authenticate_user - authenticate user
#include <auth.h> /* For error codes */
int authenticate_user(user, password, line)
struct passwd *user;
user The UID.
The password for the UID.
line The name of the login line as it is listed in
The routine authenticates a username/UID against a supplied password
and returns a nonnegative integer on success. The value returned is
the number of failed login authentication attempts since the last suc‐
cessful login authentication (or zero if this feature is not enabled).
This routine is found in the library and loaded using the -lauth
At all security levels higher than BSD, the login fail count in the
auth database is incremented if authentication fails, and cleared if it
succeeds. In addition, the account must be marked enabled for logins
as defined by the Account Mask value for A_LOGIN. See for information
about the Account Mask values.
If a non-NULL value is supplied for the argument and the argument is
not the empty string, the function also verifies that the specified
user is allowed access through that line. In particular, accounts with
UID equal to zero will only return success if the specified line is
marked secure in the file.
The process must have read access to the auth database to authenticate
users in a secure environment.
The process must have read/write access to the auth database to update
the authentication fail count.
If auth information is being served through BIND, the process is
required to obtain a Kerberos ticket for that service before invoking
extern int errno;
struct passwd *pwd;
pwd = getpwnam("root");
status = authenticate_user(pwd, "rootpass", "/dev/console");
if(status < 0)
if(errno == EPERM)
if(status > 0)
printf("%d failed attempts\n", status);
When successful, the routine returns the number of failed login authen‐
tication attempts since last successful login authentication.
When an error occurs, is set and a negative error code is returned. The
error code returned may be the same as or it may be an extended error
code defined in
On error return may be set to one of the following values:
[EPERM] Either the password is incorrect, the password is
expired, the specified line needs to be secure and is
not, or the account is disabled and a login authenti‐
cation is required.
[EINVAL] No authentication information for user.
[ENOSYS] Security subsystem not configured correctly.
[EACCES] Process does not have read access to the necessary
On error return the return value may be the same as or, if errno is
[EPERM], it may be one of the following additional values defined in
[A_EBADPASS] The supplied password was incorrect.
[A_ESOFTEXP] The accounts password has expired recently.
[A_EHARDEXP] The accounts password expired quite some time ago.
[A_ENOLOGIN] The account is not enabled.
[A_EOPENLINE] The account requires a secure line and the specified
line was not marked that way in
If the system is operating in the BSD security level, the password
expiration, login fail count, and account disabling features are not
available (and therefore are not used in authentication computations).
See Alsogetauthuid(3), getpwent(3), auth(5), passwd(5yp), ttys(5)authenticate_user(3x)