Why is it dangerous?
The select system call can be used to check if a file descriptor (usually a socket) is ready for reading or writing. The function accepts as an input three sets, each one containing the file descriptors of the entities (pipes, sockets, ...) to test for readability, writability or an error state.The largest file descriptor that can be stored in these sets is limited by the FD_SETSIZE constant, that is usually equal to 1024. If a file descriptor having a value larger than FD_SETSIZE is "stored" in one of such arrays, the effects is undetermined: but generally that operation results in an invalid memory access.
So if your program can potentially use many file descriptors, it is possible that select and its sets will make it fail. The remainder of this blog shows how to replace select with poll, that is not limited by the value of the file descriptors it can handle.
