IDA Help: attach_process

Attach the debugger to a running process
     pid - PID of the process to attach to. If NO_PROCESS, a dialog box
           will interactively ask the user for the process to attach to.
     event_id - reserved, must be -1
returns:
        -2 - impossible to find a compatible process
        -1 - impossible to attach to the given process (process died, privilege
             needed, not supported by the debugger plugin, ...)
         0 - the user cancelled the attaching to the process
         1 - the debugger properly attached to the process
See the important note to the step_into() function

long attach_process(long pid, long event_id);