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In certain situations it would be desirable to log to an existing file descriptor, the most common case being a pipe between containers or processes. The main issue with pipes is that using write() on them will randomly truncate messages. But there is a trick. By using writev(), we can atomically deliver or drop a message, which perfectly fits the purpose. The only caveat is that large messages (4096 bytes on modern operating systems) may be interleaved with messages from other processes if using nbproc for example. In practice such messages are rare and most of the time when users need such type of logging, the load is low enough for a single process to be running so this is not really a problem. This logging method thus uses unbuffered writev() calls and is uses more CPU than if it used its own buffer with large writes at once, though this is not a problem for moderate loads. Logging to a file descriptor attached to a file also works with the side effect that the process is significantly slowed down during disk accesses and that it's not possible to rotate the file without restarting the process. For this reason this option is not offered as a configuration option, since it would confuse most users, but one could decide to redirect haproxy's output to a file during debugging sessions. Two aliases "stdout" and "stderr" are provided, but keep in mind that these are closed by default in daemon mode. When logging to a pipe or socket at a high enough rate, some logs will be dropped and the number of dropped messages is reported in "show info".