SSL_read.pod revision 79998
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 not the case if a generic 29method is being used (see L<SSL_CTX_new(3)|SSL_CTX_new(3)>, so that 30L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state() 31must be used before the first call to an SSL_read() or 32L<SSL_write(3)|SSL_write(3)> function). 33 34SSL_read() works based on the SSL/TLS records. The data are received in 35records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a 36record has been completely received, it can be processed (decryption and 37check of integrity). Therefore data that was not retrieved at the last 38call of SSL_read() can still be buffered inside the SSL layer and will be 39retrieved on the next call to SSL_read(). If B<num> is higher than the 40number of bytes buffered, SSL_read() will return with the bytes buffered. 41If no more bytes are in the buffer, SSL_read() will trigger the processing 42of the next record. Only when the record has been received and processed 43completely, SSL_read() will return reporting success. At most the contents 44of the record will be returned. As the size of an SSL/TLS record may exceed 45the maximum packet size of the underlying transport (e.g. TCP), it may 46be necessary to read several packets from the transport layer before the 47record is complete and SSL_read() can succeed. 48 49If the underlying BIO is B<blocking>, SSL_read() will only return, once the 50read operation has been finished or an error occurred, except when a 51renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur. 52This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the 53L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call. 54 55If the underlying BIO is B<non-blocking>, SSL_read() will also return 56when the underlying BIO could not satisfy the needs of SSL_read() 57to continue the operation. In this case a call to 58L<SSL_get_error(3)|SSL_get_error(3)> with the 59return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or 60B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a 61call to SSL_read() can also cause write operations! The calling process 62then must repeat the call after taking appropriate action to satisfy the 63needs of SSL_read(). The action depends on the underlying BIO. When using a 64non-blocking socket, nothing is to be done, but select() can be used to check 65for the required condition. When using a buffering BIO, like a BIO pair, data 66must be written into or retrieved out of the BIO before being able to continue. 67 68=head1 WARNING 69 70When an SSL_read() operation has to be repeated because of 71B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated 72with the same arguments. 73 74=head1 RETURN VALUES 75 76The following return values can occur: 77 78=over 4 79 80=item E<gt>0 81 82The read operation was successful; the return value is the number of 83bytes actually read from the TLS/SSL connection. 84 85=item 0 86 87The read operation was not successful, probably because no data was 88available. Call SSL_get_error() with the return value B<ret> to find out, 89whether an error occurred. 90 91=item E<lt>0 92 93The read operation was not successful, because either an error occurred 94or action must be taken by the calling process. Call SSL_get_error() with the 95return value B<ret> to find out the reason. 96 97=back 98 99=head1 SEE ALSO 100 101L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>, 102L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>, 103L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)> 104L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>, 105L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)> 106 107=cut 108