pam_conv(3) — Linux manual page


PAM_CONV(3)                   Linux-PAM Manual                   PAM_CONV(3)

NAME         top

       pam_conv - PAM conversation function

SYNOPSIS         top

       #include <security/pam_appl.h>

       struct pam_message {
           int msg_style;
           const char *msg;

       struct pam_response {
           char *resp;
           int resp_retcode;

       struct pam_conv {
           int (*conv)(int num_msg, const struct pam_message **msg,
                       struct pam_response **resp, void *appdata_ptr);
           void *appdata_ptr;

DESCRIPTION         top

       The PAM library uses an application-defined callback to allow a
       direct communication between a loaded module and the application.
       This callback is specified by the struct pam_conv passed to
       pam_start(3) at the start of the transaction.

       When a module calls the referenced conv() function, the argument
       appdata_ptr is set to the second element of this structure.

       The other arguments of a call to conv() concern the information
       exchanged by module and application. That is to say, num_msg holds
       the length of the array of pointers, msg. After a successful return,
       the pointer resp points to an array of pam_response structures,
       holding the application supplied text. The resp_retcode member of
       this struct is unused and should be set to zero. It is the caller's
       responsibility to release both, this array and the responses
       themselves, using free(3). Note, *resp is a struct pam_response array
       and not an array of pointers.

       The number of responses is always equal to the num_msg conversation
       function argument. This does require that the response array is
       free(3)'d after every call to the conversation function. The index of
       the responses corresponds directly to the prompt index in the
       pam_message array.

       On failure, the conversation function should release any resources it
       has allocated, and return one of the predefined PAM error codes.

       Each message can have one of four types, specified by the msg_style
       member of struct pam_message:

           Obtain a string without echoing any text.

           Obtain a string whilst echoing text.

           Display an error message.

           Display some text.

       The point of having an array of messages is that it becomes possible
       to pass a number of things to the application in a single call from
       the module. It can also be convenient for the application that
       related things come at once: a windows based application can then
       present a single form with many messages/prompts on at once.

       In passing, it is worth noting that there is a descrepency between
       the way Linux-PAM handles the const struct pam_message **msg
       conversation function argument from the way that Solaris' PAM (and
       derivitives, known to include HP/UX, are there others?) does.
       Linux-PAM interprets the msg argument as entirely equivalent to the
       following prototype const struct pam_message *msg[] (which, in
       spirit, is consistent with the commonly used prototypes for argv
       argument to the familiar main() function: char **argv; and char
       *argv[]). Said another way Linux-PAM interprets the msg argument as a
       pointer to an array of num_msg read only 'struct pam_message'
       pointers. Solaris' PAM implementation interprets this argument as a
       pointer to a pointer to an array of num_msg pam_message structures.
       Fortunately, perhaps, for most module/application developers when
       num_msg has a value of one these two definitions are entirely
       equivalent. Unfortunately, casually raising this number to two has
       led to unanticipated compatibility problems.

       For what its worth the two known module writer work-arounds for
       trying to maintain source level compatibility with both PAM
       implementations are:

       ·   never call the conversation function with num_msg greater than

       ·   set up msg as doubly referenced so both types of conversation
           function can find the messages. That is, make

                      msg[n] = & (( *msg )[n])

RETURN VALUES         top

           Memory buffer error.

           Conversation failure. The application should not set *resp.


SEE ALSO         top

       pam_start(3), pam_set_item(3), pam_get_item(3), pam_strerror(3),

COLOPHON         top

       This page is part of the linux-pam (Pluggable Authentication Modules
       for Linux) project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this manual
       page, see ⟨//⟩.  This page was obtained from the
       tarball Linux-PAM-1.3.0.tar.bz2 fetched from
       ⟨⟩ on 2020-09-18.  If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page, or
       you have corrections or improvements to the information in this
       COLOPHON (which is not part of the original manual page), send a mail

Linux-PAM Manual                 04/01/2016                      PAM_CONV(3)

Pages that refer to this page: misc_conv(3)pam(3)pam_authenticate(3)pam_get_item(3)pam_get_user(3)pam_prompt(3)pam_set_item(3)pam_vprompt(3)