From bfd18a52e1baa4da7040b8df04a3f29e83b0d6b6 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Sun, 9 Jan 2022 20:18:52 -0800
Subject: Solve Exercise 8-4: fseek() implementation

---
 ch8/8-04_fseek.c | 34 ++++++++++++++++++++++++++++++++++
 ch8/syscalls.h   | 45 ++++++++++++++++++++++++++++++++++-----------
 2 files changed, 68 insertions(+), 11 deletions(-)
 create mode 100644 ch8/8-04_fseek.c

diff --git a/ch8/8-04_fseek.c b/ch8/8-04_fseek.c
new file mode 100644
index 0000000..b968ca2
--- /dev/null
+++ b/ch8/8-04_fseek.c
@@ -0,0 +1,34 @@
+#include <errno.h>
+#include <string.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include "syscalls.h"
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 8-4: The standard library function
+ *
+ *     int fseek(FILE *fp, long offset, int origin)
+ *
+ * is identical to `lseek` except that `fp` is a file pointer instead of a file
+ * descriptor and the return value is an `int` status, not a position. Write
+ * `fseek`. Make sure that your `fseek` coordinates properly with the buffering
+ * done for the other functions of the library.
+ *
+ * Notes: Page 174 covers lseek, where it describes fseek's return value as 0
+ * for success and non-zero for errors.
+ */
+
+int main() {
+	FILE *fp = fopen("8-04_fseek.c", "r");
+	if (fp == NULL) {
+		exit(1);
+	}
+	fseek(fp, -34L, 2);
+	puts(fp->base);
+	fflush(stdout);
+	fclose(fp);
+	return 0;
+}
+
+/* If you see this, 8-4 works! */
diff --git a/ch8/syscalls.h b/ch8/syscalls.h
index ff58db5..cda913f 100644
--- a/ch8/syscalls.h
+++ b/ch8/syscalls.h
@@ -66,7 +66,7 @@ int fempty(struct _flags flags) {
 FILE * fopen(char *name, char *mode) {
 	int fd;
 	FILE *fp;
-	if (*mode != 'r' && *mode != 'w' && *mode != 'a') {
+	if (*mode != 'r' && *mode != 'w' && *mode != 'a' && *mode != '+') {
 		return NULL;
 	}
 	for (fp = _iob; fp < _iob + FOPEN_MAX; fp++) {
@@ -77,24 +77,34 @@ FILE * fopen(char *name, char *mode) {
 	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) {
+	switch (*mode) {
+		case '+':
+			if ((fd = open(name, O_RDWR, 0)) == -1) {
+				fd = creat(name, PERMS);
+			}
+			break;
+		case 'w':
 			fd = creat(name, PERMS);
-		}
-		lseek(fd, 0L, 2);
-	} else {
-		fd = open(name, O_RDONLY, 0);
+			break;
+		case 'a':
+			if ((fd = open(name, O_WRONLY, 0)) == -1) {
+				fd = creat(name, PERMS);
+			}
+			lseek(fd, 0L, 2);
+			break;
+		default:
+			fd = open(name, O_RDONLY, 0);
 	}
 	if (fd == -1) {
 		return NULL;
 	}
 	fp->fd = fd;
 	fp->cnt = 0;
-	if (*mode == 'r') {
+	fp->flags._EOF = 0;
+	if (*mode == 'r' || *mode == '+') {
 		fp->flags._READ = 1;
-	} else {
+	}
+	if (*mode == 'w' || *mode == '+' || *mode == 'a') {
 		fp->flags._WRITE = 1;
 	}
 	return fp;
@@ -220,3 +230,16 @@ int puts(char *s) {
 	putchar('\n');
 	return i;
 }
+
+/* Set the 'cursor' position within a file for read or write operations
+ * Returns 0 on success, EOF on error, with errno set.
+ */
+int fseek(FILE *fp, long offset, int origin) {
+	long pos;
+	if ((pos = lseek(fp->fd, offset, origin)) == EOF) {
+		return EOF;
+	}
+	_fillbuf(fp);
+	lseek(fp->fd, offset, origin);
+	return 0;
+}
-- 
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