normal user uses smbpasswd

[jun xu sun]

hi,
when one normal user wants to change password using smbpasswd, system says:
machine 127.0.0.1 rejected the password change: Error was : The specified
password is invalid.
Failed to change password for xxxxxxx.

I am sure password is correct.
how does passwd work? /etc/passwd is 0444, how passwd write to it for every
user?

thanks.

rgds
sun