# getlogin

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           getlogin, getlogin_r — get login name
    


## SYNOPSIS



           #include <unistd.h>
    
           char *getlogin(void);
           int getlogin_r(char *_name_ , size_t _namesize_);
    


## DESCRIPTION



           The _getlogin_() function shall return a pointer to a string
           containing the user name associated by the login activity with the
           controlling terminal of the current process. If _getlogin_() returns
           a non-null pointer, then that pointer points to the name that the
           user logged in under, even if there are several login names with
           the same user ID.
    
           The _getlogin_() function need not be thread-safe.
    
           The _getlogin_r_() function shall put the name associated by the
           login activity with the controlling terminal of the current
           process in the character array pointed to by _name_.  The array is
           _namesize_ characters long and should have space for the name and
           the terminating null character. The maximum size of the login name
           is {LOGIN_NAME_MAX}.
    
           If _getlogin_r_() is successful, _name_ points to the name the user
           used at login, even if there are several login names with the same
           user ID.
    
           The _getlogin_() and _getlogin_r_() functions may make use of file
           descriptors 0, 1, and 2 to find the controlling terminal of the
           current process, examining each in turn until the terminal is
           found. If in this case none of these three file descriptors is
           open to the controlling terminal, these functions may fail. The
           method used to find the terminal associated with a file descriptor
           may depend on the file descriptor being open to the actual
           terminal device, not **/dev/tty**.
    


## RETURN VALUE



           Upon successful completion, _getlogin_() shall return a pointer to
           the login name or a null pointer if the user's login name cannot
           be found. Otherwise, it shall return a null pointer and set _errno_
           to indicate the error.
    
           The application shall not modify the string returned. The returned
           pointer might be invalidated or the string content might be
           overwritten by a subsequent call to _getlogin_().  The returned
           pointer and the string content might also be invalidated if the
           calling thread is terminated.
    
           If successful, the _getlogin_r_() function shall return zero;
           otherwise, an error number shall be returned to indicate the
           error.
    


## ERRORS



           These functions may fail if:
    
           **EMFILE** All file descriptors available to the process are currently
                  open.
    
           **ENFILE** The maximum allowable number of files is currently open in
                  the system.
    
           **ENOTTY** None of the file descriptors 0, 1, or 2 is open to the
                  controlling terminal of the current process.
    
           **ENXIO** The calling process has no controlling terminal.
    
           The _getlogin_r_() function may fail if:
    
           **ERANGE** The value of _namesize_ is smaller than the length of the
                  string to be returned including the terminating null
                  character.
    
           _The following sections are informative._
    


## EXAMPLES



       **Getting the User Login Name S**
           The following example calls the _getlogin_() function to obtain the
           name of the user associated with the calling process, and passes
           this information to the _getpwnam_() function to get the associated
           user database information.
    
               #include <unistd.h>
               #include <sys/types.h>
               #include <pwd.h>
               #include <stdio.h>
               ...
               char *lgn;
               struct passwd *pw;
               ...
               if ((lgn = getlogin()) == NULL || (pw = getpwnam(lgn)) == NULL) {
                   fprintf(stderr, "Get of user information failed.\n"); exit(1);
                   }
    


## APPLICATION USAGE



           Three names associated with the current process can be determined:
           _getpwuid_(_geteuid_()) shall return the name associated with the
           effective user ID of the process; _getlogin_() shall return the name
           associated with the current login activity; and _getpwuid_(_getuid_())
           shall return the name associated with the real user ID of the
           process.
    
           The _getlogin_r_() function is thread-safe and returns values in a
           user-supplied buffer instead of possibly using a static data area
           that may be overwritten by each call.
    


## RATIONALE



           The _getlogin_() function returns a pointer to the user's login
           name. The same user ID may be shared by several login names. If it
           is desired to get the user database entry that is used during
           login, the result of _getlogin_() should be used to provide the
           argument to the _getpwnam_() function. (This might be used to
           determine the user's login shell, particularly where a single user
           has multiple login shells with distinct login names, but the same
           user ID.)
    
           The information provided by the _cuserid_() function, which was
           originally defined in the POSIX.1‐1988 standard and subsequently
           removed, can be obtained by the following:
    
               getpwuid(geteuid())
    
           while the information provided by historical implementations of
           _cuserid_() can be obtained by:
    
               getpwuid(getuid())
    
           The thread-safe version of this function places the user name in a
           user-supplied buffer and returns a non-zero value if it fails. The
           non-thread-safe version may return the name in a static data area
           that may be overwritten by each call.
    


## FUTURE DIRECTIONS



           None.
    


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