The PAM library uses an application-defined callback to communicate with the user. This callback is specified by the
struct pam_conv passed to
pam_start() at the start of the transaction. It is also possible to set or change the conversation function at any point during a PAM transaction by changing the value of the
PAM_CONV item.
The conversation function's first argument specifies the number of messages (up to
PAM_NUM_MSG) to process. The second argument is a pointer to an array of pointers to
pam_message structures containing the actual messages.
Each message can have one of four types, specified by the
msg_style member of
struct pam_message:
PAM_PROMPT_ECHO_OFF
Display a prompt and accept the user's response without echoing it to the terminal. This is commonly used for passwords.
PAM_PROMPT_ECHO_ON
Display a prompt and accept the user's response, echoing it to the terminal. This is commonly used for login names and one-time passphrases.
PAM_ERROR_MSG
Display an error message.
PAM_TEXT_INFO
Display an informational message.
In each case, the prompt or message to display is pointed to by the
msg member of
struct pam_message. It can be up to
PAM_MAX_MSG_SIZE characters long, including the terminating NUL.
On success, the conversation function should allocate and fill a contiguous array of
struct pam_response, one for each message that was passed in. A pointer to the user's response to each message (or
NULL in the case of informational or error messages) should be stored in the
resp member of the corresponding
struct pam_response. Each response can be up to
PAM_MAX_RESP_SIZE characters long, including the terminating NUL.
The
resp_retcode member of
struct pam_response is unused and should be set to zero.
The conversation function should store a pointer to this array in the location pointed to by its third argument. It is the caller's responsibility to release both this array and the responses themselves, using
free(3). It is the conversation function's responsibility to ensure that it is legal to do so.
The
appdata_ptr member of
struct pam_conv is passed unmodified to the conversation function as its fourth and final argument.
On failure, the conversation function should release any resources it has allocated, and return one of the predefined PAM error codes.