GETLOGIN(3C) Standard C Library Functions GETLOGIN(3C)

getlogin, getlogin_r - get login name

#include <unistd.h>
char *getlogin(void);

char *getlogin_r(char *name, int namelen);

cc [ flag ... ] file... -D_POSIX_PTHREAD_SEMANTICS [ library ... ]
int getlogin_r(char *name, size_t namesize);

The getlogin() function returns a pointer to the login name as found in /var/adm/utmpx. It can be used in conjunction with getpwnam(3C) to locate the correct password file entry when the same user ID is shared by several login names.

The login name plus the terminating null byte can be up to 33 characters in length. Newly-compiled programs should use the LOGIN_NAME_MAX symbol, defined in <limits.h>, to size the buffer. Older programs that call getlogin() expect only the legacy 9-character length. These automatically link to a version of the getlogin() functions that truncates longer login names. It's also possible to compile new programs that link to truncating versions of these functions by defining __USE_LEGACY_LOGNAME__ in the compile environment.

Some older programs will correctly handle long login names returned by the getlogin() function. For this case, the user compatibility library /usr/lib/ redirects to a version of the getlogin() function that returns the long name. This library should be added to such an application at runtime using LD_PRELOAD.

If getlogin() is called within a process that is not attached to a terminal, it returns a null pointer. The correct procedure for determining the login name is to call cuserid(3C), or to call getlogin() and if it fails to call getpwuid(3C).

The getlogin_r() function has the same functionality as getlogin() except that the caller must supply a buffer name with length namelen to store the result. The name buffer should be at least LOGIN_NAME_MAX bytes in size (defined in <limits.h>). The POSIX version (see standards(5)) of getlogin_r() takes a namesize parameter of type size_t. If the size of the supplied buffer is less than the size of LOGIN_NAME_MAX and the name, including the null terminator, does not fit inside the buffer, than an error will be generated. Otherwise, the buffer name will be updated with the login name.

Upon successful completion, getlogin() returns a pointer to the login name or a null pointer if the user's login name cannot be found. Otherwise it returns a null pointer and sets errno to indicate the error.

The standard-conforming getlogin_r() returns 0 if successful, or the error number upon failure.

The getlogin_r() function will fail if:


The size of the buffer is smaller than the result to be returned.


And entry for the current user was not found in the /var/adm/utmpx file.

The getlogin() and getlogin_r() functions may fail if:


There are {OPEN_MAX} file descriptors currently open in the calling process.


The maximum allowable number of files is currently open in the system.


The calling process has no controlling terminal.

The getlogin_r() function may fail if:


The size of the buffer is smaller than the result to be returned.

The return value of getlogin() points to thread-specific data whose content is overwritten on each call by the same thread.

Three names associated with the current process can be determined: getpwuid(geteuid()) returns the name associated with the effective user ID of the process; getlogin() returns the name associated with the current login activity; and getpwuid(getuid()) returns the name associated with the real user ID of the process.

user access and administration information


A compatibility library that returns long login names to older applications.


A 64-bit compatibility library to return long login names.

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

Interface Stability Standard
MT-Level See below.

geteuid(2), getuid(2), cuserid(3C), getgrnam(3C), getpwnam(3C), getpwuid(3C), utmpx(4), attributes(5), standards(5)

When compiling multithreaded programs, see Intro(3).

The getlogin() function is safe to use in multithreaded applications, but is discouraged. The getlogin_r() function should be used instead.

Solaris 2.4 and earlier releases provided a getlogin_r() as specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the interface as described above. Support for the Draft 6 interface is provided for compatibility only and may not be supported in future releases. New applications and libraries should use the standard-conforming interface.

March 15, 2014