From 6498896bbbd051b799e6b69ad59b91a30bfcc5da Mon Sep 17 00:00:00 2001
From: Kaiwan N Billimoria <kaiwan.billimoria@gmail.com>
Date: Tue, 31 Oct 2023 12:05:01 +0530
Subject: [PATCH] initial commit - orig user space app to test this misc driver

---
 ch12/rdwr_test_secret.c | 133 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 133 insertions(+)
 create mode 100644 ch12/rdwr_test_secret.c

diff --git a/ch12/rdwr_test_secret.c b/ch12/rdwr_test_secret.c
new file mode 100644
index 0000000..bb82fb6
--- /dev/null
+++ b/ch12/rdwr_test_secret.c
@@ -0,0 +1,133 @@
+/*
+ * ch1/miscdrv_rdwr/rdwr_test_secret.c
+ ***************************************************************
+ * This program is part of the source code released for the book
+ *  "Linux Kernel Programming (Part 2)"
+ *  (c) Author: Kaiwan N Billimoria
+ *  Publisher:  Packt
+ *  GitHub repository:
+ *  https://github.com/PacktPublishing/Linux-Kernel-Programming-Part-2
+ *
+ * From: Ch 1 : Writing a Simple Misc Character Device Driver
+ ****************************************************************
+ * Brief Description:
+ * A simple test bed for the miscdrv_rdwr demo driver; a small user space app
+ * to issue the read(2) and write(2) system calls upon a given (device) file.
+ * Also, again as a demo, we use the read(2) to retreive the 'secret' <eye-roll>
+ * from the driver within kernel-space. Equivalently, one can use the write(2)
+ * change the 'secret' (just plain text).
+ *
+ * For details, please refer the book, Ch 1.
+ * License: Dual MIT/GPL
+ */
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+#include <sys/types.h>
+#include <limits.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+#define MAXBYTES    128		/* Must match the driver; we should actually use a
+				 * common header file for things like this */
+static int stay_alive;
+
+static inline void usage(char *prg)
+{
+	fprintf(stderr,
+		"Usage: %s opt=read/write device_file [\"secret-msg\"]\n"
+		" opt = 'r' => we shall issue the read(2), retrieving the 'secret' form the driver\n"
+		" opt = 'w' => we shall issue the write(2), writing the secret message <secret-msg>\n"
+		"  (max %d bytes)\n", prg, MAXBYTES);
+}
+
+int main(int argc, char **argv)
+{
+	char opt = 'r';
+	int fd, flags = O_RDONLY;
+	ssize_t n;
+	char *buf = NULL;
+	size_t num = 0;
+
+	if (argc < 3) {
+		usage(argv[0]);
+		exit(EXIT_FAILURE);
+	}
+
+	opt = argv[1][0];
+	if (opt != 'r' && opt != 'w') {
+		usage(argv[0]);
+		exit(EXIT_FAILURE);
+	}
+	if ((opt == 'w' && argc != 4) || (opt == 'r' && argc != 3)) {
+		usage(argv[0]);
+		exit(EXIT_FAILURE);
+	}
+	if ('w' == opt && strnlen(argv[3], MAXBYTES) > MAXBYTES) {
+		fprintf(stderr, "%s: too big a secret (%zu bytes); pl restrict"
+			" to %d bytes max\n", argv[0], strnlen(argv[3],
+							       MAXBYTES),
+			MAXBYTES);
+		exit(EXIT_FAILURE);
+	}
+
+	if ('w' == opt)
+		flags = O_WRONLY;
+	fd = open(argv[2], flags, 0);
+	if (fd == -1) {
+		fprintf(stderr, "%s: open(2) on %s failed\n", argv[0], argv[2]);
+		perror("open");
+		exit(EXIT_FAILURE);
+	}
+	printf("Device file %s opened (in %s mode): fd=%d\n",
+	       argv[2], (flags == O_RDONLY ? "read-only" : "write-only"), fd);
+
+	if ('w' == opt) {
+		num = strnlen(argv[3], MAXBYTES) + 1;	// IMP! +1 to include the NULL byte!
+		if (num > MAXBYTES)
+			num = MAXBYTES;
+	} else
+		num = MAXBYTES;
+
+	buf = malloc(num);
+	if (!buf) {
+		fprintf(stderr, "%s: out of memory!\n", argv[0]);
+		close(fd);
+		exit(EXIT_FAILURE);
+	}
+
+	if ('r' == opt) {
+		n = read(fd, buf, num);
+		if (n < 0) {
+			perror("read failed");
+			fprintf(stderr, "Tip: see kernel log\n");
+			free(buf);
+			close(fd);
+			exit(EXIT_FAILURE);
+		}
+		printf("%s: read %zd bytes from %s\n", argv[0], n, argv[2]);
+		printf("The 'secret' is:\n \"%.*s\"\n", (int)n, buf);
+	} else {
+		strncpy(buf, argv[3], num);
+		n = write(fd, buf, num);
+		if (n < 0) {
+			perror("write failed");
+			fprintf(stderr, "Tip: see kernel log\n");
+			free(buf);
+			close(fd);
+			exit(EXIT_FAILURE);
+		}
+		printf("%s: wrote %zd bytes to %s\n", argv[0], n, argv[2]);
+	}
+
+	if (stay_alive) {
+		printf("%s:%d: stayin' alive (in pause()) ... \n", argv[0],
+			getpid());
+		pause();
+	}
+
+	free(buf);
+	close(fd);
+	exit(EXIT_SUCCESS);
+}