Is there another way to truncate a string?

char out_file_name[30];  


ogSize = strlen(FileName); //i.e. - FileName_anylength.log (always.log)
ogSize -= strlen(IN_FILE_SUFFIX); //amount of chars before .log

strncpy( out_file_name, FileName, ogSize ); //out_file_name now is FileName_anylength (without the .log)  

Is this the best way to do this?

Also, how do I guard that ogSize doesn't happen to be more than 30 chars as it is coming from user input?

Thank You.