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

       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 discrepancy
       between the way Linux-PAM handles the const struct pam_message
       **msg conversation function argument and the way that Solaris'
       PAM (and derivatives, 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 one.

       •   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 project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At
       that time, the date of the most recent commit that was found in
       the repository was 2023-12-18.)  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 to

Linux-PAM Manual               12/22/2023                    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)