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CreateMailslot info  Overview  Group

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



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




Returns immediately if no message is present. (The system does not treat an immediate return as an error.)


Waits forever for a message.

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

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.


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

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:




Retrieves a client handle to a local mailslot.


Retrieves a client handle to a remote mailslot.


Retrieves a client handle to all mailslots with the specified name in the specified domain.


Retrieves a client handle to all mailslots with the specified name in the systemís primary domain.

If CreateFile specifies a domain or uses the asterisk format to specify the systemís 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

file: /Techref/os/win/api/win32/func/src/f10_2.htm, 7KB, , updated: 2000/4/7 12:19, local time: 2022/5/26 00:06,

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