The six module functions are grouped into four independent management groups. These groups are as follows: authentication, account, session and password. To be properly defined, a module must define all functions within at least one of these groups. A single module may contain the necessary functions for all four groups.
The independence of the four groups of service a module can offer means that the module should allow for the possibility that any one of these four services may legitimately be called in any order. Thus, the module writer should consider the appropriateness of performing a service without the prior success of some other part of the module.
As an informative example, consider the possibility that an application applies to change a user's authentication token, without having first requested that Linux-PAM authenticate the user. In some cases this may be deemed appropriate: when root wants to change the authentication token of some lesser user. In other cases it may not be appropriate: when joe maliciously wants to reset alice's password; or when anyone other than the user themself wishes to reset their KERBEROS authentication token. A policy for this action should be defined by any reasonable authentication scheme, the module writer should consider this when implementing a given module.
To avoid system administration problems and the poor
construction of a /etc/pam.conf file,
the module developer may define all six of the following
functions. For those functions that would not be called,
the module should return PAM_SERVICE_ERR
and write an appropriate message to the system log. When
this action is deemed inappropriate, the function would
simply return PAM_IGNORE.
The flags argument of each of
the following functions can be logically OR'd with
PAM_SILENT, which is used to inform the
module to not pass any text (errors or
warnings) application.
The argc and argv
arguments are taken from the line appropriate to this
module---that is, with the service_name
matching that of the application---in the configuration file
(see the Linux-PAM
System Administrators' Guide). Together these two parameters
provide the number of arguments and an array of pointers to
the individual argument tokens. This will be familiar to C
programmers as the ubiquitous method of passing command arguments
to the function main(). Note, however, that
the first argument (argv[0]) is a true
argument and not the name of the module.