SSL_read.pod 5.0 KB

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