| CUSERID(3) | Library Functions Manual | CUSERID(3) | 
cuserid —
#include <stdio.h>
char *
  
  cuserid(char
    *buf);
The cuserid() function returns a character
    string representation of the user name associated with the effective user ID
    of the calling process.
If buf is not the
    NULL pointer, the user name is copied into the
    memory referenced by buf. The argument
    buf is assumed to point to an array at least
    L_cuserid (as defined in the include file
    <stdio.h>) bytes long.
    Otherwise, the user name is copied to a static buffer.
NULL pointer,
  buf is returned; otherwise the address of the static
  buffer is returned.
If the user name could not be determined, if
    buf is not the NULL pointer,
    the null character ‘\0’ will be stored at
    *buf; otherwise the NULL
    pointer is returned.
cuserid() function conforms to IEEE
  Std 1003.1-1988 (“POSIX.1”).
cuserid() was removed from the IEEE
  Std 1003.1-1990 (“POSIX.1”) standard. This implementation
  exists purely for compatibility with existing programs. New programs should
  use one of the following three alternatives to obtain the user name:
getlogin() to return the user's login name.getpwuid(geteuid()) to
      return the user name associated with the calling process' effective user
      ID.getpwuid(getuid()) to
      return the user name associated with the calling process' real user
    ID.| November 28, 1993 | NetBSD 10.0 |