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Calling socket() creates a TCP socket and gives your process a file descriptor; it does not, by itself, connect to another machine. A client normally starts a connection with connect(). A server instead prepares a listening socket with bind() and listen(), then gets a separate connected socket for each client through accept().
What does socket() do?
A typical TCP socket begins with a call such as socket(AF_INET, SOCK_STREAM, IPPROTO_TCP). The arguments request an IPv4 stream socket using TCP. Linux returns a file descriptor: a process-level handle that can be passed to socket operations such as connect(), bind(), listen(), accept(), read() and write().
At this point, the socket is not a connected session. It has no peer, and socket() does not normally send a TCP SYN. The call creates the endpoint handle; what happens next depends on whether the program is acting as a client or a server. The Linux man-pages project describes this API behavior in its tcp(7) documentation.
What happens when a client calls connect()?
- Create the socket. The client calls
socket()to obtain a TCP socket descriptor. - Optionally bind a local address. A program can call
bind()to choose a local address or port. Many clients omit this step and let Linux select local endpoint details as part of setting up the connection. - Call
connect(). The client supplies the remote address and port. Linux associates the attempt with local and remote endpoint information and begins connection establishment. The chosen local port and route depend on the host and network. - Exchange TCP control packets. For an ordinary connection, the conceptual handshake is client SYN, server SYN-ACK, then client ACK. This is a packet-level model, not a claim that the system call is itself one packet or that every Linux kernel follows one identical internal call path.
- Use the connected stream. After a successful connection, each side has TCP state supporting sequence tracking, retransmission, flow control and ordered delivery. The application can then read and write bytes.
With a blocking socket, connect() normally returns when the attempt succeeds or fails. With nonblocking setup, the connection may still be pending when the call returns. Linux TCP Fast Open can allow data to accompany connection setup in supported and configured cases, so the ordinary handshake model is not the only possible behavior. See connect(2) and tcp(7).
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If connect() fails
Linux documents the socket’s state after a failed connect() as unspecified. For another attempt, close that socket and create a new one rather than assuming it can safely be reused. How long a failure takes to surface can vary; network timeouts are not a fixed universal interval. The recommendation is documented in connect(2).
What happens on the server side?
socket(): Create a TCP socket descriptor.bind(): Associate it with a local address and port, commonly the service endpoint clients will contact.listen(): Mark the socket as passive, ready to receive connection requests.accept(): Retrieve a pending connection. Linux returns a new connected descriptor for that client; the original descriptor remains the listener and can accept more connections.
If a blocking listener has no pending connection, accept() waits. The listener does not become the client connection: it remains available to receive further connections while the application handles each accepted descriptor separately. See listen(2) and accept(2).
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What does the listen() backlog mean on Linux?
The backlog argument to listen() limits the queue of fully established connections waiting for the application to accept them. It is not the limit for every connection request in progress: incomplete requests have a separate control, net.ipv4.tcp_max_syn_backlog. Linux also caps the requested backlog at net.core.somaxconn.
| Setting or stage | What it controls | Documented version detail |
|---|---|---|
listen(backlog) |
Fully established connections waiting for accept() |
The value is capped by net.core.somaxconn. |
net.ipv4.tcp_max_syn_backlog |
Incomplete connection requests | A separate control from the established-connection backlog. |
net.core.somaxconn |
Maximum effective listen backlog | Linux man-pages 6.19 documents a default of 4096 since Linux 5.4 and 128 before Linux 5.4. |
Those figures are documented defaults, not guarantees about a particular machine’s runtime configuration. The installed kernel version and configured value matter. The distinctions and defaults are described in listen(2).
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What do reads and writes mean on a TCP socket?
TCP provides a reliable, ordered, full-duplex byte stream. It does not preserve application record boundaries: one write is not guaranteed to correspond to one read, and a read may return fewer bytes than the application requested. As the Linux man-pages put it, “TCP does not preserve record boundaries.”
If an application needs messages, it must define framing above TCP—for example, a fixed-size record, a delimiter, or a length prefix—and handle partial reads and writes accordingly. This stream behavior is described in tcp(7).
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What happens “inside” Linux—and what varies?
From the application’s perspective, Linux provides a file descriptor and socket system calls. Inside the kernel, networking code maintains socket and TCP protocol state and participates in the IP and network-device path. That is the useful architectural picture; it does not imply one invariant series of internal functions or hardware events.
Exact allocation details, routing decisions, filtering traversal, interrupt behavior and driver operations depend on the Linux release, address family, configuration, namespace and environment. Without a specific kernel and configuration, those details should not be presented as a universal call trace. The observable API distinctions are more stable: socket() creates the endpoint, connect() initiates an outgoing association, and server-side accept() returns a new connected descriptor.
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