. At 60 fps the loop polls 500 times per second and pops 60, so added latency stays under about 2.5 ms including the 0.23 ms copy.
8. Shutdown order: stop acquisition in the SDK first, then bridge_shutdown, then close. Uninstalling after the device is gone leaves the driver hooked and the next run silent — the ordering trap behind acquisition that stops while buffers are still busy.

Push instead of poll, if you must

Polling keeps every thread boundary boring, which is why it is the default here. If you need push on arrival, the wrapper still cannot create or fire a LabVIEW user event from C — user events live inside LabVIEW. The classic route is LabVIEW's occurrence mechanism: the main VI creates an occurrence, passes its ID to bridge_install, and the callback triggers the occurrence after each copy; a waiting consumer then pops. The wiring lives in configuring LabVIEW to receive DLL driver callbacks. Push does not remove the queue: the SDK thread still cannot block on your consumer, so keep the ring either way.

Test with synthetic frames before the camera is in the loop

Write a 20-line console program that links the bridge DLL and calls FrameCallBack directly, no SDK involved — the same self-test idea as writing your own test adaptor for image acquisition. Set slots = 4 and frame_bytes = 64. Fill the struct with ExposureTime = 1234 and a payload where byte i equals i modulo 256. Call the callback 10 times. Expected result: bridge_pop returns 1 four times, each pop reads ExposureTime 1234 and byte 255 at index 255, then returns 0; bridge_dropped reports 6. That one test proves the struct copy, the byte copy, the ring arithmetic and the counter. Only then wire the SDK: at 60 fps the drop counter must sit at 0 across a 10-minute soak, and TimeStamp must increase frame to frame.

Common failure modes

• Crash on the first frame — convention mismatch (32-bit), a struct field out of order, or a LabVIEW call from the callback thread. The callback touches the ring, nothing else.
• Error 1097 from the node — wrong prototype: parameter count, sizes, array handling or convention. Compare the node's configuration against the export, parameter by parameter.
• Freezes under load — two threads in one queue, or a consumer that calls back into acquisition; the classic patterns are in multi-camera acquisition freezes from callbacks and buffer queues.
• Silent after restart — hook installed after acquisition started, or shutdown order inverted. Install the hook before start; stop, then shutdown, then close.
• Drops creep up — a consumer that occasionally stalls on logging or display work. Grow the ring 2× or move the slow work off the reader loop.

When to skip the wrapper

If the SDK also exports flat polling calls — acquire one frame, get the buffer — call those from the node and skip C entirely; the walkthrough of calling a camera SDK DLL from LabVIEW with callbacks, buffers and snap covers SDKs whose LabVIEW path already works. If the vendor ships an ActiveX or .NET surface, LabVIEW consumes those natively, and fixing LabVIEW DLL callbacks through ActiveX is the cheaper route when callbacks are the only sticking point. Write the wrapper only when the hook is the only road to the pixels — and then it is an afternoon, not a project.

The PC side of the frame path

The wrapper runs wherever the SDK's camera is attached, and every byte you counted crosses that PC's I/O before the copy. For networked cameras the NIC is the part worth choosing deliberately: the frame grabbers category carries the Ethernet adapters and capture cards. Do the division before sharing ports: a GigE link delivers about 118 MB/s of payload, a 10GigE link roughly 1.2 GB/s.

For a one- or two-camera GigE bench, the MV-GE2002P is a dual-port PoE adapter — one card, one driver.

When the cameras speak 10GigE, step up to the MV-GT2104, a quad-port copper NIC.

Ship checklist

• Struct and signature copied from the SDK header, not retyped from memory — the field order is the interface.
• Callback under 1 ms: copies only, no SDK calls, no LabVIEW calls, no allocation.
• Ring sized by the formula; drop counter wired to an alarm, not a file nobody reads.
• Bitness of bridge DLL, SDK DLL and LabVIEW all matched.
• Synthetic test passed: 4 kept, 6 counted, payload byte 255 reads 255.
• Soak passed: 0 drops in 10 minutes at rate, TimeStamp strictly increasing.
• Shutdown rehearsed: stop, shutdown, close, restart clean.

Once the bridge runs, the rest of the machine is ordinary LabVIEW. For a second pair of eyes on the wrapper or the capture hardware behind it, use the contact page; the systems topic page gathers the acquisition-side guides, and the guides hub indexes everything. To spec the capture card and PC side as one build: