BIO_S_ACCEPT(3ossl)

💖 BIO_S_ACCEPT(3ossl) 💖

okay bestie, gather round, because today we're learning the accept BIO from OpenSSL — it's the thing that makes your program sit there like "hi, come in, yes you, TCP connection, welcome to the group chat" 🌸. It's giving main character energy for servers, no cap.

---

✨ NAME ✨

So the whole squad of functions on this page is:

• BIO_s_accept
• BIO_set_accept_name
• BIO_set_accept_port
• BIO_get_accept_name
• BIO_get_accept_port
• BIO_new_accept
• BIO_set_nbio_accept
• BIO_set_tfo_accept
• BIO_set_accept_bios
• BIO_get_peer_name
• BIO_get_peer_port
• BIO_get_accept_ip_family
• BIO_set_accept_ip_family
• BIO_set_bind_mode
• BIO_get_bind_mode
• BIO_do_accept

…and collectively? They're the accept BIO. That's the vibe. 👑

---

🌈 SYNOPSIS 🌈

#include <openssl/bio.h>

const BIO_METHOD *BIO_s_accept(void);

long BIO_set_accept_name(BIO *b, char *name);
char *BIO_get_accept_name(BIO *b);

long BIO_set_accept_port(BIO *b, char *port);
char *BIO_get_accept_port(BIO *b);

BIO *BIO_new_accept(char *host_port);

long BIO_set_nbio_accept(BIO *b, int n);
long BIO_set_tfo_accept(BIO *b, int n);
long BIO_set_accept_bios(BIO *b, char *bio);

char *BIO_get_peer_name(BIO *b);
char *BIO_get_peer_port(BIO *b);
long BIO_get_accept_ip_family(BIO *b);
long BIO_set_accept_ip_family(BIO *b, long family);

long BIO_set_bind_mode(BIO *b, long mode);
long BIO_get_bind_mode(BIO *b);

int BIO_do_accept(BIO *b);

---

🦋 DESCRIPTION 🦋

BIO_s_accept() hands you back the accept BIO method. It's literally a wrapper round the platform's TCP/IP socket accept routines — like a cute lil jacket over something feral. 🧥

With accept BIOs, TCP/IP connections can be accepted and data transferred using only BIO routines. So every platform-specific messy detail gets hidden behind the BIO abstraction. Slay. ✨

Read and write operations on an accept BIO will perform I/O on the underlying connection. And if no connection is established yet, but the port (see below bestie) is set up properly, then the BIO just waits for an incoming connection. Patient queen behavior. 💅

Accept BIOs support BIO_puts() but not BIO_gets(). Noted. 📝

If the close flag is set on an accept BIO, then any active connection on that chain is shutdown and the socket is closed when the BIO is freed. Very "we're done here" of it.

Calling BIO_reset() on an accept BIO will close any active connection and reset the BIO into a state where it awaits another incoming connection. Fresh start, clean slate, iconic. 🎀

BIO_get_fd() and BIO_set_fd() can be called to retrieve or set the accept socket. Peep BIO_s_fd(3) for that.

💫 Setting up names and ports 💫

BIO_set_accept_name() uses the string name to set the accept name. The name is a string shaped like "host:port", where "host" is the interface to use and "port" is the port. The host can be "*" or empty, which is interpreted as meaning any interface. If the host is an IPv6 address, it has to be enclosed in brackets, for example "[::1]:https". And "port" has the same syntax as the port specified in BIO_set_conn_port() for connect BIOs — that is, it can be a numerical port string, or a string to lookup using getservbyname() and a string table.

BIO_set_accept_port() uses the string port to set the accept port of BIO b. Same deal: "port" has the same syntax as the port specified in BIO_set_conn_port() for connect BIOs, so it can be a numerical port string or a string to lookup using getservbyname() and a string table. And if the given port is 0, then a random available port is chosen. You can query it using BIO_sock_info() and BIO_ADDR_service_string(3). Surprise port! 🎁

BIO_new_accept() combines BIO_new() and BIO_set_accept_name() into a single call — that is, it creates a new accept BIO with port host_port. One-stop shop, we love efficiency. 🛍️

🫶 Non-blocking, Fast Open, and BIO chains 🫶

BIO_set_nbio_accept() sets the accept socket to blocking mode (the default) if n is 0, or non blocking mode if n is 1.

BIO_set_tfo_accept() enables TCP Fast Open on the accept socket if n is 1, or disables TCP Fast Open if n is 0 (the default). Setting the value to 1 is equivalent to setting BIO_SOCK_TFO in BIO_set_bind_mode(). Zoom zoom. 🔥

BIO_set_accept_bios() can be used to set a chain of BIOs which will be duplicated and prepended to the chain when an incoming connection is received. Super useful if, for example, a buffering or SSL BIO is required for each connection. The chain of BIOs must not be freed after this call — they will be automatically freed when the accept BIO is freed. So hands off, the BIO's got custody. 🙈

🌸 IP families and bind modes 🌸

BIO_get_accept_ip_family() returns the IP family accepted by the BIO b, which may be BIO_FAMILY_IPV4, BIO_FAMILY_IPV6, or BIO_FAMILY_IPANY.

BIO_set_accept_ip_family() sets the IP family family accepted by BIO b. The default is BIO_FAMILY_IPANY.

BIO_set_bind_mode() and BIO_get_bind_mode() set and retrieve the current bind mode:

• If BIO_BIND_NORMAL (the default) is set, then another socket cannot be bound to the same port.
• If BIO_BIND_REUSEADDR is set, then other sockets can bind to the same port.
• If BIO_BIND_REUSEADDR_IF_UNUSED is set, then an attempt is first made to use BIO_BIN_NORMAL, and if this fails and the port is not in use, then a second attempt is made using BIO_BIND_REUSEADDR.
• If BIO_SOCK_TFO is set, then the socket will be configured to accept TCP Fast Open connections.

👑 BIO_do_accept() 👑

BIO_do_accept() serves two functions. When it is first called, after the accept BIO has been setup, it will attempt to create the accept socket and bind an address to it. Second and subsequent calls to BIO_do_accept() will await an incoming connection, or request a retry in non blocking mode. Double duty, no complaints. 💅

---

🌟 NOTES 🌟

When an accept BIO is at the end of a chain, it will await an incoming connection before processing I/O calls. When an accept BIO is not at then end of a chain, it passes I/O calls to the next BIO in the chain.

When a connection is established, a new socket BIO is created for the connection and appended to the chain. So the chain is now accept->socket. Which effectively means that attempting I/O on an initial accept socket will await an incoming connection and then perform I/O on it. Sneaky but correct. 🧚

If any additional BIOs have been set using BIO_set_accept_bios(), then they are placed between the socket and the accept BIO — that is, the chain will be accept->otherbios->socket.

If a server wishes to process multiple connections (as is normally the case), then the accept BIO must be made available for further incoming connections. This can be done by waiting for a connection and then calling:

connection = BIO_pop(accept);

After this call, connection will contain a BIO for the recently established connection, and accept will now be a single BIO again, which can be used to await further incoming connections. If no further connections will be accepted, the accept can be freed using BIO_free().

If only a single connection will be processed, it is possible to perform I/O using the accept BIO itself. This is often undesirable, however, because the accept BIO will still accept additional incoming connections. This can be resolved by using BIO_pop() (see above) and freeing up the accept BIO after the initial connection.

🚨 ok listen up because this one matters: if the underlying accept socket is nonblocking and BIO_do_accept() is called to await an incoming connection, it is possible for BIO_should_io_special() with the reason BIO_RR_ACCEPT. If this happens, then it is an indication that an accept attempt would block — the application should take appropriate action to wait until the underlying socket has accepted a connection and retry the call. Don't just spin, bestie. 😭

Heads up: BIO_set_accept_name(), BIO_get_accept_name(), BIO_set_accept_port(), BIO_get_accept_port(), BIO_set_nbio_accept(), BIO_set_accept_bios(), BIO_get_peer_name(), BIO_get_peer_port(), BIO_get_accept_ip_family(), BIO_set_accept_ip_family(), BIO_set_bind_mode(), BIO_get_bind_mode() and BIO_do_accept() are macros. Yes, all of them. iykyk. ✨

---

💀 RETURN VALUES 💀

BIO_do_accept(), BIO_set_accept_name(), BIO_set_accept_port(), BIO_set_nbio_accept(), BIO_set_accept_bios(), BIO_set_accept_ip_family(), and BIO_set_bind_mode() return 1 for success and <= 0 for failure.

BIO_get_accept_name() returns the accept name or NULL on error. BIO_get_peer_name() returns the peer name or NULL on error.

BIO_get_accept_port() returns the accept port as a string or NULL on error. BIO_get_peer_port() returns the peer port as a string or NULL on error. BIO_get_accept_ip_family() returns the IP family or <= 0 on error.

BIO_get_bind_mode() returns the set of BIO_BIND flags, or <= 0 on failure.

BIO_new_accept() returns a BIO or NULL on error.

---

🍓 EXAMPLES 🍓

This example accepts two connections on port 4444, sends messages down each, and finally closes both down. Iconic behavior honestly.

BIO *abio, *cbio, *cbio2;

/* First call to BIO_do_accept() sets up accept BIO */
abio = BIO_new_accept("4444");
if (BIO_do_accept(abio) <= 0) {
    fprintf(stderr, "Error setting up accept\n");
    ERR_print_errors_fp(stderr);
    exit(1);
}

/* Wait for incoming connection */
if (BIO_do_accept(abio) <= 0) {
    fprintf(stderr, "Error accepting connection\n");
    ERR_print_errors_fp(stderr);
    exit(1);
}
fprintf(stderr, "Connection 1 established\n");

/* Retrieve BIO for connection */
cbio = BIO_pop(abio);
BIO_puts(cbio, "Connection 1: Sending out Data on initial connection\n");
fprintf(stderr, "Sent out data on connection 1\n");

/* Wait for another connection */
if (BIO_do_accept(abio) <= 0) {
    fprintf(stderr, "Error accepting connection\n");
    ERR_print_errors_fp(stderr);
    exit(1);
}
fprintf(stderr, "Connection 2 established\n");

/* Close accept BIO to refuse further connections */
cbio2 = BIO_pop(abio);
BIO_free(abio);
BIO_puts(cbio2, "Connection 2: Sending out Data on second\n");
fprintf(stderr, "Sent out data on connection 2\n");

BIO_puts(cbio, "Connection 1: Second connection established\n");

/* Close the two established connections */
BIO_free(cbio);
BIO_free(cbio2);

---

⭐ HISTORY ⭐

BIO_set_tfo_accept() was added in OpenSSL 3.2. Newer than your fave, periodt. 💅

---

🎀 COPYRIGHT 🎀

Copyright 2000-2022 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>.

---

okay that's the whole page bestie, you ate it up 💖 this was BIO_S_ACCEPT(3ossl) from OpenSSL 3.5.9, dated 2026-09-29 — go forth and accept some connections, you gorgeous server queen 👑🦋✨