debug and log file

Hi. I have create log system. It uses file debug.txt in which function "void debug(string in)" writes lines of information. The problem is: if application crashes, i have (??) to close this file before this happens, otherwise stuff gets lost. So everytime I call debug, I have to open file, write stuff in, and close it. This is very time consuming. Any suggestions? Are there any log libraries available for this? bye and thanx, ivan