BIO_S_ACCEPT(3) | Library Functions Manual | BIO_S_ACCEPT(3)

# 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` — accept BIO

# 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);

# DESCRIPTION

`BIO_s_accept`() returns the accept BIO method. This is a wrapper round the platform's TCP/IP socket accept(2) routines.

Using accept BIOs, TCP/IP connections can be accepted and data transferred using only BIO routines. In this way any platform specific operations are hidden by the BIO abstraction.

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) is set up properly then the BIO waits for an incoming connection.

Accept BIOs support BIO_puts(3) but not BIO_gets(3).

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.

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.

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

`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.

`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.

`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_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.

`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`; 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. 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. 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 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. 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.

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(3).

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.

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.

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

cmd constant | larg | corresponding macro
`BIO_C_DO_STATE_MACHINE` | 0 | `BIO_do_accept`()
`BIO_C_GET_ACCEPT` | 0 | `BIO_get_accept_port`()
`BIO_C_GET_BIND_MODE` | 0 | `BIO_get_bind_mode`()
`BIO_C_GET_FD` | 0 | BIO_get_fd(3)
`BIO_C_SET_ACCEPT` | 0 | `BIO_set_accept_port`()
| 1 | `BIO_set_nbio_accept`()
| 2 | `BIO_set_accept_bios`()
`BIO_C_SET_FD` | fd | BIO_set_fd(3)
`BIO_C_SET_NBIO` | n | BIO_set_nbio(3)
`BIO_C_SET_BIND_MODE` | mode | `BIO_set_bind_mode`()
`BIO_CTRL_GET_CLOSE` | 0 | BIO_get_close(3)
`BIO_CTRL_RESET` | 0 | BIO_reset(3)
`BIO_CTRL_SET_CLOSE` | flag | BIO_set_close(3)

# 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".

`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.

`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.

# EXAMPLES

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

    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);

# SEE ALSO

BIO_new(3)

# 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.

`BIO_set_bind_mode`() and `BIO_get_bind_mode`() first appeared in SSLeay 0.9.1 and have been available since OpenBSD 2.6.

June 8, 2025 | Debian