Encyclopedia of Computer Science and Technology

ulletin board systems 61bugs and debuggingIn general terms a bug is an error in a computer programthat leads to unexpected and unwanted behavior. (Lore hasit that the first “bug” was a burnt moth found in the relaysof the early Mark I computer in the 1940s; however, as earlyas 1878 Thomas Edison had referred to “bugs” in the designof his new inventions.)Computer bugs can be divided into two categories: syntaxerrors and logic errors. A syntax error results fromfailing to follow a language’s rules for constructing statements,or from using the wrong symbol. For example, eachstatement in the C language must end with a semicolon.This sort of syntax error is easily detected and reported bymodern compilers, so fixing it is trivial.A logic error, on the other hand, is a syntactically validstatement that does not do what was intended. For example,if a C programmer writes:if Total = 100instead ofif Total == 100the programmer may have intended to test the value of Totalto see if it is 100, but the first statement actually assigns thevalue of 100 to Total. That’s because a single equals sign inC is the assignment operator; testing for equality requiresthe double equals sign. Further, the error will result in theif statement always being true, because the truth value of anassignment is the value assigned (100 in this case) and anynonzero value is considered to be “true” (see branchingstatements).Loops and pointers are frequent sources of logical errors(see loop and pointers and indirection). The boundarycondition of a loop can be incorrectly specified (for example,< 10 when < = 10 is wanted). If a loop and a pointer orindex variable are being used to retrieve data from an array,pointing beyond the end of the array will retrieve whateverdata happens to be stored out there.Errors can also be caused in the conversion of data ofdifferent types (see data types). For example, in many languageimplementations the compiler will automatically convertan integer value to floating point if it is to be assignedto a floating point variable. However, while an integer canretain at least nine decimal digits of precision, a float mayonly be able to guarantee seven. The result could be a lossof precision sufficient to render the program’s results unreliable,particularly for scientific purposes.Debugging TechniquesThe process of debugging (identifying and fixing bugs) isaided by the debugging features integrated into most modernprogramming environments. Some typical featuresinclude the ability to set a breakpoint or place in the codewhere the running program should halt so the values of keyvariables can be examined. A watch can be set on specifiedcertain variables so their changing values will be displayedas the program executes. A trace highlights the source codeto show what statements are being executed as the programruns. (It can also be set to follow execution into and throughany procedures or subroutines called by the main code.)During the process of software development, debuggingwill usually proceed hand in hand with software testing.Indeed, the line between the two can be blurry. Essentially,debugging deals with fixing problems so that the programis doing what it intends to do, while testing determineswhether the program’s performance adequately meets theneeds and objectives of the end user.Further ReadingAgans, David J. Debugging: The Nine Indispensable Rules for FindingEven the Most Elusive Software and Hardware Problems. NewYork: AMACOM, 2002.Robbins, John. Debugging Applications. Redmond, Wash.: MicrosoftPress, 2000.Rosenberg, Jonathan B. How Debuggers Work: Algorithms, DataStructures, and Architecture. New York: Wiley, 1996.bulletin board systems (BBS)An electronic bulletin board is a computer application thatlets users access a computer (usually with a modem andphone line) and read or post messages on a variety of topics.The messages are often organized by topic, resultingin threads of postings, responses, and responses to theresponses. In addition to the message service, many bulletinboards provide files that users can download, suchas games and other programs, text documents, pictures, orsound files. Some bulletin boards expect users to uploadfiles to contribute to the board in return for the privilege ofdownloading material.The earliest form of bulletin board appeared in the late1960s in government installations and a few universities participatingin the Defense Department’s ARPANET (the ancestorto the Internet). As more universities came online in theearly 1970s, the Netnews (or Usenet) system offered a way touse UNIX file-transfer programs to store messages in topicalnewsgroups (see netnews and newsgroups). The newssystem automatically propagated messages (in the form of a“news feed”) from the site where they were originally postedto regional nodes, and from there throughout the network.By the early 1980s, a significant number of personalcomputer users were connecting modems to their PCs. Bulletinboard software was developed to allow an operator(called a “sysop”) to maintain a bulletin board on his orher PC. Users (one or a few at a time) could dial a phonenumber to connect to the bulletin board. In 1984, programmerTom Jennings developed the Fido BBS software, whichallowed participating bulletin boards to propagate postingsin a way roughly similar to the distribution of UNIX Netnewsmessages.Decline of the BBSIn the 1990s, two major developments led to a drastic declinein the number of bulletin boards. The growth of major servicessuch as America Online and CompuServe (see onlineservices) offered users a friendlier user interface, a comprehensiveselection of forums and file downloads, and