add lab answers

labcodes_answer/lab7_result/kern/libs/readline.c

@@ -0,0 +1,50 @@
+#include <stdio.h>
+
+#define BUFSIZE 1024
+static char buf[BUFSIZE];
+
+/* *
+ * readline - get a line from stdin
+ * @prompt:     the string to be written to stdout
+ *
+ * The readline() function will write the input string @prompt to
+ * stdout first. If the @prompt is NULL or the empty string,
+ * no prompt is issued.
+ *
+ * This function will keep on reading characters and saving them to buffer
+ * 'buf' until '\n' or '\r' is encountered.
+ *
+ * Note that, if the length of string that will be read is longer than
+ * buffer size, the end of string will be discarded.
+ *
+ * The readline() function returns the text of the line read. If some errors
+ * are happened, NULL is returned. The return value is a global variable,
+ * thus it should be copied before it is used.
+ * */
+char *
+readline(const char *prompt) {
+    if (prompt != NULL) {
+        cprintf("%s", prompt);
+    }
+    int i = 0, c;
+    while (1) {
+        c = getchar();
+        if (c < 0) {
+            return NULL;
+        }
+        else if (c >= ' ' && i < BUFSIZE - 1) {
+            cputchar(c);
+            buf[i ++] = c;
+        }
+        else if (c == '\b' && i > 0) {
+            cputchar(c);
+            i --;
+        }
+        else if (c == '\n' || c == '\r') {
+            cputchar(c);
+            buf[i] = '\0';
+            return buf;
+        }
+    }
+}
+