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Objective: To discuss the considerations that mustbe addressed when designing an interface. Toillustrate these design issues with a simple yetuseful example.
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“The essence of design is to balance competinggoals and constraints. Although there are manytradeoffs when one is writing a small self-contained system, the ramifications of particularchoices remain within the system and affect onlythe individual programmer. But when code is tobe used by others, decisions have widerrepercussions.”
*The examples in these slides come fromBrian W. Kernighan and Rob Pike,“The Practice of Programming”, Addison-Wesley, 1999.
CSV
Interfaces*
Themes
- Interfaces: what services and access are
provided?^ – The interface is in effect a contract between the
supplier and the customer. The desire is toprovide services that are uniform andconvenient, with enough functionality to beeasy to use but not so much as to becomeunwieldy.
Themes
- Resource Management: who is responsible
for managing memory and other resources?^ – Here, the main problems are allocating and
freeing storage, and managing shared copies ofinformation.
Themes
- Error handling: who detects errors, who
reports them, and how? When an error isdetected, what recovery is attempted?
Topics
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Interface Principles^ 1. Hide implementation details^ 2. Choose a small orthogonal set of primitives^ 3. Don't reach behind the user's back^ 4. Do the same thing the same way everywhere
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Resource Management^ 1. Free a resource in the same layer that allocated it
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Abort, Retry, Fail?^ 1. Detect errors at a low level, handle them at a high level^ 2. Use exceptions only for exceptional situations
Comma Separated Values (CSV) •^
A natural and widely used format for tabular data^ – Each row is a line of text^ – The fields on each line are separated by commas^ – Text format used by many spreadsheets
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Example^ – “Good Student”,CS265,”Advanced Programming Tools
and Techniques”,A
- “Bad Student”,CS265,”Advanced Programming Tools
and Techniques”,D
Prototype
char buf[200];
/* input line buffer */
char *field[20];
/* fields */
/* csvgetline: read and parse line, return field count / / sample input: "LU",86.25,"11/4/1998","2:19PM",+4.0625 */ int csvgetline(FILE *fin) {
int nfield; char *p, *q; if (fgets(buf, sizeof(buf), fin) == NULL)
return -1; nfield = 0; for (q = buf; (p=strtok(q, ",\n\r")) != NULL; q = NULL)
field[nfield++] = unquote(p); return nfield; }
Prototype
/* unquote: remove leading and trailing quote */ char *unquote(char *p) {
if (p[0] == '"') {
if (p[strlen(p)-1] == '"')
p[strlen(p)-1] = '\0'; p++; } return p; } /* csvtest main: test csvgetline function */ int main(void) {
int i, nf; while ((nf = csvgetline(stdin)) != -1)
for (i = 0; i < nf; i++)
printf("field[%d] = `%s'\n", i, field[i]);
return 0; }
Decisions Made in Prototype
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Global variables make code unsuitable to multi-threaded environment or interleaved calls
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Caller must open and close files
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Input and splitting are inextricably linked
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The return value is the number of fields (each linemust be split to compute this value)
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Each decision is interwoven into the code. Thereis no way to change any of these propertieswithout changing the code
Specification
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Fields are separated by commas
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A field may be enclosed in double-quotes
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A quoted field may contain commas but notnewlines
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A quoted field may contain double-quotes,represented by “”
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Fields may be empty; “” and empty string bothrepresent an empty field
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Leading and trailing white space is preserved
Specification
•^
char *csvfield(int n);^ – fields are numbered from 0.^ – returns n-th field from last line read by csvgetline
- returns NULL if n<0 or beyond last field.
- fields are separated by commas. – fields may be surrounded by “…”, such quotes are
removed; within “…”, “” is replaced by “ and comma isnot a separator
- in unquoted fields, quotes are regular characters – there can be an arbitrary number of fields of any length;
- returns NULL if memory limit exceeded.
- fields must be treated as read-only storage;
- caller must make a copy to preserve or change contents
- behavior undefined if called before csvgetline is called.
Specification
- int csvnfield(void);
- returns number of fields on last line read by
csvgetline.
- behavior undefined if called before csvgetline is
called.
Internal Variables
enum { NOMEM = -2 };
/* out of memory signal */
static char *line
= NULL; /* input chars */
static char *sline
= NULL; /* line copy used by split */
static int maxline = 0;
/* size of line[] and sline[] */
static char *field = NULL; / field pointers */ static int maxfield = 0;
/* size of field[] */
static int nfield
= 0;
/* number of fields in field[] */
static char fieldsep[] = ","; /* field separator chars */
Internal Functions
/* endofline: check for and consume \r, \n, \r\n, or EOF */ static int endofline(FILE fin, int c) / reset: set variables back to starting values / static void reset(void) / split: split line into fields / static int split(void) / advquoted: quoted field; return pointer to next separator */ static char *advquoted(char *p)
