Solve Exercise 8-1: `cat` remastered

1 files changed, 115 insertions, 0 deletions

diff --git a/ch8/8-01_cat-remastered.c b/ch8/8-01_cat-remastered.c
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+++ b/ch8/8-01_cat-remastered.c
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+#include <stdio.h> /* printf and friends */
+#include <fcntl.h> /* open, creat, etc */
+#include <unistd.h> /* read, close */
+#include <stdarg.h> /* va_* */
+#include <stdlib.h> /* exit */
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 8-1: Rewrite the program `cat` from Chapter 7 using read(),
+ * write(), open(), and close() instead of their standard library
+ * equivalents. Perferm experiments to determine the relative speeds of
+ * the two versions.
+ *
+ * Notes: The `cat` program can be found on pp 162-163 in the book.
+ */
+
+#define DEBUG 0
+
+char buf;
+/* Let's have fewer magic numbers */
+enum {
+	STDIN,
+	STDOUT,
+	STDERR
+};
+
+#if DEBUG == 1
+/* d_printf - output a debug message
+ * This was shamelessly taken from the book
+ */
+void d_printf(char *fmt, ...) {
+	va_list args;
+	va_start(args, fmt);
+	fprintf(stderr, "debug: ");
+	vfprintf(stderr, fmt, args);
+	fprintf(stderr, "\n");
+	va_end(args);
+}
+#endif
+
+void in2out() {
+	int status;
+	while ((status = read(STDIN, &buf, 1))) {
+		if (status != -1) {
+			status = write(STDOUT, &buf, 1);
+			if (status == -1) {
+				fprintf(stderr, "error: Could not write to stdout; something is very wrong.\n");
+				exit(1);
+			}
+		} else {
+			fprintf(stderr, "error: Could not read stdin; something is very wrong.\n");
+		}
+	}
+}
+
+int main(int argc, char **argv) {
+	int fd;
+	/* Remember the name we were invoked as, for messages */
+	ssize_t fstatus;
+	argv++;
+#if DEBUG == 1
+	char *prog = *argv;
+	d_printf("Starting %s", prog);
+#endif
+	if (argc == 1) {
+#if DEBUG == 1
+		d_printf("No arguments specified; copying stdin to stdout");
+#endif
+		in2out();
+		argv++;
+		exit(0);
+	}
+	/* The trick to understanding this is "x--" will run one more time than
+	 * expected, because it doesn't decrement until after it's been evaluated
+	 * for use in the expression. You can also look at it as "we've already
+	 * pushed argv ahead by one".
+	 */
+	while (argc-- > 1) {
+#if DEBUG == 1
+		d_printf("Trying to open %s", *argv);
+#endif
+		fd = open(*argv, O_RDONLY, 0);
+		if (fd != -1) {
+#if DEBUG == 1
+			d_printf("It was successfully opened. Reading.");
+#endif
+			while ((fstatus = read(fd, &buf, 1))) {
+				if (fstatus == (-1)) {
+					/* Fail early, fail often */
+					fprintf(stderr, "error: Cannot read from '%s'. \n", *argv);
+					close(fd);
+					return 1;
+				}
+				if ((fstatus = write(STDOUT, &buf, 1)) == (-1)) {
+					fprintf(stderr, "error: Cannot write to stdout.\n");
+					close(fd);
+					return 1;
+				}
+			}
+#if DEBUG == 1
+			d_printf("%s read complete.", *argv);
+#endif
+		} else {
+			fprintf(stderr, "error: Could not open file '%s' for reading.\n", *argv);
+			close(fd);
+			return 1;
+		}
+#if DEBUG == 1
+		d_printf("Closing file %s.", *argv);
+#endif
+		close(fd);
+		argv++;
+	}
+	exit(0);
+}