pam_start man page
pam_start — initialization of PAM transaction
int pam_start(const char *service_name, const char *user, const struct pam_conv *pam_conversation, pam_handle_t **pamh);
The pam_start function creates the PAM context and initiates the PAM transaction. It is the first of the PAM functions that needs to be called by an application. The transaction state is contained entirely within the structure identified by this handle, so it is possible to have multiple transactions in parallel. But it is not possible to use the same handle for different transactions, a new one is needed for every new context.
The service_name argument specifies the name of the service to apply and will be stored as PAM_SERVICE item in the new context. The policy for the service will be read from the file /etc/pam.d/service_name or, if that file does not exist, from /etc/pam.conf.
The user argument can specify the name of the target user and will be stored as PAM_USER item. If the argument is NULL, the module has to ask for this item if necessary.
The pam_conversation argument points to a struct pam_conv describing the conversation function to use. An application must provide this for direct communication between a loaded module and the application.
Following a successful return (PAM_SUCCESS) the contents of pamh is a handle that contains the PAM context for successive calls to the PAM functions. In an error case is the content of pamh undefined.
The pam_handle_t is a blind structure and the application should not attempt to probe it directly for information. Instead the PAM library provides the functions pam_set_item(3) and pam_get_item(3). The PAM handle cannot be used for mulitiple authentications at the same time as long as pam_end was not called on it before.
Memory buffer error.
Transaction was successful created.
System error, for example a NULL pointer was submitted instead of a pointer to data.
pam_get_data(3), pam_set_data(3), pam_end(3), pam_strerror(3)
pam(3), pam_acct_mgmt(3), pam_authenticate(3), pam_chauthtok(3), pam.conf(5), pam_conv(3), pam_end(3), pam_fail_delay(3), pam_getenv(3), pam_getenvlist(3), pam_get_user(3), pam_putenv(3), pam_xauth_data(3).