libssl/man/SSL_write.3

.\" $OpenBSD: SSL_write.3,v 1.7 2021/10/24 15:10:13 schwarze Exp $
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.Dd $Mdocdate: October 24 2021 $
.Dt SSL_WRITE 3
.Os
.Sh NAME
.Nm SSL_write_ex ,
.Nm SSL_write
.Nd write bytes to a TLS connection
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_write_ex "SSL *ssl" "const void *buf" "size_t num" "size_t *written"
.Ft int
.Fn SSL_write "SSL *ssl" "const void *buf" "int num"
.Sh DESCRIPTION
.Fn SSL_write_ex
and
.Fn SSL_write
write
.Fa num
bytes from the buffer
.Fa buf
into the specified
.Fa ssl
connection.
On success
.Fn SSL_write_ex
stores the number of bytes written in
.Pf * Fa written .
.Pp
In the following,
.Fn SSL_write_ex
and
.Fn SSL_write
are called
.Dq write functions .
.Pp
If necessary, a write function negotiates a TLS session,
if not already explicitly performed by
.Xr SSL_connect 3
or
.Xr SSL_accept 3 .
If the peer requests a re-negotiation,
it will be performed transparently during the
write function operation.
The behaviour of the write functions depends on the underlying
.Vt BIO .
.Pp
For the transparent negotiation to succeed, the
.Fa ssl
must have been initialized to client or server mode.
This is done by calling
.Xr SSL_set_connect_state 3
or
.Xr SSL_set_accept_state 3
before the first call to a write function.
.Pp
If the underlying
.Vt BIO
is
.Em blocking ,
the write function
will only return once the write operation has been finished or an error
occurred, except when a renegotiation takes place, in which case a
.Dv SSL_ERROR_WANT_READ
may occur.
This behaviour can be controlled with the
.Dv SSL_MODE_AUTO_RETRY
flag of the
.Xr SSL_CTX_set_mode 3
call.
.Pp
If the underlying
.Vt BIO
is
.Em non-blocking ,
the write function will also return when the underlying
.Vt BIO
could not satisfy the needs of the function to continue the operation.
In this case a call to
.Xr SSL_get_error 3
with the return value of the write function will yield
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE .
As at any time a re-negotiation is possible, a call to
a write function can also cause read operations.
The calling process then must repeat the call after taking appropriate action
to satisfy the needs of the write function.
The action depends on the underlying
.Vt BIO .
When using a non-blocking socket, nothing is to be done, but
.Xr select 2
can be used to check for the required condition.
When using a buffering
.Vt BIO ,
like a
.Vt BIO
pair, data must be written into or retrieved out of the BIO before being able
to continue.
.Pp
The write functions
will only return with success when the complete contents of
.Fa buf
of length
.Fa num
have been written.
This default behaviour can be changed with the
.Dv SSL_MODE_ENABLE_PARTIAL_WRITE
option of
.Xr SSL_CTX_set_mode 3 .
When this flag is set, the write functions will also return with
success when a partial write has been successfully completed.
In this case the write function operation is considered completed.
The bytes are sent and a new write call with a new buffer (with the
already sent bytes removed) must be started.
A partial write is performed with the size of a message block,
which is 16kB.
.Pp
When a write function call has to be repeated because
.Xr SSL_get_error 3
returned
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE ,
it must be repeated with the same arguments.
.Pp
When calling
.Fn SSL_write
with
.Fa num Ns =0
bytes to be sent, the behaviour is undefined.
.Fn SSL_write_ex
can be called with
.Fa num Ns =0 ,
but will not send application data to the peer.
.Sh RETURN VALUES
.Fn SSL_write_ex
returns 1 for success or 0 for failure.
Success means that all requested application data bytes have been
written to the TLS connection or, if
.Dv SSL_MODE_ENABLE_PARTIAL_WRITE
is in use, at least one application data byte has been written
to the TLS connection.
Failure means that not all the requested bytes have been written yet (if
.Dv SSL_MODE_ENABLE_PARTIAL_WRITE
is not in use) or no bytes could be written to the TLS connection (if
.Dv SSL_MODE_ENABLE_PARTIAL_WRITE
is in use).
Failures can be retryable (e.g. the network write buffer has temporarily
filled up) or non-retryable (e.g. a fatal network error).
In the event of a failure, call
.Xr SSL_get_error 3
to find out the reason
which indicates whether the call is retryable or not.
.Pp
For
.Fn SSL_write ,
the following return values can occur:
.Bl -tag -width Ds
.It >0
The write operation was successful.
The return value is the number of bytes actually written to the TLS
connection.
.It 0
The write operation was not successful.
Probably the underlying connection was closed.
Call
.Xr SSL_get_error 3
with the return value to find out whether an error occurred or the connection
was shut down cleanly
.Pq Dv SSL_ERROR_ZERO_RETURN .
.It <0
The write operation was not successful, because either an error occurred or
action must be taken by the calling process.
Call
.Xr SSL_get_error 3
with the return value to find out the reason.
.El
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr ssl 3 ,
.Xr SSL_accept 3 ,
.Xr SSL_connect 3 ,
.Xr SSL_CTX_new 3 ,
.Xr SSL_CTX_set_mode 3 ,
.Xr SSL_get_error 3 ,
.Xr SSL_read 3 ,
.Xr SSL_set_connect_state 3
.Sh HISTORY
.Fn SSL_write
appeared in SSLeay 0.4 or earlier and has been available since
.Ox 2.4 .
.Pp
.Fn SSL_write_ex
first appeared in OpenSSL 1.1.1 and has been available since
.Ox 7.1 .