---
name: "getlogin"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/getlogin/3"
---

# getlogin(3)

get username

## NAME

getlogin, getlogin\_r, cuserid - get username

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <unistd.h>
```

**char \*getlogin(void);** **int getlogin\_r(**size\_t bufsize;                **char** *buf***\[***bufsize***\], size\_t** *bufsize***);**

**#include \<stdio.h\>**

**char \*cuserid(char \****string***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**getlogin\_r**():

```
    _POSIX_C_SOURCE >= 199506L
```

**cuserid**():

```
    Since glibc 2.24:
        (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
            || _GNU_SOURCE
    Up to and including glibc 2.23:
        _XOPEN_SOURCE
```

## DESCRIPTION

**getlogin**() returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to **cuserid**().

**getlogin\_r**() returns this same username in the array *buf* of size *bufsize*.

**cuserid**() returns a pointer to a string containing a username associated with the effective user ID of the process. If *string* is not a null pointer, it should be an array that can hold at least **L\_cuserid** characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned. This string is statically allocated and might be overwritten on subsequent calls to this function or to **getlogin**().

The macro **L\_cuserid** is an integer constant that indicates how long an array you might need to store a username. **L\_cuserid** is declared in *\<stdio.h\>*.

These functions let your program identify positively the user who is running (**cuserid**()) or the user who logged in this session (**getlogin**()). (These can differ when set-user-ID programs are involved.)

For most purposes, it is more useful to use the environment variable **LOGNAME** to find out who the user is. This is more flexible precisely because the user can set **LOGNAME** arbitrarily.

## RETURN VALUE

**getlogin**() returns a pointer to the username when successful, and NULL on failure, with *errno* set to indicate the error. **getlogin\_r**() returns 0 when successful, and nonzero on failure.

## ERRORS

POSIX specifies:

- **EMFILE**

  The per-process limit on the number of open file descriptors has been reached.

- **ENFILE**

  The system-wide limit on the total number of open files has been reached.

- **ENXIO**

  The calling process has no controlling terminal.

- **ERANGE**

  (getlogin\_r) The length of the username, including the terminating null byte (**'\\0'**), is larger than *bufsize*.

Linux/glibc also has:

- **ENOENT**

  There was no corresponding entry in the utmp-file.

- **ENOMEM**

  Insufficient memory to allocate passwd structure.

- **ENOTTY**

  Standard input didn't refer to a terminal. (See BUGS.)

## FILES

- */etc/passwd*

  password database file

- */var/run/utmp*

  (traditionally */etc/utmp*; some libc versions used */var/adm/utmp*)

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **getlogin**() | Thread safety | MT-Unsafe race:getlogin race:utent sig:ALRM timer locale |
| **getlogin\_r**() | Thread safety | MT-Unsafe race:utent sig:ALRM timer locale |
| **cuserid**() | Thread safety | MT-Unsafe race:cuserid/!string locale |

In the above table, *utent* in *race:utent* signifies that if any of the functions [**setutent**(3)](/setutent/3.md), [**getutent**(3)](/getutent/3.md), or [**endutent**(3)](/endutent/3.md) are used in parallel in different threads of a program, then data races could occur. **getlogin**() and **getlogin\_r**() call those functions, so we use race:utent to remind users.

## VERSIONS

OpenBSD has **getlogin**() and **setlogin**(), and a username associated with a session, even if it has no controlling terminal.

## STANDARDS

- **getlogin**() **getlogin\_r**()

  POSIX.1-2008.

- **cuserid**()

  None.

## STANDARDS

- **getlogin**() **getlogin\_r**():

  POSIX.1-2001. OpenBSD.

- **cuserid**()

  System V, POSIX.1-1988. Removed in POSIX.1-1990. SUSv2. Removed in POSIX.1-2001.

-

  System V has a **cuserid**() function which uses the real user ID rather than the effective user ID.

## BUGS

Unfortunately, it is often rather easy to fool **getlogin**(). Sometimes it does not work at all, because some program messed up the utmp file. Often, it gives only the first 8 characters of the login name. The user currently logged in on the controlling terminal of our program need not be the user who started it. Avoid **getlogin**() for security-related purposes.

Note that glibc does not follow the POSIX specification and uses *stdin* instead of */dev/tty*. A bug. (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8 all return the login name also when *stdin* is redirected.)

Nobody knows precisely what **cuserid**() does; avoid it in portable programs. Or avoid it altogether: use *getpwuid(geteuid())* instead, if that is what you meant. **Do not use** **cuserid**().

## SEE ALSO

[**logname**(1)](/logname/1.md), [**geteuid**(2)](/geteuid/2.md), [**getuid**(2)](/getuid/2.md), [**utmp**(5)](/utmp/5.md)