BIO_S_ACCEPT(3)

💅 BIO_S_ACCEPT(3) 💅

okay bestie, so this is a libcrypto manpage, and it's literally all about accept BIOs — like the glow-up wrapper around your platform's accept(2) socket magic ✨

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✨ NAME ✨

BIO_s_accept, BIO_set_accept_port, BIO_get_accept_port, BIO_new_accept, BIO_set_nbio_accept, BIO_set_accept_bios, BIO_set_bind_mode, BIO_get_bind_mode, BIO_do_accept — the accept BIO squad 🫶

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🌸 SYNOPSIS 🌸

library "libcrypto"
#include <openssl/bio.h>

const BIO_METHOD *
BIO_s_accept(void);

long
BIO_set_accept_port(BIO *b, char *name);

char *
BIO_get_accept_port(BIO *b);

BIO *
BIO_new_accept(const char *host_port);

long
BIO_set_nbio_accept(BIO *b, int n);

long
BIO_set_accept_bios(BIO *b, char *bio);

long
BIO_set_bind_mode(BIO *b, long mode);

long
BIO_get_bind_mode(BIO *b, long dummy);

#define BIO_BIND_NORMAL                 0
#define BIO_BIND_REUSEADDR_IF_UNUSED    1
#define BIO_BIND_REUSEADDR              2

long
BIO_do_accept(BIO *b);

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🦋 DESCRIPTION 🦋

So BIO_s_accept() returns the accept BIO method — it's basically a cute little wrapper round the platform's TCP/IP socket accept(2) routines.

Using accept BIOs, you can accept TCP/IP connections and transfer data using only BIO routines. The way any platform-specific operations are hidden by the BIO abstraction? Iconic. Main character energy fr fr 👑

Read and write operations on an accept BIO will perform I/O on the underlying connection. If no connection is established and 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(3) but not BIO_gets(3) — that's the vibe check and it failed, sorry not sorry 🙈

If the close flag is set on an accept BIO, then any active connection on that chain is shut down and the socket closed when the BIO is freed. Clean exits only ✨

Calling BIO_reset(3) on an accept BIO will close any active connection and reset the BIO into a state where it awaits another incoming connection — it's giving refresh and reset, periodt 🔄

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

BIO_set_accept_port() uses the string name to set the accept port. The port is represented as a string of the form host:port, where host is the interface to use and port is the port. The host can be "*", which is interpreted as meaning any interface; port has the same syntax as the port specified in BIO_set_conn_port(3) for connect BIOs. It can be a numerical port string or a string to look up using getservbyname(3) and a string table. Flexible and fierce 💖

BIO_new_accept() combines BIO_new(3) and BIO_set_accept_port() into a single call. It creates a new accept BIO with port host_port — one-stop shop, we love efficiency 🛍️

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. Pick your fighter 🥊

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. This is 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. Don't do it bestie, the cleanup is handled 💀

BIO_set_bind_mode() and BIO_get_bind_mode() set and retrieve the current bind mode. Here's the tea ☕:

• 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; if this fails and the port is not in use, then a second attempt is made using BIO_BIND_REUSEADDR.

BIO_do_accept() serves two purposes, lowkey multitasking icon:

1. When it is first called, after the accept BIO has been set up, it will attempt to create the accept socket and bind an address to it.
2. Second and subsequent calls to BIO_do_accept() will await an incoming connection, or request a retry in non-blocking mode.

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🌈 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 passes I/O calls to the next BIO in the chain. Know your place in the chain, queen 👑

When a connection is established, a new socket BIO is created for the connection and appended to the chain. That is, the chain is now accept->socket. This effectively means that attempting I/O on an initial accept socket will await an incoming connection then perform I/O on it. It's giving pipeline realness 🔥

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. Sandwich architecture, we stan 🥪

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(3). Pop off, literally 💅

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(3) (see above) and freeing up the accept BIO after the initial connection. One-and-done energy, but read the fine print bestie 📖

If the underlying accept socket is non-blocking and BIO_do_accept() is called to await an incoming connection, it is possible for BIO_should_io_special(3) 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 panic, just retry — it's giving "try again" energy 🔁

BIO_ctrl(3) cmd and larg arguments correspond to macros as follows:

• BIO_C_DO_STATE_MACHINE with larg 0 → BIO_do_accept()
• BIO_C_GET_ACCEPT with larg 0 → BIO_get_accept_port()
• BIO_C_GET_BIND_MODE with larg 0 → BIO_get_bind_mode()
• BIO_C_GET_FD with larg 0 → BIO_get_fd(3)
• BIO_C_SET_ACCEPT with larg 0 → BIO_set_accept_port()
• BIO_C_SET_ACCEPT with larg 1 → BIO_set_nbio_accept()
• BIO_C_SET_ACCEPT with larg 2 → BIO_set_accept_bios()
• BIO_C_SET_FD with larg fd → BIO_set_fd(3)
• BIO_C_SET_NBIO with larg n → BIO_set_nbio(3)
• BIO_C_SET_BIND_MODE with larg mode → BIO_set_bind_mode()
• BIO_CTRL_GET_CLOSE with larg 0 → BIO_get_close(3)
• BIO_CTRL_RESET with larg 0 → BIO_reset(3)
• BIO_CTRL_SET_CLOSE with larg flag → BIO_set_close(3)

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💜 RETURN VALUES 💜

When called on an accept BIO object, BIO_method_type(3) returns the constant BIO_TYPE_ACCEPT and BIO_method_name(3) returns a pointer to the static string "socket accept". Cute little identifiers 🏷️

BIO_do_accept(), BIO_set_accept_port(), BIO_set_nbio_accept(), BIO_set_accept_bios(), and BIO_set_bind_mode() return 1 for success or 0 or -1 for failure. Success slays, failure is a no 💀

BIO_get_accept_port() returns the port as a string or NULL on error.

BIO_get_bind_mode() returns the set of BIO_BIND flags or -1 on failure.

BIO_new_accept() returns a BIO or NULL on error.

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🍓 EXAMPLES 🍓

This example accepts two connections on port 4444, sends messages down each and finally closes both down. It's giving full lifecycle, no crumbs left behind 🧹✨

BIO *abio, *cbio, *cbio2;
ERR_load_crypto_strings();
abio = BIO_new_accept("4444");

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

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

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🧚 SEE ALSO 🧚

BIO_new(3) — go read it bestie, it's linked for a reason 💖

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👑 HISTORY 👑

BIO_s_accept(), BIO_set_accept_port(), BIO_new_accept(), BIO_set_accept_bios(), and BIO_do_accept() first appeared in SSLeay 0.8.0. BIO_set_nbio_accept() and BIO_get_accept_port() first appeared in SSLeay 0.9.0. All these functions have been available since OpenBSD 2.4. Vintage but still thriving 💅

BIO_set_bind_mode() and BIO_get_bind_mode() first appeared in SSLeay 0.9.1 and have been available since OpenBSD 2.6. Late bloomers, we love to see it 🌸

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Debian · June 8, 2025 · that's the tea, bestie — this page came straight from the Debian libcrypto manpage vault, and honestly? She ate. No cap, periodt, hope this helped you understand accept BIOs fr fr 🫶✨💖🎀🦋