How to match a field of a struct which is passed by reference to a mocked function?

Question

I have the following struct :

    struct can_frame {
        canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
        __u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
        __u8    __pad;   /* padding */
        __u8    __res0;  /* reserved / padding */
        __u8    __res1;  /* reserved / padding */
        __u8    data[CAN_MAX_DLEN] __attribute__((aligned(8)));
    };

I pass the struct by reference to the mocked function : MOCK_METHOD1(write, int(can_frame* frame));

I would like to match if the passes struct has a given number in the datafield: EXPECT_CALL(*canSocketMock_, write(**if the value of can_frame->data[1] is equal to 10, else assert false**))).WillOnce(Return(16));

I tried to combine the Pointee, Field and ArrayElement matchers, but failed to create the thing I wanted. The syntax of matchers is a bit too confusing for me.

Edit: The test:

TEST_F(SchunkDeviceShould, applyBreakWritesRightMessage) {
ASSERT_NO_THROW(sut_.reset(new SchunkDevice(
        canSocketMock_, 3)));
can_frame frame;
EXPECT_CALL(*canSocketMock_, write(FrameDataEquals(&frame, 1, CMD_STOP)))).WillOnce(Return(16));
ASSERT_TRUE(sut_->applyBreak());
}

The function we call:

bool SchunkDevice::applyBreak() {
    can_frame frame;
    frame.can_id = MESSAGE_TO_DEVICE+canID_;
    frame.can_dlc = 0x02;
    frame.data[0] = frame.can_dlc - 1;
    frame.data[1] = CMD_STOP;
    if (int len = socket_->write(&frame) != 16) {
        return false;
    }
    return true;
}

Output of the test:

Unexpected mock function call - taking default action specified at:
/home/../SchunkDeviceTests.cpp:47:
    Function call: write(0x7ffc27c4de60)
          Returns: 16
Google Mock tried the following 1 expectation, but it didn't match:

/home/../SchunkDeviceTests.cpp:457: EXPECT_CALL(*canSocketMock_, write(FrameDataEquals(&frame, 1, CMD_STOP)))...
  Expected arg #0: frame data equals (0x7ffc27c4def0, 1, 145)
           Actual: 0x7ffc27c4de60
         Expected: to be called once
           Actual: never called - unsatisfied and active

Answer 1

You could define a custom matcher to examine the structure contents.

MATCHER_P2(FrameDataEquals, index, value, "") { return (arg->data[index] == value); }

You would then use it like this:

EXPECT_CALL(mock, write(FrameDataEquals(1, 10))).WillOnce(Return(16));