Writing your first Ouroboros C program

Here we guide you to write your first ouroboros program. It will use the basic Ouroboros IPC Application Programming Interface. It will has a client and a server that send a small “Hello World!” message from the client to the server.

We will explain how to connect two applications. The server application uses the flow_accept() call to accept incoming connections and the client uses the flow_alloc() call to connect to the server. The flow_accept and flow_alloc call have the following definitions:

int flow_accept(qosspec_t * qs, const struct timespec * timeo);
int flow_alloc(const char * dst, qosspec_t * qs, const struct timespec * timeo);

On the server side, the flow_accept() call is a blocking call that will wait for an incoming flow from a client. On the client side, the flow_alloc() call is a blocking call that allocates a flow to dst. Both calls return an non-negative integer number describing a “flow descriptor”, which is very similar to a file descriptor. On error, they will return a negative error code. (See the man page for all details). If the timeo parameter supplied is NULL, the calls will block indefinitely, otherwise flow_alloc() will return -ETIMEDOUT when the time interval provided by timeo expires. We are working on implementing non-blocking versions if the provided timeo is 0.

After the flow is allocated, the flow_read() and flow_write() calls are used to read from the flow descriptor. They operate just like the read() and write() POSIX calls. The default behaviour is that these calls will block. To release the resource, the flow can be deallocated using flow_dealloc.

ssize_t flow_write(int fd, const void * buf, size_t count);
ssize_t flow_read(int fd, void * buf, size_t count); int
flow_dealloc(int fd);

So a very simple application would just need a couple of lines of code for both the server and the client:

/* server side */
char msg[BUF_LEN];
int fd = flow_accept(NULL, NULL);
flow_read(fd, msg, BUF_LEN);
flow_dealloc(fd);

/* client side */
char * msg = "Hello World!";
int fd = flow_alloc("server", NULL, NULL);
flow_write(fd, msg, strlen(msg));
flow_dealloc(fd);

The full code for an example is the oecho application in the tools directory.

To compile your C program from the command line, you have to link -louroboros-dev. For instance, in the Ouroboros repository, you can do

cd src/tools/oecho
gcc -louroboros-dev oecho.c -o oecho