LZ4F_decompress() returning 0 is the signal that the current LZ4 frame is complete.

That means the frame footer has been consumed too: the zero-size block / EndMark, and the optional content checksum if the frame has one. The public header says:

/* Call repeatedly until the return value is 0 (frame fully decoded) ... */

and for the 0 case:

/* The current frame is fully decoded. If *srcSizePtr is less than the provided
 * value, the unconsumed tail is the start of another frame (if any). */

So the usual loop is:

size_t ret = 1;
while (ret != 0) {
    size_t srcSize = available_input;
    size_t dstSize = dstCapacity;

    ret = LZ4F_decompress(dctx, dst, &dstSize, src, &srcSize, NULL);
    if (LZ4F_isError(ret)) {
        // handle error
    }

    // consume srcSize bytes from input
    // write/use dstSize bytes from dst
}

// ret == 0: one frame is complete

The distinction is frame end vs stream/file end.

The LZ4 frame format allows multiple frames to be concatenated, and the spec explicitly leaves multi-frame stream handling to the decoder/application. The reference CLI decodes concatenated frames in sequence, but the frame API reports completion one frame at a time.

So if your project has the concept of "one compressed payload is exactly one LZ4 frame", then ret == 0 is your decompression-complete condition. You may also want to reject any unconsumed bytes after that if trailing data is invalid for your format.

If your input can contain concatenated frames, then after ret == 0 you should check whether there are unconsumed bytes or more bytes from the file/socket. If there are, feed them as the start of the next frame, optionally after LZ4F_resetDecompressionContext(dctx). If there are no remaining bytes and your underlying source reports EOF, then the whole stream is done.

For a live stream/socket, the EndMark cannot tell you that the transport itself is done; it only tells you the current LZ4 frame is done. You still need EOF, a content length, a higher-level message boundary, or your own framing if you need to know there will not be another LZ4 frame after it.