SSL_read.pod 6.1 KB

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152
  1. =pod
  2. =head1 NAME
  3. SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek
  4. - read bytes from a TLS/SSL connection
  5. =head1 SYNOPSIS
  6. #include <openssl/ssl.h>
  7. int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
  8. int SSL_read(SSL *ssl, void *buf, int num);
  9. int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
  10. int SSL_peek(SSL *ssl, void *buf, int num);
  11. =head1 DESCRIPTION
  12. SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl>
  13. into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes
  14. actually read in B<*readbytes>.
  15. SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read()
  16. respectively except no bytes are actually removed from the underlying BIO during
  17. the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield
  18. at least the same bytes.
  19. =head1 NOTES
  20. In the paragraphs below a "read function" is defined as one of SSL_read_ex(),
  21. SSL_read(), SSL_peek_ex() or SSL_peek().
  22. If necessary, a read function will negotiate a TLS/SSL session, if not already
  23. explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the
  24. peer requests a re-negotiation, it will be performed transparently during
  25. the read function operation. The behaviour of the read functions depends on the
  26. underlying BIO.
  27. For the transparent negotiation to succeed, the B<ssl> must have been
  28. initialized to client or server mode. This is being done by calling
  29. L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first
  30. invocation of a read function.
  31. The read functions work based on the SSL/TLS records. The data are received in
  32. records (with a maximum record size of 16kB). Only when a record has been
  33. completely received, can it be processed (decryption and check of integrity).
  34. Therefore, data that was not retrieved at the last read call can still be
  35. buffered inside the SSL layer and will be retrieved on the next read
  36. call. If B<num> is higher than the number of bytes buffered then the read
  37. functions will return with the bytes buffered. If no more bytes are in the
  38. buffer, the read functions will trigger the processing of the next record.
  39. Only when the record has been received and processed completely will the read
  40. functions return reporting success. At most the contents of one record will
  41. be returned. As the size of an SSL/TLS record may exceed the maximum packet size
  42. of the underlying transport (e.g. TCP), it may be necessary to read several
  43. packets from the transport layer before the record is complete and the read call
  44. can succeed.
  45. If B<SSL_MODE_AUTO_RETRY> has been switched off and a non-application data
  46. record has been processed, the read function can return and set the error to
  47. B<SSL_ERROR_WANT_READ>.
  48. In this case there might still be unprocessed data available in the B<BIO>.
  49. If read ahead was set using L<SSL_CTX_set_read_ahead(3)>, there might also still
  50. be unprocessed data available in the B<SSL>.
  51. This behaviour can be controlled using the L<SSL_CTX_set_mode(3)> call.
  52. If the underlying BIO is B<blocking>, a read function will only return once the
  53. read operation has been finished or an error occurred, except when a
  54. non-application data record has been processed and B<SSL_MODE_AUTO_RETRY> is
  55. not set.
  56. Note that if B<SSL_MODE_AUTO_RETRY> is set and only non-application data is
  57. available the call will hang.
  58. If the underlying BIO is B<nonblocking>, a read function will also return when
  59. the underlying BIO could not satisfy the needs of the function to continue the
  60. operation.
  61. In this case a call to L<SSL_get_error(3)> with the
  62. return value of the read function will yield B<SSL_ERROR_WANT_READ> or
  63. B<SSL_ERROR_WANT_WRITE>.
  64. As at any time it's possible that non-application data needs to be sent,
  65. a read function can also cause write operations.
  66. The calling process then must repeat the call after taking appropriate action
  67. to satisfy the needs of the read function.
  68. The action depends on the underlying BIO.
  69. When using a nonblocking socket, nothing is to be done, but select() can be
  70. used to check for the required condition.
  71. When using a buffering BIO, like a BIO pair, data must be written into or
  72. retrieved out of the BIO before being able to continue.
  73. L<SSL_pending(3)> can be used to find out whether there
  74. are buffered bytes available for immediate retrieval.
  75. In this case the read function can be called without blocking or actually
  76. receiving new data from the underlying socket.
  77. =head1 RETURN VALUES
  78. SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
  79. Success means that 1 or more application data bytes have been read from the SSL
  80. connection.
  81. Failure means that no bytes could be read from the SSL connection.
  82. Failures can be retryable (e.g. we are waiting for more bytes to
  83. be delivered by the network) or non-retryable (e.g. a fatal network error).
  84. In the event of a failure call L<SSL_get_error(3)> to find out the reason which
  85. indicates whether the call is retryable or not.
  86. For SSL_read() and SSL_peek() the following return values can occur:
  87. =over 4
  88. =item E<gt> 0
  89. The read operation was successful.
  90. The return value is the number of bytes actually read from the TLS/SSL
  91. connection.
  92. =item Z<><= 0
  93. The read operation was not successful, because either the connection was closed,
  94. an error occurred or action must be taken by the calling process.
  95. Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
  96. Old documentation indicated a difference between 0 and -1, and that -1 was
  97. retryable.
  98. You should instead call SSL_get_error() to find out if it's retryable.
  99. =back
  100. =head1 SEE ALSO
  101. L<SSL_get_error(3)>, L<SSL_write_ex(3)>,
  102. L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
  103. L<SSL_connect(3)>, L<SSL_accept(3)>
  104. L<SSL_set_connect_state(3)>,
  105. L<SSL_pending(3)>,
  106. L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>,
  107. L<ssl(7)>, L<bio(7)>
  108. =head1 HISTORY
  109. The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1.
  110. =head1 COPYRIGHT
  111. Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
  112. Licensed under the Apache License 2.0 (the "License"). You may not use
  113. this file except in compliance with the License. You can obtain a copy
  114. in the file LICENSE in the source distribution or at
  115. L<https://www.openssl.org/source/license.html>.
  116. =cut