Why g++ giver: "error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]"

Question

I'm doing an UDP connection, between a microcontroller and a computer. The framework I'm using is c++ based and has a function to send an UDP packet with the following prototype:

bool UdpConnection::send(const char *data, int length)

int length is the number of bytes that the pointer contains.

But I'm doing some input reading using a function that returns a uint16_t type.

I cannot change anything directly in those two functions.

Then I did the following:

UdpConnection udp;
uint16_t dummy = 256;
udp.send(reinterpret_cast<char*>(dummy),2);

But I'm a curious guy so I tried the following:

UdpConnection udp;
uint16_t dummy = 256;
udp.send((char*)dummy,2);

When I compile this last code I get:

error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]

In my analysis both snippets do the same thing, why I get an error in the last one but not in the first?

EDIT:

The first snippet of code compiles but gives segmentation fault when the code runs. So neither code works.

EDIT 2:

An efficient and tested solution for the problem, but not an answer to the original question, is this:

union Shifter {
    uint16_t b16;
    char b8[2];
} static shifter;

shifter.b16 = 256;
udp.send(shifter.b8,2);

This solution is widely used, but it's not portable as it's dependent on CPU byte ordering, so do a test in your application before to be sure.

Answer 1

I would assume this is correct:

udp.send(reinterpret_cast<char*>(&dummy),2);

Note ampersand. Otherwise you are sending two byes from address 256, which are probably random (at best). This is microcontroller, so it might not crash. Second version should be:

udp.send((char*)&dummy,2);