The pam_authenticate() function is called to authenticate the current user. The user is usually required to enter a password or similar authentication token depending upon the authentication service configured within the system. The user in question should have been specified by a prior call to pam_start() or pam_set_item().

The following flags may be set in the flags field:

PAM_SILENT

Authentication service should not generate any messages.

PAM_DISALLOW_NULL_AUTHTOK

The authentication service should return PAM_AUTH_ERROR if the user has a null authentication token.

Upon successful completion, PAM_SUCCESS is returned. In addition to the error return values described in pam(3PAM), the following values may be returned:

PAM_AUTH_ERR

Authentication failure.

PAM_CRED_INSUFFICIENT

Cannot access authentication data due to insufficient credentials.

PAM_AUTHINFO_UNAVAIL

Underlying authentication service cannot retrieve authentication information.

PAM_USER_UNKNOWN

User not known to the underlying authentication module.

PAM_MAXTRIES

An authentication service has maintained a retry count which has been reached. No further retries should be attempted.

See attributes(5) for description of the following attributes:

pam(3PAM), pam_open_session(3PAM), pam_set_item(3PAM), pam_setcred(3PAM), pam_start(3PAM), libpam(3LIB), attributes(5)

In the case of authentication failures due to an incorrect username or password, it is the responsibility of the application to retry pam_authenticate() and to maintain the retry count. An authentication service module may implement an internal retry count and return an error PAM_MAXTRIES if the module does not want the application to retry.

If the PAM framework cannot load the authentication module, then it will return PAM_ABORT. This indicates a serious failure, and the application should not attempt to retry the authentication.

For security reasons, the location of authentication failures is hidden from the user. Thus, if several authentication services are stacked and a single service fails, pam_authenticate() requires that the user re-authenticate each of the services.

A null authentication token in the authentication database will result in successful authentication unless PAM_DISALLOW_NULL_AUTHTOK was specified. In such cases, there will be no prompt to the user to enter an authentication token.

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