lindelof lindelof - 3 months ago 18
C Question

How to stub a socket in C?

I've written client code that's supposed to send some data through a socket and read back an answer from the remote server.

I would like to unit-test that code. The function's signature is something along the lines of:

double call_remote(double[] args, int fd);

is the file descriptor of the socket to the remote server.

Now the
function will, after sending the data, block on reading the answer from the server. How can I stub such a remote server for unit-testing the code?

Ideally I would like something like:

int main() {
int stub = /* initialize stub */
double expected = 42.0;

assert(expected == call_remote(/* args */, stub);

return 0;

double stub_behavior(double[] args) {
return 42.0;

I would like
to be called and send the
value down the stubbed file descriptor.

Any easy way I can do that?

caf caf

If this is a POSIX system, you can use fork() and socketpair():

double stub_behaviour(double []);

int initialize_stub(void)
    int sock[2];
    double data[N_DOUBLES_EXPECTED];

    socketpair(AF_UNIX, SOCK_STREAM, 0, p);

    if (fork()) {
        return sock[1];

    /* Child process */


    /* read N_DOUBLES_EXPECTED in */
    read(sock[0], data, sizeof data);

    /* execute stub */
    data[0] = stub_behaviour(data);

    /* write one double back */
    write(sock[0], data, sizeof data[0]);

int main()
  int stub = initialize_stub();
  double expected = 42.0;

  assert(expected == call_remote(/* args */, stub);

  return 0;

double stub_behavior(double args[])
  return 42.0;

...of course, you will probably want to add some error checking, and alter the logic that reads the request.

The file descriptor created by socketpair() is a normal socket, and thus socket calls like send() and recv() will work fine on it.