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186 Programming

“It Can’t Be a Bug, My Makefile Depends on It!” ....................

The programmers at BBN were generally the exception. Most Unix pro-

grammers don’t fix bugs: most don’t have source code. Those with the

code know that fixing bugs won’t help. That’s why when most Unix pro-

grammers encounter a bug, they simply program around it.

It’s a sad state of affairs: if one is going to solve a problem, why not solve

it once and for all instead of for a single case that will have to repeated for

each new program ad infinitum? Perhaps early Unix programmers were

closet metaphysicians that believed in Nietzche’s doctrine of Eternal

Recurrence.

There are two schools of debugging thought. One is the “debugger as phy-

sician” school, which was popularized in early ITS and Lisp systems. In

these environments, the debugger is always present in the running program

and when the program crashes, the debugger/physician can diagnose the

problem and make the program well again.

Unix follows the older “debugging as autopsy” model. In Unix, a broken

program dies, leaving a core file, that is like a dead body in more ways than

one. A Unix debugger then comes along and determines the cause of death.

Interestingly enough, Unix programs tend to die from curable diseases,

accidents, and negligence, just as people do.

Dealing with the Core

After your program has written out a core file, your first task is to find it.

This shouldn’t be too difficult a task, because the core file is quite large—

4, 8, and even 12 megabyte core files are not uncommon.

Core files are large because they contain almost everything you need to

debug your program from the moment it died: stack, data, pointers to

code... everything, in fact, except the program’s dynamic state. If you were

debugging a network program, by the time your core file is created, it’s too

late; the program’s network connections are gone. As an added slap, any

files it might have had opened are now closed.

Unfortunately, under Unix, it has to be that way.

For instance, one cannot run a debugger as a command-interpreter or trans-

fer control to a debugger when the operating system generates an excep-

tion. The only way to have a debugger take over from your program when