diff --git a/ulib.c b/ulib.c
index decab69..ce4e13d 100644
--- a/ulib.c
+++ b/ulib.c
@@ -88,3 +88,14 @@ stat(char *n, struct stat *st)
   close(fd);
   return r;
 }
+
+int
+atoi(const char *s)
+{
+  int n;
+
+  n = 0;
+  while('0' <= *s && *s <= '9')
+    n = n*10 + *s++ - '0';
+  return n;
+}
diff --git a/user.h b/user.h
index bda6e6c..1bd512d 100644
--- a/user.h
+++ b/user.h
@@ -31,3 +31,4 @@ unsigned int strlen(char*);
 void* memset(void*, int, unsigned int);
 void* malloc(uint);
 void free(void*);
+int atoi(const char*);