wolfSSL (formerly CyaSSL) has support for debugging through log messages in environments where debugging is limited. To turn logging on use the functionwolfSSL_Debugging_ON() and to turn it off usewolfSSL_Debugging_OFF(). In a normal build (release mode) these functions will have no effect. In a debug build, defineDEBUG_WOLFSSL to ensure these functions are turned on.

As of wolfSSL 2.0, logging callback functions may be registered at runtime to provide more flexibility with how logging is done. The logging callback can be registered with the functionwolfSSL_SetLoggingCb():

int wolfSSL_SetLoggingCb(wolfSSL_Logging_cb log_function);typedef void (*wolfSSL_Logging_cb)(const int logLevel,                  const char *const logMessage);

The log levels can be found inwolfssl/wolfcrypt/logging.h, and the implementation is located inlogging.c. By default, wolfSSL logs tostderr withfprintf.

Error Codes

wolfSSL tries to provide informative error messages in order to help with debugging.

EachwolfSSL_read() andwolfSSL_write() call will return the number of bytes written upon success, 0 upon connection closure, and -1 for an error, just likeread() andwrite(). In the event of an error you can use two calls to get more information about the error.

The functionwolfSSL_get_error() will return the current error code. It takes the currentWOLFSSL object, andwolfSSL_read() orwolfSSL_write() result value as arguments and returns the current error code.

int err = wolfSSL_get_error(ssl, result);

To get a more human-readable error code description, thewolfSSL_ERR_error_string() function can be used. It takes the return code fromwolfSSL_get_error and a storage buffer as arguments, and places the corresponding error description into the storage buffer (errorString in the example below).

char errorString[80];wolfSSL_ERR_error_string(err, errorString);

If you are using non blocking sockets, you can test for errno EAGAIN/EWOULDBLOCK or more correctly you can test the specific error code forSSL_ERROR_WANT_READ orSSL_ERROR_WANT_WRITE.

For a list of wolfSSL and wolfCrypt error codes, please see Appendix C (Error Codes).