The getpwuid function searches the user database for an entry
    with the specified uid. The function returns the first user entry
    in the database with a pw_uid member of the passwd structure that
    matches the uid argument.

    The passwd structure is defined in the <pwd.h> header file as
    follows:

    pw_name    The user's login name.
    pw_uid     The numerical user ID.
    pw_gid     The numerical group ID.
    pw_dir     The home directory of the user.
    pw_shell   The initial program for the user.

                                   NOTE

       All information generated by the getpwuid function is stored
       in a per-thread static area and is overwritten on subsequent
       calls to the function.

    The getpwuid_r function is the reentrant version of getpwuid.
    The getpwuid_r function updates the passwd structure pointed to
    by pwd and stores a pointer to that structure at the location
    pointed to by result. The structure will contain an entry from
    the user database with a matching uid. Storage referenced by the
    structure is allocated from the memory provided with the buffer
    argument, which is bufsize characters in size. The maximum size
    needed for this buffer can be determined with the _SC_GETPW_R_
    SIZE_MAX parameter of the sysconf function. On error or if the
    requested entry is not found, a NULL pointer is returned at the
    location pointed to by result.

    Applications wishing to check for error situations should set
    errno to 0 before calling getpwuid. If getpwuid returns a NULL
    pointer and errno is nonzero, an error occurred.

    See also getuid to know how UIC is represented.