CreateMailslot

The CreateMailslot function creates a mailslot with the specified name and returns a handle that a mailslot server can use to perform operations on the mailslot. The mailslot is local to the computer that creates it. An error occurs if a mailslot with the specified name already exists.

HANDLE CreateMailslot(

LPCTSTR lpName,	// pointer to string for mailslot name
DWORD nMaxMessageSize,	// maximum message size
DWORD lReadTimeout,	// milliseconds before read time-out
LPSECURITY_ATTRIBUTES lpSecurityAttributes	// pointer to security structure
);

Parameters

lpName

Points to a null-terminated string specifying the name of the mailslot. This name must have the following form:

\\.\mailslot\[path]name

The name field must be unique. The name may include multiple levels of pseudodirectories separated by backslashes. For example, both \\.\mailslot\example_mailslot_name and \\.\mailslot\abc\def\ghi are valid names.

nMaxMessageSize

Specifies the maximum size, in bytes, of a single message that can be written to the mailslots. To specify that the message can be of any size, set this value to zero.

lReadTimeout

Specifies the amount of time, in milliseconds, a read operation can wait for a message to be written to the mailslot before a time-out occurs. The following values have special meanings:

Value	Meaning
0	Returns immediately if no message is present. (The system does not treat an immediate return as an error.)
MAILSLOT_WAIT_FOREVER	Waits forever for a message.

This time-out value applies to all subsequent read operations and all inherited mailslot handles.

lpSecurityAttributes

Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpSecurityAttributes is NULL, the handle cannot be inherited.

Windows NT: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new mailslot. If lpSecurityAttributes is NULL, the mailslot gets a default security descriptor.

Windows 95: The lpSecurityDescriptor member of the structure is ignored.

Return Values

If the function succeeds, the return value is a handle to the mailslot, for use in server mailslot operations.

If the function fails, the return value is INVALID_HANDLE_VALUE. To get extended error information, call GetLastError.

Remarks

The mailslot exists until one of the following conditions is true:

• The last (possibly inherited or duplicated) handle to it is closed using the CloseHandle function.

• The process owning the last (possibly inherited or duplicated) handle exits.

Both Windows NT and Windows 95 use the second method to destroy mailslots.

To write a message to a mailslot, a process uses the CreateFile function, specifying the mailslot name by using one of the following formats:

Format	Usage
\\.\mailslot\name	Retrieves a client handle to a local mailslot.
\\computername\mailslot\name	Retrieves a client handle to a remote mailslot.
\\domainname\mailslot\name	Retrieves a client handle to all mailslots with the specified name in the specified domain.
\\*\mailslot\name	Retrieves a client handle to all mailslots with the specified name in the system's primary domain.

If CreateFile specifies a domain or uses the asterisk format to specify the system's primary domain, the application cannot write more than 400 bytes at a time to the mailslot. If the application attempts to do so, the WriteFile function fails and GetLastError returns ERROR_BAD_NETPATH.

An application must specify the FILE_SHARE_READ flag when using CreateFile to retrieve a client handle to a mailslot.

See Also

CloseHandle, CreateFile, GetMailslotInfo, SECURITY_ATTRIBUTES, SetMailslotInfo, WriteFile

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