Solve Exercise 8-2: fopen and fillbuf

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diff --git a/ch8/8-02_fopen-and-fillbuf.c b/ch8/8-02_fopen-and-fillbuf.c
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+++ b/ch8/8-02_fopen-and-fillbuf.c
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+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include "8-02_syscalls.h"
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 8-2: Rewrite `fopen` and `_fillbuf` with fields instead of explicit
+ * bit operations. Compare code size and execution speed.
+ *
+ * Notes: Fields, in this case, are struct fields. I implemented them as 1-bit
+ * integers to simulate bit fields via struct.
+ *
+ * The resulting binary of this file (plus the custom header) runs through this
+ * source file in about 0.001s. However, the binary is 1KB larger.
+ */
+
+#define PERMS 0666
+
+int getc(FILE *);
+
+int my__fillbuf(my_FILE *fp) {
+	int bufsize;
+	if (fp->flags._READ == 0) {
+		return EOF;
+	}
+	bufsize = (fp->flags._UNBUF != 0) ? 1 : BUFSIZ;
+	if (fp->base == NULL) {
+		if ((fp->base = (char *) calloc(bufsize, sizeof (char))) == NULL) {
+			return EOF;
+		}
+	}
+	fp->ptr = fp->base;
+	fp->cnt = read(fp->fd, fp->ptr, bufsize);
+	if (--fp->cnt < 0) {
+		if (fp->cnt == -1) {
+			fp->flags._EOF = 1;
+		} else {
+			fp->flags._ERR = 1;
+		}
+		fp->cnt = 0;
+		return EOF;
+	}
+	return (unsigned char) *fp->ptr++;
+}
+
+my_FILE * my_fopen(char *name, char *mode) {
+	int fd;
+	my_FILE *fp;
+	if (*mode != 'r' && *mode != 'w' && *mode != 'a') {
+		return NULL;
+	}
+	for (fp = _iob; fp < _iob + FOPEN_MAX; fp++) {
+		if (fp->flags._READ == 0 && fp->flags._WRITE == 0) {
+			break;
+		}
+	}
+	if (fp >= _iob + FOPEN_MAX) {
+		return NULL;
+	}
+	if (*mode == 'w') {
+		fd = creat(name, PERMS);
+	} else if (*mode == 'a') {
+		if ((fd = open(name, O_WRONLY, 0)) == -1) {
+			fd = creat(name, PERMS);
+		}
+		lseek(fd, 0L, 2);
+	} else {
+		fd = open(name, O_RDONLY, 0);
+	}
+	if (fd == -1) {
+		return NULL;
+	}
+	fp->fd = fd;
+	fp->cnt = 0;
+	if (*mode == 'r') {
+		fp->flags._READ = 1;
+	} else {
+		fp->flags._WRITE = 1;
+	}
+	return fp;
+}
+
+int main() {
+	my_FILE *fp = my_fopen("8-02_fopen-and-fillbuf.c", "r");
+	if (fp != NULL) {
+		puts("We could read the file.\n");
+		/* pro tip: don't declare this as 'unsigned' or you'll create an
+		 * endless loop thanks to EOF typically being negative. Don't waste
+		 * half an hour on a newbie mistake like I did. :)
+		 */
+		char c;
+		while ((c = my_getc(fp)) != EOF) {
+			putchar(c);
+		}
+	} else {
+		puts("Could not open file for reading.");
+	}
+	return 0;
+}