'\" t
.\"     Title: pam_conv
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: 06/08/2020
.\"    Manual: Linux-PAM Manual
.\"    Source: Linux-PAM Manual
.\"  Language: English
.\"
.TH "PAM_CONV" "3" "06/08/2020" "Linux-PAM Manual" "Linux-PAM Manual"
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.SH "NAME"
pam_conv \- PAM conversation function
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <security/pam_appl\&.h>
.fi
.ft
.sp
.nf
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;
};
    
.fi
.SH "DESCRIPTION"
.PP
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
\fIstruct pam_conv\fR
passed to
\fBpam_start\fR(3)
at the start of the transaction\&.
.PP
When a module calls the referenced conv() function, the argument
\fIappdata_ptr\fR
is set to the second element of this structure\&.
.PP
The other arguments of a call to conv() concern the information exchanged by module and application\&. That is to say,
\fInum_msg\fR
holds the length of the array of pointers,
\fImsg\fR\&. After a successful return, the pointer
\fIresp\fR
points to an array of pam_response structures, holding the application supplied text\&. The
\fIresp_retcode\fR
member of this struct is unused and should be set to zero\&. It is the caller\*(Aqs responsibility to release both, this array and the responses themselves, using
\fBfree\fR(3)\&. Note,
\fI*resp\fR
is a
\fIstruct pam_response\fR
array and not an array of pointers\&.
.PP
The number of responses is always equal to the
\fInum_msg\fR
conversation function argument\&. This does require that the response array is
\fBfree\fR(3)\*(Aqd after every call to the conversation function\&. The index of the responses corresponds directly to the prompt index in the pam_message array\&.
.PP
On failure, the conversation function should release any resources it has allocated, and return one of the predefined PAM error codes\&.
.PP
Each message can have one of four types, specified by the
\fImsg_style\fR
member of
\fIstruct pam_message\fR:
.PP
PAM_PROMPT_ECHO_OFF
.RS 4
Obtain a string without echoing any text\&.
.RE
.PP
PAM_PROMPT_ECHO_ON
.RS 4
Obtain a string whilst echoing text\&.
.RE
.PP
PAM_ERROR_MSG
.RS 4
Display an error message\&.
.RE
.PP
PAM_TEXT_INFO
.RS 4
Display some text\&.
.RE
.PP
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\&.
.PP
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\*(Aq 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 \*(Aqstruct pam_message\*(Aq pointers\&. Solaris\*(Aq 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\&.
.PP
For what its worth the two known module writer work\-arounds for trying to maintain source level compatibility with both PAM implementations are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
never call the conversation function with num_msg greater than one\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
set up msg as doubly referenced so both types of conversation function can find the messages\&. That is, make
.sp
.if n \{\
.RS 4
.\}
.nf
       msg[n] = & (( *msg )[n])
       
.fi
.if n \{\
.RE
.\}
.RE
.SH "RETURN VALUES"
.PP
PAM_BUF_ERR
.RS 4
Memory buffer error\&.
.RE
.PP
PAM_CONV_ERR
.RS 4
Conversation failure\&. The application should not set
\fI*resp\fR\&.
.RE
.PP
PAM_SUCCESS
.RS 4
Success\&.
.RE
.SH "SEE ALSO"
.PP
\fBpam_start\fR(3),
\fBpam_set_item\fR(3),
\fBpam_get_item\fR(3),
\fBpam_strerror\fR(3),
\fBpam\fR(8)