.\" $OpenBSD: BIO_s_accept.3,v 1.16 2023/04/29 13:06:10 schwarze Exp $
.\" full merge up to: OpenSSL c03726ca Thu Aug 27 12:28:08 2015 -0400
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.Dd $Mdocdate: April 29 2023 $
.Dt BIO_S_ACCEPT 3
.Os
.Sh NAME
.Nm BIO_s_accept ,
.Nm BIO_set_accept_port ,
.Nm BIO_get_accept_port ,
.Nm BIO_new_accept ,
.Nm BIO_set_nbio_accept ,
.Nm BIO_set_accept_bios ,
.Nm BIO_set_bind_mode ,
.Nm BIO_get_bind_mode ,
.Nm BIO_do_accept
.Nd accept BIO
.Sh SYNOPSIS
.In openssl/bio.h
.Ft const BIO_METHOD *
.Fo BIO_s_accept
.Fa void
.Fc
.Ft long
.Fo BIO_set_accept_port
.Fa "BIO *b"
.Fa "char *name"
.Fc
.Ft char *
.Fo BIO_get_accept_port
.Fa "BIO *b"
.Fc
.Ft BIO *
.Fo BIO_new_accept
.Fa "const char *host_port"
.Fc
.Ft long
.Fo BIO_set_nbio_accept
.Fa "BIO *b"
.Fa "int n"
.Fc
.Ft long
.Fo BIO_set_accept_bios
.Fa "BIO *b"
.Fa "char *bio"
.Fc
.Ft long
.Fo BIO_set_bind_mode
.Fa "BIO *b"
.Fa "long mode"
.Fc
.Ft long
.Fo BIO_get_bind_mode
.Fa "BIO *b"
.Fa "long dummy"
.Fc
.Fd #define BIO_BIND_NORMAL				0
.Fd #define BIO_BIND_REUSEADDR_IF_UNUSED	1
.Fd #define BIO_BIND_REUSEADDR			2
.Ft long
.Fo BIO_do_accept
.Fa "BIO *b"
.Fc
.Sh DESCRIPTION
.Fn BIO_s_accept
returns the accept BIO method.
This is a wrapper round the platform's TCP/IP socket
.Xr accept 2
routines.
.Pp
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.
.Pp
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.
.Pp
Accept BIOs support
.Xr BIO_puts 3
but not
.Xr BIO_gets 3 .
.Pp
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.
.Pp
Calling
.Xr 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.
.Pp
.Xr BIO_get_fd 3
and
.Xr BIO_set_fd 3
can be called to retrieve or set the accept socket.
See
.Xr BIO_s_fd 3 .
.Pp
.Fn BIO_set_accept_port
uses the string
.Fa name
to set the accept port.
The port is represented as a string of the form
.Ar host : Ns Ar port ,
where
.Ar host
is the interface to use and
.Ar port
is the port.
The host can be
.Qq * ,
which is interpreted as meaning any interface;
.Ar port
has the same syntax as the port specified in
.Xr BIO_set_conn_port 3
for connect BIOs.
It can be a numerical port string or a string to look up using
.Xr getservbyname 3
and a string table.
.Pp
.Fn BIO_new_accept
combines
.Xr BIO_new 3
and
.Fn BIO_set_accept_port
into a single call.
It creates a new accept BIO with port
.Fa host_port .
.Pp
.Fn BIO_set_nbio_accept
sets the accept socket to blocking mode (the default) if
.Fa n
is 0 or non-blocking mode if
.Fa n
is 1.
.Pp
.Fn 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.
.Pp
.Fn BIO_set_bind_mode
and
.Fn BIO_get_bind_mode
set and retrieve the current bind mode.
If
.Dv BIO_BIND_NORMAL Pq the default
is set, then another socket cannot be bound to the same port.
If
.Dv BIO_BIND_REUSEADDR
is set, then other sockets can bind to the same port.
If
.Dv BIO_BIND_REUSEADDR_IF_UNUSED
is set, then an attempt is first made to use
.Dv BIO_BIN_NORMAL ;
if this fails and the port is not in use,
then a second attempt is made using
.Dv BIO_BIND_REUSEADDR .
.Pp
.Fn 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
.Fn BIO_do_accept
will await an incoming connection, or request a retry in non-blocking mode.
.Sh 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.
.Pp
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.
.Pp
If any additional BIOs have been set using
.Fn BIO_set_accept_bios ,
then they are placed between the socket and the accept BIO;
that is, the chain will be accept->otherbios->socket.
.Pp
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:
.Pp
.Dl connection = BIO_pop(accept);
.Pp
After this call,
.Sy connection
will contain a BIO for the recently established connection and
.Sy 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
.Sy accept
can be freed using
.Xr BIO_free 3 .
.Pp
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
.Xr BIO_pop 3
(see above) and freeing up the accept BIO after the initial connection.
.Pp
If the underlying accept socket is non-blocking and
.Fn BIO_do_accept
is called to await an incoming connection, it is possible for
.Xr BIO_should_io_special 3
with the reason
.Dv 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.
.Pp
.Xr BIO_ctrl 3
.Fa cmd
and
.Fa larg
arguments correspond to macros as follows:
.Bl -column BIO_C_DO_STATE_MACHINE larg BIO_get_accept_port(3) -offset 3n
.It Fa cmd No constant        Ta Fa larg Ta corresponding macro
.It Dv BIO_C_DO_STATE_MACHINE Ta 0       Ta Fn BIO_do_accept
.It Dv BIO_C_GET_ACCEPT       Ta 0       Ta Fn BIO_get_accept_port
.It Dv BIO_C_GET_BIND_MODE    Ta 0       Ta Fn BIO_get_bind_mode
.It Dv BIO_C_GET_FD           Ta 0       Ta Xr BIO_get_fd 3
.It Dv BIO_C_SET_ACCEPT       Ta 0       Ta Fn BIO_set_accept_port
.It                           Ta 1       Ta Fn BIO_set_nbio_accept
.It                           Ta 2       Ta Fn BIO_set_accept_bios
.It Dv BIO_C_SET_FD           Ta Fa fd   Ta Xr BIO_set_fd 3
.It Dv BIO_C_SET_NBIO         Ta Fa n    Ta Xr BIO_set_nbio 3
.It Dv BIO_C_SET_BIND_MODE    Ta Fa mode Ta Fn BIO_set_bind_mode
.It Dv BIO_CTRL_GET_CLOSE     Ta 0       Ta Xr BIO_get_close 3
.It Dv BIO_CTRL_RESET         Ta 0       Ta Xr BIO_reset 3
.It Dv BIO_CTRL_SET_CLOSE     Ta Fa flag Ta Xr BIO_set_close 3
.El
.Sh RETURN VALUES
When called on an accept BIO object,
.Xr BIO_method_type 3
returns the constant
.Dv BIO_TYPE_ACCEPT
and
.Xr BIO_method_name 3
returns a pointer to the static string
.Qq socket accept .
.Pp
.Fn BIO_do_accept ,
.Fn BIO_set_accept_port ,
.Fn BIO_set_nbio_accept ,
.Fn BIO_set_accept_bios ,
and
.Fn BIO_set_bind_mode
return 1 for success or 0 or -1 for failure.
.Pp
.Fn BIO_get_accept_port
returns the port as a string or
.Dv NULL
on error.
.Pp
.Fn BIO_get_bind_mode
returns the set of BIO_BIND flags or -1 on failure.
.Pp
.Fn BIO_new_accept
returns a
.Vt BIO
or
.Dv NULL
on error.
.Sh EXAMPLES
This example accepts two connections on port 4444,
sends messages down each and finally closes both down.
.Bd -literal -offset 2n
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\en");
	ERR_print_errors_fp(stderr);
	exit(0);
}

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

/* Retrieve BIO for connection */
cbio = BIO_pop(abio);

BIO_puts(cbio, "Connection 1: Sending out Data on initial connection\en");
fprintf(stderr, "Sent out data on connection 1\en");

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

/* Close accept BIO to refuse further connections */
cbio2 = BIO_pop(abio);
BIO_free(abio);

BIO_puts(cbio2, "Connection 2: Sending out Data on second\en");
fprintf(stderr, "Sent out data on connection 2\en");
BIO_puts(cbio, "Connection 1: Second connection established\en");

/* Close the two established connections */
BIO_free(cbio);
BIO_free(cbio2);
.Ed
.Sh SEE ALSO
.Xr BIO_new 3
.Sh HISTORY
.Fn BIO_s_accept ,
.Fn BIO_set_accept_port ,
.Fn BIO_new_accept ,
.Fn BIO_set_accept_bios ,
and
.Fn BIO_do_accept
first appeared in SSLeay 0.8.0.
.Fn BIO_set_nbio_accept
and
.Fn BIO_get_accept_port
first appeared in SSLeay 0.9.0.
All these functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_set_bind_mode
and
.Fn BIO_get_bind_mode
first appeared in SSLeay 0.9.1 and have been available since
.Ox 2.6 .