CreateMailslotA function (winbase.h)

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 CreateMailslotA(
  [in]           LPCSTR                lpName,
  [in]           DWORD                 nMaxMessageSize,
  [in]           DWORD                 lReadTimeout,
  [in, optional] LPSECURITY_ATTRIBUTES lpSecurityAttributes


[in] lpName

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 pseudo directories separated by backslashes. For example, both \\.\mailslot\example_mailslot_name and \\.\mailslot\abc\def\ghi are valid names.

[in] nMaxMessageSize

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

[in] lReadTimeout

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

Value Meaning
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.

[in, optional] lpSecurityAttributes

A pointer to a SECURITY_ATTRIBUTES structure. The bInheritHandle member of the structure determines whether the returned handle can be inherited by child processes. If lpSecurityAttributes is NULL, the handle cannot be inherited.

Return value

If the function succeeds, the return value is a handle to the mailslot, for use in server mailslot operations. The handle returned by this function is asynchronous, or overlapped.

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:

  • 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.
The system uses 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 424 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.

If CreateFile is called to access a non-existent mailslot, the ERROR_FILE_NOT_FOUND error code will be set.


For an example, see Creating a Mailslot.


The winbase.h header defines CreateMailslot as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winbase.h (include Windows.h)
Library Kernel32.lib
DLL Kernel32.dll

See also




Mailslot Functions

Mailslots Overview