Editing Mbox (section)

== File locking ==
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Because more than one messages are stored in a single file, some form of [[file locking]] is needed to avoid the corruption that can result from two or more processes modifying the mailbox simultaneously. This could happen if a network email delivery program delivers a new message at the same time as a mail reader is deleting an existing message.

Various mutually incompatible mechanisms have been used by different mbox formats to enable message file locking, including <code>fcntl()</code> and <code>lockf()</code>. This does not work well with network mounted file systems, such as the [[Network File System (protocol)|Network File System (NFS)]], which is why traditionally Unix used additional "dot lock" files, which could be created atomically even over NFS.

Mbox files should also be locked while they are being read. Otherwise, the reader may see corrupted message contents if another process is modifying the mbox at the same time, even though no actual file corruption occurs.