.\" $OpenBSD: BIO_s_bio.3,v 1.16 2022/12/21 15:08:37 schwarze Exp $ .\" full merge up to: OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400 .\" .\" This file was written by .\" Lutz Jaenicke , .\" Dr. Stephen Henson , .\" Bodo Moeller , .\" and Richard Levitte . .\" Copyright (c) 2000, 2002, 2015, 2016 The OpenSSL Project. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: December 21 2022 $ .Dt BIO_S_BIO 3 .Os .Sh NAME .Nm BIO_s_bio , .Nm BIO_make_bio_pair , .Nm BIO_destroy_bio_pair , .Nm BIO_shutdown_wr , .Nm BIO_set_write_buf_size , .Nm BIO_get_write_buf_size , .Nm BIO_new_bio_pair , .Nm BIO_get_write_guarantee , .Nm BIO_ctrl_get_write_guarantee , .Nm BIO_get_read_request , .Nm BIO_ctrl_get_read_request , .Nm BIO_ctrl_reset_read_request .\" The following non-copying I/O functions are intentionally undocumented .\" because they seem fragile and unused by anything: .\" .Nm BIO_nread0 .\" .Nm BIO_nread .\" .Nm BIO_nwrite0 .\" .Nm BIO_nwrite .Nd BIO pair BIO .Sh SYNOPSIS .In openssl/bio.h .Ft const BIO_METHOD * .Fo BIO_s_bio .Fa void .Fc .Ft int .Fo BIO_make_bio_pair .Fa "BIO *b1" .Fa "BIO *b2" .Fc .Ft int .Fo BIO_destroy_bio_pair .Fa "BIO *b" .Fc .Ft int .Fo BIO_shutdown_wr .Fa "BIO *b" .Fc .Ft int .Fo BIO_set_write_buf_size .Fa "BIO *b" .Fa "long size" .Fc .Ft size_t .Fo BIO_get_write_buf_size .Fa "BIO *b" .Fa "long size" .Fc .Ft int .Fo BIO_new_bio_pair .Fa "BIO **bio1" .Fa "size_t writebuf1" .Fa "BIO **bio2" .Fa "size_t writebuf2" .Fc .Ft size_t .Fo BIO_get_write_guarantee .Fa "BIO *b" .Fc .Ft size_t .Fo BIO_ctrl_get_write_guarantee .Fa "BIO *b" .Fc .Ft int .Fo BIO_get_read_request .Fa "BIO *b" .Fc .Ft size_t .Fo BIO_ctrl_get_read_request .Fa "BIO *b" .Fc .Ft int .Fo BIO_ctrl_reset_read_request .Fa "BIO *b" .Fc .Sh DESCRIPTION .Fn BIO_s_bio returns the method for a BIO pair. A BIO pair is a pair of source/sink BIOs where data written to either half of the pair is buffered and can be read from the other half. Both halves must usually be handled by the same application thread since no locking is done on the internal data structures. .Pp Since BIO chains typically end in a source/sink BIO, it is possible to make this one half of a BIO pair and have all the data processed by the chain under application control. .Pp One typical use of BIO pairs is to place TLS/SSL I/O under application control. This can be used when the application wishes to use a non-standard transport for TLS/SSL or the normal socket routines are inappropriate. .Pp Calls to .Xr BIO_read 3 will read data from the buffer or request a retry if no data is available. .Pp Calls to .Xr BIO_write 3 will place data in the buffer or request a retry if the buffer is full. .Pp The standard calls .Xr BIO_ctrl_pending 3 and .Xr BIO_ctrl_wpending 3 can be used to determine the amount of pending data in the read or write buffer. .Pp .Xr BIO_reset 3 clears any data in the write buffer. .Pp .Fn BIO_make_bio_pair joins two separate BIOs into a connected pair. .Pp .Fn BIO_destroy_pair destroys the association between two connected BIOs. Freeing up any half of the pair will automatically destroy the association. .Pp .Fn BIO_shutdown_wr is used to close down a BIO .Fa b . After this call no further writes on BIO .Fa b are allowed; they will return an error. Reads on the other half of the pair will return any pending data or EOF when all pending data has been read. .Pp .Fn BIO_set_write_buf_size sets the write buffer size of BIO .Fa b to .Fa size . If the size is not initialized, a default value is used. This is currently 17K, sufficient for a maximum size TLS record. When a chain containing a BIO pair is copied with .Xr BIO_dup_chain 3 , the write buffer size is automatically copied from the original BIO object to the new one. .Pp .Fn BIO_get_write_buf_size returns the size of the write buffer. .Pp .Fn BIO_new_bio_pair combines the calls to .Xr BIO_new 3 , .Fn BIO_make_bio_pair and .Fn BIO_set_write_buf_size to create a connected pair of BIOs .Fa bio1 and .Fa bio2 with write buffer sizes .Fa writebuf1 and .Fa writebuf2 . If either size is zero, then the default size is used. .Fn BIO_new_bio_pair does not check whether .Fa bio1 or .Fa bio2 point to some other BIO; the values are overwritten and .Xr BIO_free 3 is not called. .Pp .Fn BIO_get_write_guarantee and .Fn BIO_ctrl_get_write_guarantee return the maximum length of data that can be currently written to the BIO. Writes larger than this value will return a value from .Xr BIO_write 3 less than the amount requested or if the buffer is full request a retry. .Fn BIO_ctrl_get_write_guarantee is a function whereas .Fn BIO_get_write_guarantee is a macro. .Pp .Fn BIO_get_read_request and .Fn BIO_ctrl_get_read_request return the amount of data requested, or the buffer size if it is less, if the last read attempt at the other half of the BIO pair failed due to an empty buffer. This can be used to determine how much data should be written to the BIO so the next read will succeed: this is most useful in TLS/SSL applications where the amount of data read is usually meaningful rather than just a buffer size. After a successful read this call will return zero. It also will return zero once new data has been written satisfying the read request or part of it. Note that .Fn BIO_get_read_request never returns an amount larger than that returned by .Fn BIO_get_write_guarantee . .Pp .Fn BIO_ctrl_reset_read_request can also be used to reset the value returned by .Fn BIO_get_read_request to zero. .Pp Both halves of a BIO pair should be freed. Even if one half is implicitly freed due to a .Xr BIO_free_all 3 or .Xr SSL_free 3 call, the other half still needs to be freed. .Pp When used in bidirectional applications (such as TLS/SSL), care should be taken to flush any data in the write buffer. This can be done by calling .Xr BIO_pending 3 on the other half of the pair and, if any data is pending, reading it and sending it to the underlying transport. This must be done before any normal processing (such as calling .Xr select 2 ) due to a request and .Xr BIO_should_read 3 being true. .Pp To see why this is important, consider a case where a request is sent using .Xr BIO_write 3 and a response read with .Xr BIO_read 3 , this can occur during a TLS/SSL handshake for example. .Xr BIO_write 3 will succeed and place data in the write buffer. .Xr BIO_read 3 will initially fail and .Xr BIO_should_read 3 will be true. If the application then waits for data to become available on the underlying transport before flushing the write buffer, it will never succeed because the request was never sent. .Pp .Xr BIO_eof 3 is true if no data is in the peer BIO and the peer BIO has been shutdown. .Pp .Fn BIO_make_bio_pair , .Fn BIO_destroy_bio_pair , .Fn BIO_shutdown_wr , .Fn BIO_set_write_buf_size , .Fn BIO_get_write_buf_size , .Fn BIO_get_write_guarantee , and .Fn BIO_get_read_request are implemented as macros. .Sh RETURN VALUES .Fn BIO_new_bio_pair returns 1 on success, with the new BIOs available in .Fa bio1 and .Fa bio2 , or 0 on failure, with NULL pointers stored into the locations for .Fa bio1 and .Fa bio2 . Check the error stack for more information. .\" XXX More return values need to be added here. .Sh EXAMPLES The BIO pair can be used to have full control over the network access of an application. The application can call .Xr select 2 on the socket as required without having to go through the SSL interface. .Bd -literal -offset 2n BIO *internal_bio, *network_bio; \&... BIO_new_bio_pair(&internal_bio, 0, &network_bio, 0); SSL_set_bio(ssl, internal_bio, internal_bio); SSL_operations(); /* e.g. SSL_read() and SSL_write() */ \&... application | TLS-engine | | +----------> SSL_operations() | /\e || | || \e/ | BIO-pair (internal_bio) | BIO-pair (network_bio) | || /\e | \e/ || +-----------< BIO_operations() | | socket | \&... SSL_free(ssl); /* implicitly frees internal_bio */ BIO_free(network_bio); \&... .Ed .Pp As the BIO pair will only buffer the data and never directly access the connection, it behaves non-blocking and will return as soon as the write buffer is full or the read buffer is drained. Then the application has to flush the write buffer and/or fill the read buffer. .Pp Use .Xr BIO_ctrl_pending 3 to find out whether data is buffered in the BIO and must be transferred to the network. Use .Fn BIO_ctrl_get_read_request to find out how many bytes must be written into the buffer before the SSL operations can successfully be continued. .Sh SEE ALSO .Xr BIO_new 3 , .Xr BIO_read 3 , .Xr BIO_should_retry 3 , .Xr ssl 3 , .Xr SSL_set_bio 3 .Sh HISTORY .Fn BIO_s_bio , .Fn BIO_make_bio_pair , .Fn BIO_destroy_bio_pair , .Fn BIO_set_write_buf_size , .Fn BIO_get_write_buf_size , .Fn BIO_new_bio_pair , .Fn BIO_get_write_guarantee , .Fn BIO_ctrl_get_write_guarantee , .Fn BIO_get_read_request , and .Fn BIO_ctrl_reset_read_request first appeared in OpenSSL 0.9.4 and have been available since .Ox 2.6 . .Pp .Fn BIO_ctrl_reset_read_request first appeared in OpenSSL 0.9.5 and has been available since .Ox 2.7 . .Pp .Fn BIO_shutdown_wr first appeared in OpenSSL 0.9.6 and has been available since .Ox 2.9 . .Sh CAVEATS As the data is buffered, SSL operations may return with an .Dv ERROR_SSL_WANT_READ condition, but there is still data in the write buffer. An application must not rely on the error value of the SSL operation but must assure that the write buffer is always flushed first. Otherwise a deadlock may occur as the peer might be waiting for the data before being able to continue.