# PR_OpenTCPSocket

Creates a new TCP socket of the specified address family.

## Syntax

```{code}
#include <prio.h>

PRFileDesc* PR_OpenTCPSocket(PRIntn af);
```

### Parameters

The function has the following parameters:

`af`

: The address family of the new TCP socket. Can be `PR_AF_INET`
  (IPv4), `PR_AF_INET6` (IPv6), or `PR_AF_LOCAL` (Unix domain,
  supported on POSIX systems only).

### Returns

The function returns one of the following values:

- Upon successful completion, a pointer to the {ref}`PRFileDesc` object
  created for the newly opened TCP socket.
- If the creation of a new TCP socket failed, `NULL`.

## Description

TCP (Transmission Control Protocol) is a connection-oriented, reliable
byte-stream protocol of the TCP/IP protocol suite. {ref}`PR_OpenTCPSocket`
creates a new TCP socket of the address family `af`. A TCP connection
is established by a passive socket (the server) accepting a connection
setup request from an active socket (the client). Typically, the server
binds its socket to a well-known port with {ref}`PR_Bind`, calls
{ref}`PR_Listen` to start listening for connection setup requests, and
calls {ref}`PR_Accept` to accept a connection. The client makes a
connection request using {ref}`PR_Connect`.

After a connection is established, the client and server may send and
receive data between each other. To receive data, one can call
{ref}`PR_Read` or {ref}`PR_Recv`. To send data, one can call {ref}`PR_Write`,
{ref}`PR_Writev`, {ref}`PR_Send`, or {ref}`PR_TransmitFile`. {ref}`PR_AcceptRead` is
suitable for use by the server to accept a new client connection and
read the client's first request in one function call.

A TCP connection can be shut down by {ref}`PR_Shutdown`, and the sockets
should be closed by {ref}`PR_Close`.