---
name: "BIO_connect"
section: "3ssl"
platform: "linux"
package: "openssl"
version: "3.6.5-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/BIO_connect"
---

# BIO\_connect(3ssl)

BIO socket communication setup routines

## NAME

BIO\_socket, BIO\_bind, BIO\_connect, BIO\_listen, BIO\_accept\_ex, BIO\_closesocket - BIO socket communication setup routines

## SYNOPSIS

```
 #include <openssl/bio.h>
 int BIO_socket(int domain, int socktype, int protocol, int options);
 int BIO_bind(int sock, const BIO_ADDR *addr, int options);
 int BIO_connect(int sock, const BIO_ADDR *addr, int options);
 int BIO_listen(int sock, const BIO_ADDR *addr, int options);
 int BIO_accept_ex(int accept_sock, BIO_ADDR *peer, int options);
 int BIO_closesocket(int sock);
```

## DESCRIPTION

**BIO\_socket()** creates a socket in the domain **domain**, of type **socktype** and **protocol**.  Socket **options** are currently unused, but is present for future use.

**BIO\_bind()** binds the source address and service to a socket and may be useful before calling **BIO\_connect()**.  The options may include **BIO\_SOCK\_REUSEADDR**, which is described in "FLAGS" below.

**BIO\_connect()** connects **sock** to the address and service given by **addr**.  Connection **options** may be zero or any combination of **BIO\_SOCK\_KEEPALIVE**, **BIO\_SOCK\_NONBLOCK** and **BIO\_SOCK\_NODELAY**. The flags are described in "FLAGS" below.

**BIO\_listen()** has **sock** start listening on the address and service given by **addr**.  Connection **options** may be zero or any combination of **BIO\_SOCK\_KEEPALIVE**, **BIO\_SOCK\_NONBLOCK**, **BIO\_SOCK\_NODELAY**, **BIO\_SOCK\_REUSEADDR** and **BIO\_SOCK\_V6\_ONLY**. The flags are described in "FLAGS" below.

**BIO\_accept\_ex()** waits for an incoming connections on the given socket **accept\_sock**.  When it gets a connection, the address and port of the peer gets stored in **peer** if that one is non-NULL. Accept **options** may be zero or **BIO\_SOCK\_NONBLOCK**, and is applied on the accepted socket.  The flags are described in "FLAGS" below.

**BIO\_closesocket()** closes **sock**.

## FLAGS

- BIO\_SOCK\_KEEPALIVE

  Enables regular sending of keep-alive messages.

- BIO\_SOCK\_NONBLOCK

  Sets the socket to nonblocking mode.

- BIO\_SOCK\_NODELAY

  Corresponds to **TCP\_NODELAY**, and disables the Nagle algorithm.  With this set, any data will be sent as soon as possible instead of being buffered until there's enough for the socket to send out in one go.

- BIO\_SOCK\_REUSEADDR

  Try to reuse the address and port combination for a recently closed port.

- BIO\_SOCK\_V6\_ONLY

  When creating an IPv6 socket, make it only listen for IPv6 addresses and not IPv4 addresses mapped to IPv6.

- BIO\_SOCK\_TFO

  Enables TCP Fast Open on the socket. Uses appropriate APIs on supported operating systems, including Linux, macOS and FreeBSD. Can be used with **BIO\_connect()**, **BIO\_set\_conn\_mode()**, **BIO\_set\_bind\_mode()**, and **BIO\_listen()**. On Linux kernels before 4.14, use **BIO\_set\_conn\_address()** to specify the peer address before starting the TLS handshake.

These flags are bit flags, so they are to be combined with the `"|"` operator, for example:

```
 BIO_connect(sock, addr, BIO_SOCK_KEEPALIVE | BIO_SOCK_NONBLOCK);
```

## RETURN VALUES

**BIO\_socket()** returns the socket number on success or **INVALID\_SOCKET** (-1) on error.  When an error has occurred, the OpenSSL error stack will hold the error data and errno has the system error.

**BIO\_bind()**, **BIO\_connect()** and **BIO\_listen()** return 1 on success or 0 on error. When an error has occurred, the OpenSSL error stack will hold the error data and errno has the system error.

**BIO\_accept\_ex()** returns the accepted socket on success or **INVALID\_SOCKET** (-1) on error.  When an error has occurred, the OpenSSL error stack will hold the error data and errno has the system error.

## SEE ALSO

**BIO\_ADDR**(3)

## HISTORY

**BIO\_gethostname()**, **BIO\_get\_port()**, **BIO\_get\_host\_ip()**, **BIO\_get\_accept\_socket()** and **BIO\_accept()** were deprecated in OpenSSL 1.1.0. Use the functions described above instead.

## COPYRIGHT

Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use this file except in compliance with the License.  You can obtain a copy in the file LICENSE in the source distribution or at \<https://www.openssl.org/source/license.html\>.