Code as Part of the Software Development Process

Code as Exemplar

Reasons

Understand how it works

Create compatible software

Strategy

Try multiple approaches

Search
Browse
Read documentation
Run
Run under a debugger
Trace

Ignore irrelevant details

Example: tail -f (NetBSD)

        while ((ch = getopt(argc, argv, "b:c:fn:r")) != -1)
                switch(ch) {
                [...]
                case 'f':
                        fflag = 1;
                        break;
                }

[...]

        for (;;) {
                while ((ch = getc(fp)) != EOF)
                        if (putchar(ch) == EOF)
                                oerr();
                [...]
                if (!fflag)
                        break;
                /*
                 * We pause for one second after displaying any data that has
                 * accumulated since we read the file.  Since sleep(3) takes
                 * eight system calls, use select() instead.
                 */
                second.tv_sec = 1;
                second.tv_usec = 0;
                if (select(0, NULL, NULL, NULL, &second) == -1)
                        err(1, "select: %s", strerror(errno));
                clearerr(fp);
        }

Example: tail -f (FreeBSD)

        if (fflag) {
                kq = kqueue();
                if (kq < 0)
                        err(1, "kqueue");
                action = ADD_EVENTS;
        }
        for (;;) {
                while ((ch = getc(fp)) != EOF)
                        if (putchar(ch) == EOF)
                                oerr();
[...]
                if (! fflag)
                        break;
                clearerr(fp);

                switch (action) {
                case ADD_EVENTS:
                        n = 0;
                        ts.tv_sec = 0;
                        ts.tv_nsec = 0;

                        if (Fflag && fileno(fp) != STDIN_FILENO) {
                                EV_SET(&ev[n], fileno(fp), EVFILT_VNODE,
                                    EV_ADD | EV_ENABLE | EV_CLEAR,
                                    NOTE_DELETE | NOTE_RENAME, 0, 0);
                                n++;
                        }
                        EV_SET(&ev[n], fileno(fp), EVFILT_READ,
                            EV_ADD | EV_ENABLE | EV_CLEAR, 0, 0, 0);
                        n++;

                        if (kevent(kq, ev, n, NULL, 0, &ts) < 0) {
                                action = USE_SLEEP;
                        } else {
                                action = USE_KQUEUE;
                        }
                        break;

Maintenance and its Tools

Use tools

Compiler warnings
Examine symbolic code
System call tracing
Debugger
SQL log
Packet dump
Message spy

Work from problem manifestation to problem source

Eschew unrelated paths

Compile with debugging options

Run under a debugger

Single step
Stack trace
Code breakpoint
Data breakpoint

Add print statements

Code as Part of the Software Development Process

Code as a Scientific Communication Vehicle

Gains from High-quality Code

Recognize Low-quality Code

Requirements Can Lead to Strange Code

How to Familiarize Yourself with a Body of Code

Code Reading Reasons

Code as Exemplar

Reasons

Strategy

Example: tail -f (NetBSD)

Example: tail -f (FreeBSD)

Maintenance and its Tools

Evolution

Maintenance Activities

Strategy

Porting

Refactoring

Reuse

Inspections

Further Reading

Exercises and Discussion Topics