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SSL_READ(3) Library Functions Manual SSL_READ(3)

SSL_read, SSL_peek
read bytes from a TLS/SSL connection

#include <openssl/ssl.h>
int
SSL_read(SSL *ssl, void *buf, int num);
int
SSL_peek(SSL *ssl, void *buf, int num);

SSL_read() tries to read num bytes from the specified ssl into the buffer buf.
SSL_peek() is identical to SSL_read() except that no bytes are removed from the underlying BIO during the read, such that a subsequent call to SSL_read() will yield at least the same bytes once again.
In the following, SSL_read() and SSL_peek() are called “read functions”.
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 done by calling SSL_set_connect_state(3) or SSL_set_accept_state(3) before the first call to a read function.
The read functions works 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, 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 SSL layer and will be retrieved on the next read call. If 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 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 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 renegotiation takes place, in which case an SSL_ERROR_WANT_READ may occur. This behavior can be controlled with the SSL_MODE_AUTO_RETRY flag of the SSL_CTX_set_mode(3) call.
If the underlying BIO is non-blocking, 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 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 BIO. When using a non-blocking socket, nothing is to be done, but select(2) 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 a read function can be called without blocking or actually receiving new data from the underlying socket.
When a read function operation has to be repeated because of SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with the same arguments.

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. The reason may either be a clean shutdown due to a “close notify” alert sent by the peer (in which case the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set (see SSL_shutdown(3) and SSL_set_shutdown(3)). It is also possible that the peer simply shut down the underlying transport and the shutdown is incomplete. Call SSL_get_error() with the return value to find out whether an error occurred or the connection was shut down cleanly (SSL_ERROR_ZERO_RETURN).
 
 
<0
The read operation was not successful, because either an error occurred or action must be taken by the calling process. Call SSL_get_error() with the return value to find out the reason.

BIO_new(3), ssl(3), SSL_accept(3), SSL_connect(3), SSL_CTX_new(3), SSL_CTX_set_mode(3), SSL_get_error(3), SSL_pending(3), SSL_set_connect_state(3), SSL_set_shutdown(3), SSL_shutdown(3), SSL_write(3)

SSL_read() appeared in SSLeay 0.4 or earlier. SSL_peek() first appeared in SSLeay 0.6.6. Both functions have been available since OpenBSD 2.4.
March 27, 2018 OpenBSD-current