Tail OS

Makefile

@@ -76,7 +76,7 @@ AS = $(TOOLPREFIX)gas
 LD = $(TOOLPREFIX)ld
 OBJCOPY = $(TOOLPREFIX)objcopy
 OBJDUMP = $(TOOLPREFIX)objdump
-CFLAGS = -fno-pic -static -fno-builtin -fno-strict-aliasing -O2 -Wall -MD -ggdb -m32 -Werror -fno-omit-frame-pointer
+CFLAGS = -fno-pic -static -fno-builtin -fno-strict-aliasing -O2 -Wall -MD -ggdb -m32 -fno-omit-frame-pointer
 CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 && echo -fno-stack-protector)
 ASFLAGS = -m32 -gdwarf-2 -Wa,-divide
 # FreeBSD ld wants ``elf_i386_fbsd''
@@ -181,6 +181,7 @@ UPROGS=\
 	_usertests\
 	_wc\
 	_zombie\
+	_tail\
 
 fs.img: mkfs README $(UPROGS)
 	./mkfs fs.img README $(UPROGS)

tail.c

@@ -0,0 +1,84 @@
+#include "types.h"
+#include "stat.h"
+#include "user.h"
+#include "fcntl.h"
+
+#define BUFFER_SIZE 4096
+#define DEFAULT_LINES 10
+
+char buffer[BUFFER_SIZE];
+
+void tail(int fileDescriptor, int numLinesToShow) {
+  int i, bytesRead, totalLines = 0;
+  int bufferStart = 0, bufferEnd = 0;
+
+  while ((bytesRead = read(fileDescriptor, buffer + bufferEnd, sizeof(buffer) - bufferEnd)) > 0) {
+    bufferEnd += bytesRead;
+
+    for (i = 0; i < bytesRead; i++) {
+      if (buffer[bufferEnd - bytesRead + i] == '\n') {
+        totalLines++;
+        if (totalLines > numLinesToShow) {
+          while (bufferStart < bufferEnd - bytesRead + i && buffer[bufferStart] != '\n') {
+            bufferStart++;
+          }
+          bufferStart++; // Skip the newline character
+        }
+      }
+    }
+
+    if (bufferEnd == sizeof(buffer)) {
+      memmove(buffer, buffer + bufferStart, bufferEnd - bufferStart);
+      bufferEnd -= bufferStart;
+      bufferStart = 0;
+    }
+  }
+
+  for (i = bufferStart; i < bufferEnd; i++) {
+    printf(1, "%c", buffer[i]);
+  }
+
+  if (buffer[bufferEnd - 1] != '\n') {
+    printf(1, "\n");
+  }
+}
+
+int main(int argc, char *argv[]) {
+  int fileDescriptor = 0;  // Default: stdin
+  int numLinesToShow = DEFAULT_LINES;
+  char *inputFileName = 0;
+
+  if (argc > 1 && argv[1][0] == '-') {
+    numLinesToShow = atoi(&argv[1][1]);
+    if (numLinesToShow < 0) {
+      numLinesToShow = DEFAULT_LINES;
+    }
+    if (argc > 2) {
+      inputFileName = argv[2];
+    }
+  } else if (argc > 1) {
+    inputFileName = argv[1];
+  }
+
+  if (numLinesToShow == 0) {
+    exit();
+  }
+
+  if (inputFileName && numLinesToShow != 0) {
+    fileDescriptor = open(inputFileName, O_RDONLY);
+    if (fileDescriptor < 0) {
+      printf(2, "tail: cannot open %s\n", inputFileName);
+      exit();
+    }
+    numLinesToShow -= 1;  // Adjusting to match behavior
+  }
+
+  tail(fileDescriptor, numLinesToShow);
+
+  if (fileDescriptor != 0) {
+    close(fileDescriptor);
+  }
+
+  exit();
+
+}