terminate called after throwing an instance of 'std::length_error

主要原因：对一个空指针进行操作。

例如：

char* p = NULL:
 string str(p);              //运行时报错
 
     FILE* install_log = fopen_path(install_file, "w");
     if (install_log) {
         fputs(path, install_log);
         fputc('\n', install_log);
     } else {
         LOGE("failed to open last_install: %s\n", strerror(errno));
     }
     fclose(install_log); return;