24.txt

一個計算機系教授的上課講義 主要是教導學生使用C語言編寫程序

CS 1355
Introduction to Programming in C
Thursday 2006.12.7
Lecture notes (at http://r638-2.cs.nthu.edu.tw/C/notes/24.txt)

Today: Chapter 11 File I/O (continued)

- Sequential access vs Random access

File handle contains the "current position" (also called "file offset")
of accessing a file
- initially at position 0
- sequential access:
  - read/write n bytes => increase offset by n
  - the only way to change offset is by reading/writing sequentially.
- random access:
  - move the file offset using
	int fseek(FILE*, long offset, int whence)
  - find out about current position using
	long ftell(FILE *stream);
  - three ways (whence)
    - SEEK_CUR: relative to current position
    - SEEK_END: relative to the end of the file
    - SEEK_SET: absolute position (relative to the beginning)

Example: interactive file reading
#include <stdio.h>
int main(int argc, char **argv) {
	FILE *fh;
	int end = 0;
	long n = 1;
	if (argc != 2) { 
		fprintf(stderr, "usage: %s file", argv[0]); return 1; 
	}
	if ((fh = fopen(argv[1], "r")) == NULL) {
		fprintf(stderr, "can't open file %s\n", argv[1]);
		return 2;
	}
	/* now interpret command */
	while (!end) {
		char c;
		int i;
		char line[100];
		printf("%ld> ", ftell(fh));
		if (fgets(line, 100, stdin) == NULL) {
			end = 1;
			break;
		}
		switch(sscanf(line, "%c%li", &c, &n)) {
		case 1: /* just got %c; leave n as default */
		case 2: /* c n */
			switch (c) {
			case 'r':
				for (i = 0; i < n; i++) {
					char ch;
					if (!fread(&ch, 1, 1, fh)) {
						end = 1;
						break;
					} else {
						printf("%c", ch);
					}
				}
				printf("\n");
				break;
			case 'g': /* SEEK_SET */
				fseek(fh, n, SEEK_SET); break;
			case 'c': /* SEEK_CUR */
				fseek(fh, n, SEEK_CUR); break;
			case 'e': /* SEEK_END */
				fseek(fh, n, SEEK_END); break;
			case 'q': /* quit */
				end = 1;
				break;
			default: 
				printf("unknown command %c\n", c);
			}
			break;
		case 0: 
			printf("syntax error\n");
		case EOF:
			end = 1;
			break;
		}
	}
	fclose(fh);
}
-------
Case study: a (database) transaction processing system
(Section 11.10, page 450, fig 11.16)
=>  don't worry about the example. just try running it.
How it works
- create a blank file named "credit.dat" (fill 0's)
	struct clientData {
		int acctNum;
		char lastName[15];
		char firstName[10];
		double balance;
	};
- menu to read/write files
  => one option is to read all records in, update in array, then write all out.
  => random access: one record at a time, do not affect other records.

Points with the example
- you can write structures or any data types, not just 
  char, int, or other basic types
- reading structures from the file can be done in
  exactly the same way for structures.
- important restriction:
  - you should not store pointers in files!!
    all fields should be data containers, rather than pointer variables
  that is, char lastname[15] etc is ok, but
           char *lastname;  would not be ok!
  Why not? because the pointer will probably not be meaningful
  at all when you read it back into memory.