BIO_S_ACCEPT(3SSL)

🌸 NAME 🌸

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 — bestie, it's the accept BIO 💅

---

✨ 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 🦋

So BIO_s_accept() hands you back the accept BIO method. It's basically a cute lil wrapper round your platform's TCP/IP socket accept routines — like, all the messy platform-specific stuff gets hidden behind the BIO abstraction, and honestly? iconic. 👑

With accept BIOs you can accept TCP/IP connections and move data around using only BIO routines. The abstraction does the covering-up, you do the vibes.

Read and write ops on an accept BIO will do I/O on the underlying connection. And if no connection is set up yet but the port (see below, bestie) is configured properly, the BIO just… waits. Patiently. For an incoming connection. 💖

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 gets shutdown and the socket closed when the BIO is freed. Slay but make it clean.

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 energy. 🧚

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

BIO_set_accept_name() uses the string name to set the accept name. That name looks like "host:port", where "host" is the interface to use and "port" is the port. The host can be "*" or empty, which means "any interface, don't care, whatever". If the host is an IPv6 address it has to be wrapped in brackets, for example "[::1]:https". And "port" has the same syntax as the port you'd pass to BIO_set_conn_port() for connect BIOs — so it can be a numerical port string, or a string to look up 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" matches the syntax of the port in BIO_set_conn_port() for connect BIOs, so numerical string or getservbyname() lookup string-table situation. And ooh — if the given port is 0, then a random available port gets chosen. You can query it using BIO_sock_info() and BIO_ADDR_service_string(3). ✨

BIO_new_accept() is the combo meal: BIO_new() plus BIO_set_accept_name() in a single call. It creates a new accept BIO with port host_port. One call, no stress. 🫶

BIO_set_nbio_accept() puts the accept socket in blocking mode (that's the default, btw) 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 it to 1 is equivalent to setting BIO_SOCK_TFO in BIO_set_bind_mode(). Same thing, different door. 🚪

BIO_set_accept_bios() lets you set a chain of BIOs that will be duplicated and prepended to the chain when an incoming connection arrives. Super useful if, say, you need a buffering or SSL BIO for each connection. Important: the chain of BIOs must not be freed after this call — they get automatically freed when the accept BIO is freed. So hands off, bestie. 🙈

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. Let's break it down:

• 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_BIND_NORMAL; 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() is doing double duty, main character behavior honestly. When it's 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.

---

💜 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 the end of a chain, it just passes I/O calls along to the next BIO in the chain. Middle-of-the-chain behavior, real supporting-cast energy.

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. The way that works?? Obsessed.

If any additional BIOs have been set using BIO_set_accept_bios(), then they get placed between the socket and the accept BIO — so the chain becomes accept->otherbios->socket.

If a server wants to process multiple connections (which, yeah, that's normally the whole point), then the accept BIO must be made available for further incoming connections. Do it 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, ready to await further incoming connections. And if you're not accepting any more connections, the accept can be freed using BIO_free().

If you're only processing a single connection, it is possible to perform I/O using the accept BIO itself. But this is often undesirable, because the accept BIO will still accept additional incoming connections. 😭 The fix: use BIO_pop() (see above) and free up the accept BIO after the initial connection.

If the underlying accept socket is nonblocking and BIO_do_accept() is called to await an incoming connection, it's possible for BIO_should_io_special() to return true with the reason BIO_RR_ACCEPT. If that happens, it's an indication that an accept attempt would block — so the application should take appropriate action to wait until the underlying socket has accepted a connection and then retry the call. Not a bug, just a heads up. 🔥

Lowkey important: 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.

---

🎀 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. Two for two, we love to see it. 💖

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 the rest, and it knows it. 💅

---

👑 COPYRIGHT 👑

Copyright 2000-2026 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 bestie that's the whole page, no cap, nothing skipped 🫶

This was the BIO_S_ACCEPT(3SSL) page from OpenSSL 4.0.3, dated 2026-09-29 — stay safe out there, go accept some connections, and remember to BIO_pop() before you free anything 💖✨