SSL_read.pod revision 312826
1=pod 2 3=head1 NAME 4 5SSL_read - read bytes from a TLS/SSL connection. 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 int SSL_read(SSL *ssl, void *buf, int num); 12 13=head1 DESCRIPTION 14 15SSL_read() tries to read B<num> bytes from the specified B<ssl> into the 16buffer B<buf>. 17 18=head1 NOTES 19 20If necessary, SSL_read() will negotiate a TLS/SSL session, if 21not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or 22L<SSL_accept(3)|SSL_accept(3)>. If the 23peer requests a re-negotiation, it will be performed transparently during 24the SSL_read() operation. The behaviour of SSL_read() depends on the 25underlying BIO. 26 27For the transparent negotiation to succeed, the B<ssl> must have been 28initialized to client or server mode. This is being done by calling 29L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state() 30before the first call to an SSL_read() or L<SSL_write(3)|SSL_write(3)> 31function. 32 33SSL_read() works based on the SSL/TLS records. The data are received in 34records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a 35record has been completely received, it can be processed (decryption and 36check of integrity). Therefore data that was not retrieved at the last 37call of SSL_read() can still be buffered inside the SSL layer and will be 38retrieved on the next call to SSL_read(). If B<num> is higher than the 39number of bytes buffered, SSL_read() will return with the bytes buffered. 40If no more bytes are in the buffer, SSL_read() will trigger the processing 41of the next record. Only when the record has been received and processed 42completely, SSL_read() will return reporting success. At most the contents 43of the record will be returned. As the size of an SSL/TLS record may exceed 44the maximum packet size of the underlying transport (e.g. TCP), it may 45be necessary to read several packets from the transport layer before the 46record is complete and SSL_read() can succeed. 47 48If the underlying BIO is B<blocking>, SSL_read() will only return, once the 49read operation has been finished or an error occurred, except when a 50renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur. 51This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the 52L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call. 53 54If the underlying BIO is B<non-blocking>, SSL_read() will also return 55when the underlying BIO could not satisfy the needs of SSL_read() 56to continue the operation. In this case a call to 57L<SSL_get_error(3)|SSL_get_error(3)> with the 58return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or 59B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a 60call to SSL_read() can also cause write operations! The calling process 61then must repeat the call after taking appropriate action to satisfy the 62needs of SSL_read(). The action depends on the underlying BIO. When using a 63non-blocking socket, nothing is to be done, but select() can be used to check 64for the required condition. When using a buffering BIO, like a BIO pair, data 65must be written into or retrieved out of the BIO before being able to continue. 66 67L<SSL_pending(3)|SSL_pending(3)> can be used to find out whether there 68are buffered bytes available for immediate retrieval. In this case 69SSL_read() can be called without blocking or actually receiving new 70data from the underlying socket. 71 72=head1 WARNING 73 74When an SSL_read() operation has to be repeated because of 75B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated 76with the same arguments. 77 78=head1 RETURN VALUES 79 80The following return values can occur: 81 82=over 4 83 84=item E<gt> 0 85 86The read operation was successful. 87The return value is the number of bytes actually read from the TLS/SSL 88connection. 89 90=item Z<><= 0 91 92 93=item E<lt>0 94 95The read operation was not successful, because either the connection was closed, 96an error occurred or action must be taken by the calling process. 97Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason. 98 99SSLv2 (deprecated) does not support a shutdown alert protocol, so it can 100only be detected, whether the underlying connection was closed. It cannot 101be checked, whether the closure was initiated by the peer or by something 102else. 103 104Old documentation indicated a difference between 0 and -1, and that -1 was 105retryable. 106You should instead call SSL_get_error() to find out if it's retryable. 107 108=back 109 110=head1 SEE ALSO 111 112L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>, 113L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>, 114L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)> 115L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>, 116L<SSL_pending(3)|SSL_pending(3)>, 117L<SSL_shutdown(3)|SSL_shutdown(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>, 118L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)> 119 120=cut 121