(Since Linux 2.6.23) Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl F_SETFD operations to set the FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate fcntl F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor at the same time as another thread does a fork plus execve.