setlogin(2) get/set login name

Other Alias

getlogin, getlogin_r


Lb libc


In unistd.h Ft char * Fn getlogin void In sys/param.h Ft int Fn getlogin_r char *name int len Ft int Fn setlogin const char *name


The Fn getlogin routine returns the login name of the user associated with the current session, as previously set by Fn setlogin . The name is normally associated with a login shell at the time a session is created, and is inherited by all processes descended from the login shell. (This is true even if some of those processes assume another user ID, for example when su(1) is used).

The Fn getlogin_r function provides the same service as Fn getlogin except the caller must provide the buffer Fa name with length Fa len bytes to hold the result. The buffer should be at least MAXLOGNAME bytes in length.

The Fn setlogin system call sets the login name of the user associated with the current session to Fa name . This system call is restricted to the super-user, and is normally used only when a new session is being created on behalf of the named user (for example, at login time, or when a remote shell is invoked).

NOTE There is only one login name per session.

It is CRITICALLY important to ensure that Fn setlogin is only ever called after the process has taken adequate steps to ensure that it is detached from its parent's session. Making a Fn setsid system call is the ONLY way to do this. The daemon(3) function calls Fn setsid which is an ideal way of detaching from a controlling terminal and forking into the background.

In particular, doing a Fn ioctl ttyfd TIOCNOTTY ... or Fn setpgrp ... is NOT sufficient.

Once a parent process does a Fn setsid system call, it is acceptable for some child of that process to then do a Fn setlogin even though it is not the session leader, but beware that ALL processes in the session will change their login name at the same time, even the parent.

This is not the same as the traditional UNIX behavior of inheriting privilege.

Since the Fn setlogin system call is restricted to the super-user, it is assumed that (like all other privileged programs) the programmer has taken adequate precautions to prevent security violations.


If a call to Fn getlogin succeeds, it returns a pointer to a null-terminated string in a static buffer, or NULL if the name has not been set. The Fn getlogin_r function returns zero if successful, or the error number upon failure.

Rv -std setlogin


The following errors may be returned by these calls:

The Fa name argument gave an invalid address.
The Fa name argument pointed to a string that was too long. Login names are limited to MAXLOGNAME (from In sys/param.h ) characters, currently 17 including null.
The caller tried to set the login name and was not the super-user.
The size of the buffer is smaller than the result to be returned.


The Fn getlogin system call and the Fn getlogin_r function conform to St -p1003.1-96 .


The Fn getlogin system call first appeared in BSD 4.4 The return value of Fn getlogin_r was changed from earlier versions of Fx to be conformant with St -p1003.1-96 .


In earlier versions of the system, Fn getlogin failed unless the process was associated with a login terminal. The current implementation (using Fn setlogin ) allows getlogin to succeed even when the process has no controlling terminal. In earlier versions of the system, the value returned by Fn getlogin could not be trusted without checking the user ID. Portable programs should probably still make this check.