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copied!<p>Select cannot safely be used with programs that have more than 1024 file descriptors on a Linux system. This is because the underlying fd_set that the select system call uses is a fixed sized buffer i.e. its size is allocated at compile time, not run time.</p> <p>From man 2 select:</p> <pre><code>int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); </code></pre> <blockquote> <p>An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET() with a value of fd that is negative or is equal to or larger than FD_SETSIZE will result in undefined behavior. Moreover, POSIX requires fd to be a valid file descriptor.</p> </blockquote> <p>This means that if you have more than 1024 file descriptors in your program, and you use the select system call, you will end up with memory corruption.</p> <p>If you want to use more than 1024 file descriptors in your program, you <em>must</em> use poll or epoll, and ensure that you never use select, or you will get random memory corruption. Changing the size of the file descriptor table through <code>ulimit</code> is very dangerous if you are using <code>select</code>. Don't do it.</p> <p>Ruby's select does seem to be actually implemented with the select system call, so while it may look like increasing <code>ulimit</code> works, under the hood corruption is happening: <a href="https://github.com/ruby/ruby/blob/trunk/thread.c" rel="nofollow">https://github.com/ruby/ruby/blob/trunk/thread.c</a></p> <p>Furthermore, some unrelated API's in ruby seem to use select (see thread_pthread.c) so it's probably also unsafe to use those, or any code that uses those API's within a ruby program running with a file descriptor table larger than 1024.</p>

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