libssl/man/SSL_read.3

.\" $OpenBSD: SSL_read.3,v 1.7 2020/05/26 19:45:58 schwarze Exp $
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.\" Matt Caswell <matt@openssl.org>.
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.Dd $Mdocdate: May 26 2020 $
.Dt SSL_READ 3
.Os
.Sh NAME
.Nm SSL_read ,
.Nm SSL_peek
.Nd read bytes from a TLS connection
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_read "SSL *ssl" "void *buf" "int num"
.Ft int
.Fn SSL_peek "SSL *ssl" "void *buf" "int num"
.Sh DESCRIPTION
.Fn SSL_read
tries to read
.Fa num
bytes from the specified
.Fa ssl
into the buffer
.Fa buf .
.Pp
.Fn SSL_peek
is identical to
.Fn SSL_read
except that no bytes are removed from the underlying BIO during
the read, such that a subsequent call to
.Fn SSL_read
will yield at least the same bytes once again.
.Pp
In the following,
.Fn SSL_read
and
.Fn SSL_peek
are called
.Dq read functions .
.Pp
If necessary, a read function will negotiate 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 read function operation.
The behaviour of the read 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 read function.
.Pp
The read functions works based on the TLS records.
The data are received in records (with a maximum record size of 16kB).
Only when a record has been completely received, it can be processed
(decrypted and checked for integrity).
Therefore data that was not retrieved at the last read call can
still be buffered inside the TLS layer and will be retrieved on the
next read call.
If
.Fa num
is higher than the number of bytes buffered, the read functions
will return with the bytes buffered.
If no more bytes are in the buffer, the read functions will trigger
the processing of the next record.
Only when the record has been received and processed completely
will the read functions return reporting success.
At most the contents of the record will be returned.
As the size of a TLS record may exceed the maximum packet size
of the underlying transport (e.g., TCP), it may be necessary to
read several packets from the transport layer before the record is
complete and the read call can succeed.
.Pp
If the underlying
.Vt BIO
is blocking,
a read function will only return once the read operation has been
finished or an error occurred, except when a renegotiation takes
place, in which case an
.Dv SSL_ERROR_WANT_READ
may occur.
This behavior 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 non-blocking, a read 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 read function will yield
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE .
As at any time a re-negotiation is possible, a read function may
also cause write operations.
The calling process must then repeat the call after taking appropriate
action to satisfy the needs of the read 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
.Vt BIO
before being able to continue.
.Pp
.Xr SSL_pending 3
can be used to find out whether there are buffered bytes available for
immediate retrieval.
In this case a read function can be called without blocking or
actually receiving new data from the underlying socket.
.Pp
When a read function operation has to be repeated because of
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE ,
it must be repeated with the same arguments.
.Sh RETURN VALUES
The following return values can occur:
.Bl -tag -width Ds
.It >0
The read operation was successful.
The return value is the number of bytes actually read from the
TLS connection.
.It 0
The read operation was not successful.
The reason may either be a clean shutdown due to a
.Dq close notify
alert sent by the peer (in which case the
.Dv SSL_RECEIVED_SHUTDOWN
flag in the ssl shutdown state is set (see
.Xr SSL_shutdown 3
and
.Xr SSL_set_shutdown 3 ) .
It is also possible that the peer simply shut down the underlying transport and
the shutdown is incomplete.
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 read 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_pending 3 ,
.Xr SSL_set_connect_state 3 ,
.Xr SSL_set_shutdown 3 ,
.Xr SSL_shutdown 3 ,
.Xr SSL_write 3
.Sh HISTORY
.Fn SSL_read
appeared in SSLeay 0.4 or earlier.
.Fn SSL_peek
first appeared in SSLeay 0.6.6.
Both functions have been available since
.Ox 2.4 .