---
name: "libressl-BIO_s_accept"
section: "3"
platform: "linux"
package: "libressl"
version: "4.4.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libressl-BIO_s_accept"
---

# libressl-BIO\_s\_accept(3)

accept BIO

## 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

Crypto Library (libcrypto, -lcrypto)

**#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)](/accept/2.md) 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)](/getservbyname/3.md) 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.