Add a uc_backtrace_buf() functio

include/ucore/ucore_backtrace.h

@@ -2,7 +2,23 @@
 #define UC_BACKTRACE_H_
 
 #define UC_BACTRACE_STDERR uc_backtrace_fd(2)
+#include <stddef.h>
 
+/**
+ * Print a stacktrace to the given file descriptor.
+ *
+ * @param fd filedescriptor to write to.
+ */
 void uc_backtrace_fd(int fd);
+/**
+ * Print a stactrace to the given buffer
+ * (Note that unlike uc_backtrace_fd(), this function
+ * does malloc() call and may fail in a signal handler or
+ * if memory is corrupted.
+ *
+ * @param buf buffer to place the stacktrace in.
+ * @param len length of buffer
+ */
+void uc_backtrace_buf(char *buf, size_t len);
 
 #endif