3. Common Functions
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## 3. Common Functions

### 3.1. Managing Endpoints

         COMSTACK cs_create(CS_TYPE type, int blocking, int protocol);

Creates an instance of the protocol stack - a communications endpoint. The `type` parameter determines the mode of communication. At present the following values are supported:

`tcpip_type`

  TCP/IP (BER over TCP/IP or HTTP over TCP/IP)

`ssl_type`

  Secure Socket Layer (SSL). This COMSTACK is experimental and is not fully implemented. If HTTP is used, this effectively is HTTPS.

`unix_type`

  Unix socket (unix only). Local Transfer via file socket. See unix(7).

The `cs_create` function returns a null-pointer if a system error occurs. The `blocking` parameter should be one if you wish the association to operate in blocking mode, zero otherwise. The `protocol` field should be `PROTO_Z3950` or `PROTO_HTTP`. Protocol `PROTO_SR` is no longer supported.

         void cs_close(COMSTACK handle);

Closes the connection (as elegantly as the lower layers will permit), and releases the resources pointed to by the `handle` parameter. The `handle` should not be referenced again after this call.

### Note

We really need a soft disconnect, don't we?

### 3.2. Data Exchange

         int cs_put(COMSTACK handle, char *buf, int len);

Sends `buf` down the wire. In blocking mode, this function will return only when a full buffer has been written, or an error has occurred. In nonblocking mode, it's possible that the function will be unable to send the full buffer at once, which will be indicated by a return value of 1. The function will keep track of the number of octets already written; you should call it repeatedly with the same values of `buf` and `len`, until the buffer has been transmitted. When a full buffer has been sent, the function will return 0 for success. -1 indicates an error condition (see below).

         int cs_get(COMSTACK handle, char **buf, int *size);

Receives a PDU or HTTP Response from the peer. Returns the number of bytes read. In nonblocking mode, it is possible that not all of the packet can be read at once. In this case, the function returns 1. To simplify the interface, the function is responsible for managing the size of the buffer. It will be reallocated if necessary to contain large packages, and will sometimes be moved around internally by the subsystem when partial packages are read. Before calling `cs_get` for the fist time, the buffer can be initialized to the null pointer, and the length should also be set to 0 - cs_get will perform a `malloc(2)` on the buffer for you. When a full buffer has been read, the size of the package is returned (which will always be greater than 1). -1 indicates an error condition.

See also the `cs_more()` function below.

         int cs_more(COMSTACK handle);

The `cs_more()` function should be used in conjunction with `cs_get` and `select(2)`. The `cs_get()` function will sometimes (notably in the TCP/IP mode) read more than a single protocol package off the network. When this happens, the extra package is stored by the subsystem. After calling `cs_get()`, and before waiting for more input, You should always call `cs_more()` to check if there's a full protocol package already read. If `cs_more()` returns 1, `cs_get()` can be used to immediately fetch the new package. For the mOSI subsystem, the function should always return 0, but if you want your stuff to be protocol independent, you should use it.

### Note

The `cs_more()` function is required because the RFC1729-method does not provide a way of separating individual PDUs, short of partially decoding the BER. Some other implementations will carefully nibble at the packet by calling `read(2)` several times. This was felt to be too inefficient (or at least clumsy) - hence the call for this extra function.

         int cs_look(COMSTACK handle);

This function is useful when you're operating in nonblocking mode. Call it when `select(2)` tells you there's something happening on the line. It returns one of the following values:

CS_NONE

  No event is pending. The data found on the line was not a complete package.

CS_CONNECT

  A response to your connect request has been received. Call `cs_rcvconnect` to process the event and to finalize the connection establishment.

CS_DISCON

  The other side has closed the connection (or maybe sent a disconnect request - but do we care? Maybe later). Call `cs_close` to close your end of the association as well.

CS_LISTEN

  A connect request has been received. Call `cs_listen` to process the event.

CS_DATA

  There's data to be found on the line. Call `cs_get` to get it.

### Note

You should be aware that even if `cs_look()` tells you that there's an event event pending, the corresponding function may still return and tell you there was nothing to be found. This means that only part of a package was available for reading. The same event will show up again, when more data has arrived.

         int cs_fileno(COMSTACK h);

Returns the file descriptor of the association. Use this when file-level operations on the endpoint are required (`select(2)` operations, specifically).

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