MessageBoxA function (winuser.h)

Displays a modal dialog box that contains a system icon, a set of buttons, and a brief application-specific message, such as status or error information. The message box returns an integer value that indicates which button the user clicked.

Syntax

int MessageBoxA(
  [in, optional] HWND   hWnd,
  [in, optional] LPCSTR lpText,
  [in, optional] LPCSTR lpCaption,
  [in]           UINT   uType
);

Parameters

[in, optional] hWnd

Type: HWND

A handle to the owner window of the message box to be created. If this parameter is NULL, the message box has no owner window.

[in, optional] lpText

Type: LPCTSTR

The message to be displayed. If the string consists of more than one line, you can separate the lines using a carriage return and/or linefeed character between each line.

[in, optional] lpCaption

Type: LPCTSTR

The dialog box title. If this parameter is NULL, the default title is Error.

[in] uType

Type: UINT

The contents and behavior of the dialog box. This parameter can be a combination of flags from the following groups of flags.

To indicate the buttons displayed in the message box, specify one of the following values.

Value Meaning
MB_ABORTRETRYIGNORE
0x00000002L
The message box contains three push buttons: Abort, Retry, and Ignore.
MB_CANCELTRYCONTINUE
0x00000006L
The message box contains three push buttons: Cancel, Try Again, Continue. Use this message box type instead of MB_ABORTRETRYIGNORE.
MB_HELP
0x00004000L
Adds a Help button to the message box. When the user clicks the Help button or presses F1, the system sends a WM_HELP message to the owner.
MB_OK
0x00000000L
The message box contains one push button: OK. This is the default.
MB_OKCANCEL
0x00000001L
The message box contains two push buttons: OK and Cancel.
MB_RETRYCANCEL
0x00000005L
The message box contains two push buttons: Retry and Cancel.
MB_YESNO
0x00000004L
The message box contains two push buttons: Yes and No.
MB_YESNOCANCEL
0x00000003L
The message box contains three push buttons: Yes, No, and Cancel.
 

To display an icon in the message box, specify one of the following values.

Value Meaning
MB_ICONEXCLAMATION
0x00000030L
An exclamation-point icon appears in the message box.
MB_ICONWARNING
0x00000030L
An exclamation-point icon appears in the message box.
MB_ICONINFORMATION
0x00000040L
An icon consisting of a lowercase letter i in a circle appears in the message box.
MB_ICONASTERISK
0x00000040L
An icon consisting of a lowercase letter i in a circle appears in the message box.
MB_ICONQUESTION
0x00000020L
A question-mark icon appears in the message box. The question-mark message icon is no longer recommended because it does not clearly represent a specific type of message and because the phrasing of a message as a question could apply to any message type. In addition, users can confuse the message symbol question mark with Help information. Therefore, do not use this question mark message symbol in your message boxes. The system continues to support its inclusion only for backward compatibility.
MB_ICONSTOP
0x00000010L
A stop-sign icon appears in the message box.
MB_ICONERROR
0x00000010L
A stop-sign icon appears in the message box.
MB_ICONHAND
0x00000010L
A stop-sign icon appears in the message box.
 

To indicate the default button, specify one of the following values.

Value Meaning
MB_DEFBUTTON1
0x00000000L
The first button is the default button.

MB_DEFBUTTON1 is the default unless MB_DEFBUTTON2, MB_DEFBUTTON3, or MB_DEFBUTTON4 is specified.

MB_DEFBUTTON2
0x00000100L
The second button is the default button.
MB_DEFBUTTON3
0x00000200L
The third button is the default button.
MB_DEFBUTTON4
0x00000300L
The fourth button is the default button.
 

To indicate the modality of the dialog box, specify one of the following values.

Value Meaning
MB_APPLMODAL
0x00000000L
The user must respond to the message box before continuing work in the window identified by the hWnd parameter. However, the user can move to the windows of other threads and work in those windows.

Depending on the hierarchy of windows in the application, the user may be able to move to other windows within the thread. All child windows of the parent of the message box are automatically disabled, but pop-up windows are not.

MB_APPLMODAL is the default if neither MB_SYSTEMMODAL nor MB_TASKMODAL is specified.

MB_SYSTEMMODAL
0x00001000L
Same as MB_APPLMODAL except that the message box has the WS_EX_TOPMOST style. Use system-modal message boxes to notify the user of serious, potentially damaging errors that require immediate attention (for example, running out of memory). This flag has no effect on the user's ability to interact with windows other than those associated with hWnd.
MB_TASKMODAL
0x00002000L
Same as MB_APPLMODAL except that all the top-level windows belonging to the current thread are disabled if the hWnd parameter is NULL. Use this flag when the calling application or library does not have a window handle available but still needs to prevent input to other windows in the calling thread without suspending other threads.
 

To specify other options, use one or more of the following values.

Value Meaning
MB_DEFAULT_DESKTOP_ONLY
0x00020000L
Same as desktop of the interactive window station. For more information, see Window Stations.

If the current input desktop is not the default desktop, MessageBox does not return until the user switches to the default desktop.

MB_RIGHT
0x00080000L
The text is right-justified.
MB_RTLREADING
0x00100000L
Displays message and caption text using right-to-left reading order on Hebrew and Arabic systems.
MB_SETFOREGROUND
0x00010000L
The message box becomes the foreground window. Internally, the system calls the SetForegroundWindow function for the message box.
MB_TOPMOST
0x00040000L
The message box is created with the WS_EX_TOPMOST window style.
MB_SERVICE_NOTIFICATION
0x00200000L
The caller is a service notifying the user of an event. The function displays a message box on the current active desktop, even if there is no user logged on to the computer.

Terminal Services: If the calling thread has an impersonation token, the function directs the message box to the session specified in the impersonation token.

If this flag is set, the hWnd parameter must be NULL. This is so that the message box can appear on a desktop other than the desktop corresponding to the hWnd.

For information on security considerations in regard to using this flag, see Interactive Services. In particular, be aware that this flag can produce interactive content on a locked desktop and should therefore be used for only a very limited set of scenarios, such as resource exhaustion.

Return value

Type: int

If a message box has a Cancel button, the function returns the IDCANCEL value if either the ESC key is pressed or the Cancel button is selected. If the message box has no Cancel button, pressing ESC will no effect - unless an MB_OK button is present. If an MB_OK button is displayed and the user presses ESC, the return value will be IDOK.

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

If the function succeeds, the return value is one of the following menu-item values.

Return code/value Description
IDABORT
3
The Abort button was selected.
IDCANCEL
2
The Cancel button was selected.
IDCONTINUE
11
The Continue button was selected.
IDIGNORE
5
The Ignore button was selected.
IDNO
7
The No button was selected.
IDOK
1
The OK button was selected.
IDRETRY
4
The Retry button was selected.
IDTRYAGAIN
10
The Try Again button was selected.
IDYES
6
The Yes button was selected.

Remarks

The following system icons can be used in a message box by setting the uType parameter to the corresponding flag value.

Icon Flag values
Icon for MB_ICONHAND, MB_ICONSTOP, and MB_ICONERROR MB_ICONHAND, MB_ICONSTOP, or MB_ICONERROR
Icon for MB_ICONQUESTION MB_ICONQUESTION
Icon for MB_ICONEXCLAMATION and MB_ICONWARNING MB_ICONEXCLAMATION or MB_ICONWARNING
Icon for MB_ICONASTERISK and MB_ICONINFORMATION MB_ICONASTERISK or MB_ICONINFORMATION
 

Adding two right-to-left marks (RLMs), represented by Unicode formatting character U+200F, in the beginning of a MessageBox display string is interpreted by the MessageBox rendering engine so as to cause the reading order of the MessageBox to be rendered as right-to-left (RTL).

When you use a system-modal message box to indicate that the system is low on memory, the strings pointed to by the lpText and lpCaption parameters should not be taken from a resource file because an attempt to load the resource may fail.

If you create a message box while a dialog box is present, use a handle to the dialog box as the hWnd parameter. The hWnd parameter should not identify a child window, such as a control in a dialog box.

Examples

In the following example, the application displays a message box that prompts the user for an action after an error condition has occurred. The message box displays the message that describes the error condition and how to resolve it. The MB_CANCELTRYCONTINUE style directs MessageBox to provide three buttons with which the user can choose how to proceed. The MB_DEFBUTTON2 style sets the default focus on the second button of the message box, in this case, the Try Again button.

int DisplayResourceNAMessageBox()
{
    int msgboxID = MessageBox(
        NULL,
        (LPCWSTR)L"Resource not available\nDo you want to try again?",
        (LPCWSTR)L"Account Details",
        MB_ICONWARNING | MB_CANCELTRYCONTINUE | MB_DEFBUTTON2
    );

    switch (msgboxID)
    {
    case IDCANCEL:
        // TODO: add code
        break;
    case IDTRYAGAIN:
        // TODO: add code
        break;
    case IDCONTINUE:
        // TODO: add code
        break;
    }

    return msgboxID;
}

The following image shows the output from the preceding code example:

Message box

For another message box example, see Displaying a Message Box.

Note

The winuser.h header defines MessageBox 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.

Requirements

Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winuser.h (include Windows.h)
Library User32.lib
DLL User32.dll
API set ext-ms-win-ntuser-dialogbox-l1-1-0 (introduced in Windows 8)

See also

Conceptual

Dialog Boxes

FlashWindow

MessageBeep

MessageBoxEx

MessageBoxIndirect

Other Resources

Reference

SetForegroundWindow