Strange descriptor closing in some linux programs

Question

While stracing some linux daemons (eg. sendmail) I noticed that some of them will call close() on a number of descriptors (usually ranging from 3 to 255) right at the beginning. Is this being done on purpose or is this some sort of a side effect of doing something else?

Answer 1

It is usually done as part of making a process a daemon.

All file descriptors are closed so that the long-running daemon does not unnecessarily hold any resources. For example, if a daemon were to inherit an open file and the daemon did not close it then the file could not be deleted (the storage for it would remain allocated until close) and the filesystem that the file is on could not be unmounted.

Daemonizing a process will also take a number of other actions, but those actions are beyond the scope of this question.