1 =pod 2 3 =head1 NAME 4 5 SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek 6 - read bytes from a TLS/SSL connection 7 8 =head1 SYNOPSIS 9 10 #include <openssl/ssl.h> 11 12 int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes); 13 int SSL_read(SSL *ssl, void *buf, int num); 14 15 int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes); 16 int SSL_peek(SSL *ssl, void *buf, int num); 17 18 =head1 DESCRIPTION 19 20 SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl> 21 into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes 22 actually read in B<*readbytes>. 23 24 SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read() 25 respectively except no bytes are actually removed from the underlying BIO during 26 the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield 27 at least the same bytes. 28 29 =head1 NOTES 30 31 In the paragraphs below a "read function" is defined as one of SSL_read_ex(), 32 SSL_read(), SSL_peek_ex() or SSL_peek(). 33 34 If necessary, a read function will negotiate a TLS/SSL session, if not already 35 explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the 36 peer requests a re-negotiation, it will be performed transparently during 37 the read function operation. The behaviour of the read functions depends on the 38 underlying BIO. 39 40 For the transparent negotiation to succeed, the B<ssl> must have been 41 initialized to client or server mode. This is being done by calling 42 L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first 43 invocation of a read function. 44 45 The read functions work based on the SSL/TLS records. The data are received in 46 records (with a maximum record size of 16kB). Only when a record has been 47 completely received, can it be processed (decryption and check of integrity). 48 Therefore, data that was not retrieved at the last read call can still be 49 buffered inside the SSL layer and will be retrieved on the next read 50 call. If B<num> is higher than the number of bytes buffered then the read 51 functions will return with the bytes buffered. If no more bytes are in the 52 buffer, the read functions will trigger the processing of the next record. 53 Only when the record has been received and processed completely will the read 54 functions return reporting success. At most the contents of one record will 55 be returned. As the size of an SSL/TLS record may exceed the maximum packet size 56 of the underlying transport (e.g. TCP), it may be necessary to read several 57 packets from the transport layer before the record is complete and the read call 58 can succeed. 59 60 If B<SSL_MODE_AUTO_RETRY> has been switched off and a non-application data 61 record has been processed, the read function can return and set the error to 62 B<SSL_ERROR_WANT_READ>. 63 In this case there might still be unprocessed data available in the B<BIO>. 64 If read ahead was set using L<SSL_CTX_set_read_ahead(3)>, there might also still 65 be unprocessed data available in the B<SSL>. 66 This behaviour can be controlled using the L<SSL_CTX_set_mode(3)> call. 67 68 If the underlying BIO is B<blocking>, a read function will only return once the 69 read operation has been finished or an error occurred, except when a 70 non-application data record has been processed and B<SSL_MODE_AUTO_RETRY> is 71 not set. 72 Note that if B<SSL_MODE_AUTO_RETRY> is set and only non-application data is 73 available the call will hang. 74 75 If the underlying BIO is B<nonblocking>, a read function will also return when 76 the underlying BIO could not satisfy the needs of the function to continue the 77 operation. 78 In this case a call to L<SSL_get_error(3)> with the 79 return value of the read function will yield B<SSL_ERROR_WANT_READ> or 80 B<SSL_ERROR_WANT_WRITE>. 81 As at any time it's possible that non-application data needs to be sent, 82 a read function can also cause write operations. 83 The calling process then must repeat the call after taking appropriate action 84 to satisfy the needs of the read function. 85 The action depends on the underlying BIO. 86 When using a nonblocking socket, nothing is to be done, but select() can be 87 used to check for the required condition. 88 When using a buffering BIO, like a BIO pair, data must be written into or 89 retrieved out of the BIO before being able to continue. 90 91 L<SSL_pending(3)> can be used to find out whether there 92 are buffered bytes available for immediate retrieval. 93 In this case the read function can be called without blocking or actually 94 receiving new data from the underlying socket. 95 96 =head1 RETURN VALUES 97 98 SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure. 99 Success means that 1 or more application data bytes have been read from the SSL 100 connection. 101 Failure means that no bytes could be read from the SSL connection. 102 Failures can be retryable (e.g. we are waiting for more bytes to 103 be delivered by the network) or non-retryable (e.g. a fatal network error). 104 In the event of a failure call L<SSL_get_error(3)> to find out the reason which 105 indicates whether the call is retryable or not. 106 107 For SSL_read() and SSL_peek() the following return values can occur: 108 109 =over 4 110 111 =item E<gt> 0 112 113 The read operation was successful. 114 The return value is the number of bytes actually read from the TLS/SSL 115 connection. 116 117 =item Z<><= 0 118 119 The read operation was not successful, because either the connection was closed, 120 an error occurred or action must be taken by the calling process. 121 Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason. 122 123 Old documentation indicated a difference between 0 and -1, and that -1 was 124 retryable. 125 You should instead call SSL_get_error() to find out if it's retryable. 126 127 =back 128 129 =head1 SEE ALSO 130 131 L<SSL_get_error(3)>, L<SSL_write_ex(3)>, 132 L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>, 133 L<SSL_connect(3)>, L<SSL_accept(3)> 134 L<SSL_set_connect_state(3)>, 135 L<SSL_pending(3)>, 136 L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>, 137 L<ssl(7)>, L<bio(7)> 138 139 =head1 HISTORY 140 141 The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1. 142 143 =head1 COPYRIGHT 144 145 Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved. 146 147 Licensed under the OpenSSL license (the "License"). You may not use 148 this file except in compliance with the License. You can obtain a copy 149 in the file LICENSE in the source distribution or at 150 L<https://www.openssl.org/source/license.html>. 151 152 =cut 153
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