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Using maildir format

Why should I use maildir?

Two words:no locks.An MUA can read and delete messages while new mail is being delivered:each message is stored in a separate file with a unique name,so it isn't affected by operations on other messages.An MUA doesn't have to worry about partially delivered mail:each message is safely written to disk in thetmp subdirectorybefore it is moved tonew.The maildir format is reliable even over NFS.

How are unique names created?

Unless you're writing messages to a maildir,the format of a unique name is none of your business.A unique name can be anythingthat doesn't contain a colon (or slash)and doesn't start with a dot.Do not try to extract information from unique names.

Okay, so you're writing messages.A unique name has three pieces, separated by dots.On the left is the result of time() or the second counter from gettimeofday().On the right is the result of gethostname().(To deal with invalid host names,replace / with \057 and : with \072.)In the middle is a delivery identifier, discussed below.

As the terminology suggests,every delivery to this maildir must have its own unique name.When a maildir is shared through NFS,every machine that delivers to the maildirmust have its own hostname.Within one machine,every delivery within the same secondmust have a different delivery identifier.

Modern delivery identifiers are created by concatenatingenough of the following strings to guarantee uniqueness:

#n,wheren is (in hexadecimal) the output ofthe operating system's unix_sequencenumber() system call,which returns a number that increases by 1 every time it is called,starting from 0 after reboot.
Xn,wheren is (in hexadecimal) the output ofthe operating system's unix_bootnumber() system call,which reports the number of times that the system has been booted.Together with #, this guarantees uniqueness;unfortunately, most operating systems don't supportunix_sequencenumber() and unix_bootnumber.
Rn,wheren is (in hexadecimal) the output ofthe operating system's unix_cryptorandomnumber() system call,or an equivalent source such as /dev/urandom.Unfortunately,some operating systems don't include cryptographic random number generators.
In,wheren is (in hexadecimal) the UNIX inode number of this file.Unfortunately, inode numbers aren't always available through NFS.
Vn,wheren is (in hexadecimal) the UNIX device number of this file.Unfortunately, device numbers aren't always available through NFS.(Device numbers are also not helpful with the standard UNIX filesystem:a maildir has to be within a single UNIX devicefor link() and rename() to work.)
Mn,wheren is (in decimal) the microsecond counterfrom the same gettimeofday() used for the left part of the unique name.
Pn,wheren is (in decimal) the process ID.
Qn,wheren is (in decimal) the number of deliveries made by this process.

Old-fashioned delivery identifiers use the following formats:

n, wheren is the process ID,and where this process has been forked to make one delivery.Unfortunately,some foolish operating systems repeat process IDs quickly,breaking the standard time+pid combination.
n_m, wheren is the process IDandm is the number of deliveries made by this process.

What can I put in info?

When you move a file fromnew tocur,you have to change its name fromuniq touniq:info.Make sure to preserve theuniq string,so that separate messages can't bump into each other.

info is morally equivalent to the Status field used by mbox readers.It'd be useful to have MUAs agree on the meaning ofinfo,so I'm keeping a list ofinfo semantics.Here it is.

info starting with "1,": Experimental semantics.

info starting with "2,":Each character after the comma is an independent flag.

Flag "P" (passed): the user has resent/forwarded/bounced this messageto someone else.
Flag "R" (replied): the user has replied to this message.
Flag "S" (seen): the user has viewed this message, though perhapshe didn't read all the way through it.
Flag "T" (trashed): the user has moved this message to the trash;the trash will be emptied by a later user action.
Flag "D" (draft): the user considers this message a draft;toggled at user discretion.
Flag "F" (flagged): user-defined flag; toggled at user discretion.

New flags may be defined later.Flags must be stored in ASCII order: e.g., "2,FRS".

Can a maildir contain more than tmp, new, cur?

Yes:

.qmail: used to do direct deliveries with qmail-local.
bulletintime: empty file, used by system-wide bulletin programs.
bulletinlock: empty file, used by system-wide bulletin programs.
seriallock: empty file, used to serialize AutoTURN.

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