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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | SEE ALSO | COLOPHON |
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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 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])
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 project's upstream Git repository
⟨https://github.com/linux-pam/linux-pam.git⟩ 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
[email protected]
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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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