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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | SEE ALSO | COLOPHON |
PAM_CONV(3) Linux-PAM Manual PAM_CONV(3)
pam_conv - PAM conversation function
#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;
};
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:
PAM_PROMPT_ECHO_OFF
Obtain a string without echoing any text.
PAM_PROMPT_ECHO_ON
Obtain a string whilst echoing text.
PAM_ERROR_MSG
Display an error message.
PAM_TEXT_INFO
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
one.
· set up msg as doubly referenced so both types of conversation
function can find the messages. That is, make
msg[n] = & (( *msg )[n])
PAM_BUF_ERR
Memory buffer error.
PAM_CONV_ERR
Conversation failure. The application should not set *resp.
PAM_SUCCESS
Success.
pam_start(3), pam_set_item(3), pam_get_item(3), pam_strerror(3),
pam(8)
This page is part of the linux-pam (Pluggable Authentication Modules
for Linux) project. Information about the project can be found at
⟨http://www.linux-pam.org/⟩. If you have a bug report for this manual
page, see ⟨//www.linux-pam.org/⟩. This page was obtained from the
tarball Linux-PAM-1.3.0.tar.gz fetched from
⟨http://www.linux-pam.org/library/⟩ on 2017-07-05. 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 man-pages@man7.org
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)