PAM(3PAM) PAM Library Functions PAM(3PAM)


pam - PAM (Pluggable Authentication Module)


#include <security/pam_appl.h>
cc [ flag... ] file ... -lpam [ library ... ]


The PAM framework, libpam, consists of an interface library and multiple
authentication service modules. The PAM interface library is the layer
implementing the Application Programming Interface (API). The
authentication service modules are a set of dynamically loadable objects
invoked by the PAM API to provide a particular type of user
authentication. PAM gives system administrators the flexibility of
choosing any authentication service available on the system to perform
authentication. This framework also allows new authentication service
modules to be plugged in and made available without modifying the

Interface Overview

The PAM library interface consists of six categories of functions, the
names for which all start with the prefix pam_.

The first category contains functions for establishing and terminating an
authentication activity, which are pam_start(3PAM) and pam_end(3PAM). The
functions pam_set_data(3PAM) and pam_get_data(3PAM) maintain module
specific data. The functions pam_set_item(3PAM) and pam_get_item(3PAM)
maintain state information. pam_strerror(3PAM) is the function that
returns error status information.

The second category contains the functions that authenticate an
individual user and set the credentials of the user,
pam_authenticate(3PAM) and pam_setcred(3PAM).

The third category of PAM interfaces is account management. The function
pam_acct_mgmt(3PAM) checks for password aging and access-hour

Category four contains the functions that perform session management
after access to the system has been granted. See pam_open_session(3PAM)
and pam_close_session(3PAM)

The fifth category consists of the function that changes authentication
tokens, pam_chauthtok(3PAM). An authentication token is the object used
to verify the identity of the user. In UNIX, an authentication token is a
user's password.

The sixth category of functions can be used to set values for PAM
environment variables. See pam_putenv(3PAM), pam_getenv(3PAM), and

The pam_*() interfaces are implemented through the library libpam. For
each of the categories listed above, excluding categories one and six,
dynamically loadable shared modules exist that provides the appropriate
service layer functionality upon demand. The functional entry points in
the service layer start with the pam_sm_ prefix. The only difference
between the pam_sm_*() interfaces and their corresponding pam_ interfaces
is that all the pam_sm_*() interfaces require extra parameters to pass
service-specific options to the shared modules. Refer to pam_sm(3PAM)
for an overview of the PAM service module APIs.

Stateful Interface

A sequence of calls sharing a common set of state information is referred
to as an authentication transaction. An authentication transaction
begins with a call to pam_start(). pam_start() allocates space, performs
various initialization activities, and assigns a PAM authentication
handle to be used for subsequent calls to the library.

After initiating an authentication transaction, applications can invoke
pam_authenticate() to authenticate a particular user, and pam_acct_mgmt()
to perform system entry management. For example, the application may want
to determine if the user's password has expired.

If the user has been successfully authenticated, the application calls
pam_setcred() to set any user credentials associated with the
authentication service. Within one authentication transaction (between
pam_start() and pam_end()), all calls to the PAM interface should be
made with the same authentication handle returned by pam_start(). This is
necessary because certain service modules may store module-specific data
in a handle that is intended for use by other modules. For example,
during the call to pam_authenticate(), service modules may store data in
the handle that is intended for use by pam_setcred().

To perform session management, applications call pam_open_session().
Specifically, the system may want to store the total time for the
session. The function pam_close_session() closes the current session.

When necessary, applications can call pam_get_item() and pam_set_item()
to access and to update specific authentication information. Such
information may include the current username.

To terminate an authentication transaction, the application simply calls
pam_end(), which frees previously allocated space used to store
authentication information.

Application-Authentication Service Interactive Interface
The authentication service in PAM does not communicate directly with the
user; instead it relies on the application to perform all such
interactions. The application passes a pointer to the function, conv(),
along with any associated application data pointers, through a pam_conv
structure to the authentication service when it initiates an
authentication transaction, via a call to pam_start(). The service will
then use the function, conv(), to prompt the user for data, output error
messages, and display text information. Refer to pam_start(3PAM) for more

Stacking Multiple Schemes

The PAM architecture enables authentication by multiple authentication
services through stacking. System entry applications, such as login(1),
stack multiple service modules to authenticate users with multiple
authentication services. The order in which authentication service
modules are stacked is specified in the configuration file, pam.conf(5).
A system administrator determines this ordering, and also determines
whether the same password can be used for all authentication services.

Administrative Interface

The authentication library, /usr/lib/, implements the
framework interface. Various authentication services are implemented by
their own loadable modules whose paths are specified through the
pam.conf(5) file.


The PAM functions may return one of the following generic values, or one
of the values defined in the specific man pages:

The function returned successfully.

dlopen() failed when dynamically loading a service

Symbol not found.

Error in service module.

System error.

Memory buffer error.

Conversation failure.

Permission denied.


See attributes(7) for description of the following attributes:

|MT Level | MT-Safe with exceptions |


login(1), pam_authenticate(3PAM), pam_chauthtok(3PAM),
pam_open_session(3PAM), pam_set_item(3PAM), pam_setcred(3PAM),
pam_sm(3PAM), pam_start(3PAM), pam_strerror(3PAM), pam.conf(5),


The interfaces in libpam(3LIB) are MT-Safe only if each thread within the
multithreaded application uses its own PAM handle.

August 19, 2023 PAM(3PAM)