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VersionsSSL_READ(3ossl) OpenSSL SSL_READ(3ossl)
NAME
SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_read(SSL *ssl, void *buf, int num);
int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_peek(SSL *ssl, void *buf, int num);
DESCRIPTION
SSL_read_ex() and SSL_read() try to read num bytes from the specified ssl into the buffer buf. On success
SSL_read_ex() will store the number of bytes actually read in *readbytes.
SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read() respectively except no bytes are
actually removed from the underlying BIO during the read, so that a subsequent call to SSL_read_ex() or
SSL_read() will yield at least the same bytes.
NOTES
In the paragraphs below a "read function" is defined as one of SSL_read_ex(), SSL_read(), SSL_peek_ex() or
SSL_peek().
If necessary, a read function will negotiate a TLS/SSL session, if not already explicitly performed by
SSL_connect(3) or 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 BIO.
For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is
being done by calling SSL_set_connect_state(3) or SSL_set_accept_state() before the first invocation of a read
function.
The read functions work based on the SSL/TLS records. The data are received in records (with a maximum record
size of 16kB). Only when a record has been completely received, can it be processed (decryption and check of
integrity). Therefore, data that was not retrieved at the last read call can still be buffered inside the SSL
layer and will be retrieved on the next read call. If num is higher than the number of bytes buffered then 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 one record will be returned. As the size of
an SSL/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.
If SSL_MODE_AUTO_RETRY has been switched off and a non-application data record has been processed, the read
function can return and set the error to SSL_ERROR_WANT_READ. In this case there might still be unprocessed data
available in the BIO. If read ahead was set using SSL_CTX_set_read_ahead(3), there might also still be
unprocessed data available in the SSL. This behaviour can be controlled using the SSL_CTX_set_mode(3) call.
If the underlying BIO is blocking, a read function will only return once the read operation has been finished or
an error occurred, except when a non-application data record has been processed and SSL_MODE_AUTO_RETRY is not
set. Note that if SSL_MODE_AUTO_RETRY is set and only non-application data is available the call will hang.
If the underlying BIO is nonblocking, a read function will also return when the underlying BIO could not satisfy
the needs of the function to continue the operation. In this case a call to SSL_get_error(3) with the return
value of the read function will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time it's possible
that non-application data needs to be sent, a read function can also cause write operations. The calling process
then must repeat the call after taking appropriate action to satisfy the needs of the read function. The action
depends on the underlying BIO. When using a nonblocking socket, nothing is to be done, but select() can be used
to check for the required condition. When using a buffering BIO, like a BIO pair, data must be written into or
retrieved out of the BIO before being able to continue.
SSL_pending(3) can be used to find out whether there are buffered bytes available for immediate retrieval. In
this case the read function can be called without blocking or actually receiving new data from the underlying
socket.
RETURN VALUES
SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure. Success means that 1 or more
application data bytes have been read from the SSL connection. Failure means that no bytes could be read from
the SSL connection. Failures can be retryable (e.g. we are waiting for more bytes to be delivered by the
network) or non-retryable (e.g. a fatal network error). In the event of a failure call SSL_get_error(3) to find
out the reason which indicates whether the call is retryable or not.
For SSL_read() and SSL_peek() the following return values can occur:
> 0 The read operation was successful. The return value is the number of bytes actually read from the TLS/SSL
connection.
<= 0
The read operation was not successful, because either the connection was closed, an error occurred or action
must be taken by the calling process. Call SSL_get_error(3) with the return value ret to find out the
reason.
Old documentation indicated a difference between 0 and -1, and that -1 was retryable. You should instead
call SSL_get_error() to find out if it's retryable.
SEE ALSO
SSL_get_error(3), SSL_write_ex(3), SSL_CTX_set_mode(3), SSL_CTX_new(3), SSL_connect(3), SSL_accept(3)
SSL_set_connect_state(3), SSL_pending(3), SSL_shutdown(3), SSL_set_shutdown(3), ssl(7), bio(7)
HISTORY
The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the
License. You can obtain a copy in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.1.3 2023-09-19 SSL_READ(3ossl)
NAME
SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_read(SSL *ssl, void *buf, int num);
int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_peek(SSL *ssl, void *buf, int num);
DESCRIPTION
SSL_read_ex() and SSL_read() try to read num bytes from the specified ssl into the buffer buf. On success
SSL_read_ex() will store the number of bytes actually read in *readbytes.
SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read() respectively except no bytes are
actually removed from the underlying BIO during the read, so that a subsequent call to SSL_read_ex() or
SSL_read() will yield at least the same bytes.
NOTES
In the paragraphs below a "read function" is defined as one of SSL_read_ex(), SSL_read(), SSL_peek_ex() or
SSL_peek().
If necessary, a read function will negotiate a TLS/SSL session, if not already explicitly performed by
SSL_connect(3) or 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 BIO.
For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is
being done by calling SSL_set_connect_state(3) or SSL_set_accept_state() before the first invocation of a read
function.
The read functions work based on the SSL/TLS records. The data are received in records (with a maximum record
size of 16kB). Only when a record has been completely received, can it be processed (decryption and check of
integrity). Therefore, data that was not retrieved at the last read call can still be buffered inside the SSL
layer and will be retrieved on the next read call. If num is higher than the number of bytes buffered then 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 one record will be returned. As the size of
an SSL/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.
If SSL_MODE_AUTO_RETRY has been switched off and a non-application data record has been processed, the read
function can return and set the error to SSL_ERROR_WANT_READ. In this case there might still be unprocessed data
available in the BIO. If read ahead was set using SSL_CTX_set_read_ahead(3), there might also still be
unprocessed data available in the SSL. This behaviour can be controlled using the SSL_CTX_set_mode(3) call.
If the underlying BIO is blocking, a read function will only return once the read operation has been finished or
an error occurred, except when a non-application data record has been processed and SSL_MODE_AUTO_RETRY is not
set. Note that if SSL_MODE_AUTO_RETRY is set and only non-application data is available the call will hang.
If the underlying BIO is nonblocking, a read function will also return when the underlying BIO could not satisfy
the needs of the function to continue the operation. In this case a call to SSL_get_error(3) with the return
value of the read function will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time it's possible
that non-application data needs to be sent, a read function can also cause write operations. The calling process
then must repeat the call after taking appropriate action to satisfy the needs of the read function. The action
depends on the underlying BIO. When using a nonblocking socket, nothing is to be done, but select() can be used
to check for the required condition. When using a buffering BIO, like a BIO pair, data must be written into or
retrieved out of the BIO before being able to continue.
SSL_pending(3) can be used to find out whether there are buffered bytes available for immediate retrieval. In
this case the read function can be called without blocking or actually receiving new data from the underlying
socket.
RETURN VALUES
SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure. Success means that 1 or more
application data bytes have been read from the SSL connection. Failure means that no bytes could be read from
the SSL connection. Failures can be retryable (e.g. we are waiting for more bytes to be delivered by the
network) or non-retryable (e.g. a fatal network error). In the event of a failure call SSL_get_error(3) to find
out the reason which indicates whether the call is retryable or not.
For SSL_read() and SSL_peek() the following return values can occur:
> 0 The read operation was successful. The return value is the number of bytes actually read from the TLS/SSL
connection.
<= 0
The read operation was not successful, because either the connection was closed, an error occurred or action
must be taken by the calling process. Call SSL_get_error(3) with the return value ret to find out the
reason.
Old documentation indicated a difference between 0 and -1, and that -1 was retryable. You should instead
call SSL_get_error() to find out if it's retryable.
SEE ALSO
SSL_get_error(3), SSL_write_ex(3), SSL_CTX_set_mode(3), SSL_CTX_new(3), SSL_connect(3), SSL_accept(3)
SSL_set_connect_state(3), SSL_pending(3), SSL_shutdown(3), SSL_set_shutdown(3), ssl(7), bio(7)
HISTORY
The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the
License. You can obtain a copy in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.1.3 2023-09-19 SSL_READ(3ossl)