MessageBox

The MessageBox function creates, displays, and operates a message box. The message box contains an application-defined message and title, plus any combination of predefined icons and push buttons.

int MessageBox(HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType);

• hWnd - Handle to the owner window of the message box to be created. If this parameter is NULL, the message box has no owner window.
• lpText - Pointer to a null-terminated string containing 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.
• lpCaption - Pointer to a null-terminated string used for the dialog box title. If this parameter is NULL, the default title “Error” is used.
• uType - Contains a set of bit flags that determine the content and behavior of the dialog box. This parameter can be a combination of flags from the following groups:

Buttons:

• MB_ABORTRETRYIGNORE - Contains Abort, Retry, and Ignore buttons.
• MB_OK - Contains one push button: OK (default).
• MB_OKCANCEL - Contains OK and Cancel buttons.
• MB_RETRYCANCEL - Contains Retry and Cancel buttons.
• MB_YESNO - Contains Yes and No buttons.
• MB_YESNOCANCEL - Contains Yes, No, and Cancel buttons.

Icons:

• MB_ICONEXCLAMATION / MB_ICONWARNING - Exclamation-point icon.
• MB_ICONINFORMATION / MB_ICONASTERISK - Information icon (i in a circle).
• MB_ICONQUESTION - Question-mark icon.
• MB_ICONSTOP / MB_ICONERROR / MB_ICONHAND - Stop-sign icon.

Default Button:

• MB_DEFBUTTON1 - First button is default (default).
• MB_DEFBUTTON2 - Second button is default.
• MB_DEFBUTTON3 - Third button is default.

Modality:

• MB_APPLMODAL - Application-modal (default). User must respond to the message box before continuing work in the current window.
• MB_SYSTEMMODAL - System-modal (topmost). All applications are suspended until user responds.
• MB_TASKMODAL - Task-modal. Similar to MB_APPLMODAL but used when the calling application has no window handle.

Other:

• MB_SETFOREGROUND - Message box becomes the foreground window.
• MB_TOPMOST - Message box is placed above all non-topmost windows.

The function returns zero if there is not enough memory to create the message box. If the call succeeds, the return value is one of the following menu-item values:

• IDABORT - Abort button was selected.
• IDCANCEL - Cancel button was selected.
• IDIGNORE - Ignore button was selected.
• IDNO - No button was selected.
• IDOK - OK button was selected.
• IDRETRY - Retry button was selected.
• IDYES - Yes button was selected.

• The system supports only a limited number of message boxes simultaneously.
• When using the MB_SYSTEMMODAL flag, the message box appears on top of all windows and is typically used for critical errors requiring immediate user attention.
• If a message box is created without an owner window (hWnd = NULL), it behaves as an independent window.
• The message box is modal and does not return control until the user closes it by pressing a button.

#include <windows.h>
 
int PASCAL WinMain(HANDLE hInstance, HANDLE hPrevInstance,
                   LPSTR lpCmdLine, int nCmdShow)
{
    // Simple information message box
    MessageBox(NULL,
               "Hello, world! This is a message from Janus.",
               "Win16 Example",
               MB_OK | MB_ICONINFORMATION);
 
    // Question message box with Yes/No/Cancel
    int result = MessageBox(NULL,
                            "Do you want to save changes?",
                            "Confirmation",
                            MB_YESNOCANCEL | MB_ICONQUESTION | MB_DEFBUTTON1);
 
    // Handle the result
    if (result == IDYES)
    {
        MessageBox(NULL, "You chose Yes", "Result", MB_OK | MB_ICONINFORMATION);
    }
    else if (result == IDNO)
    {
        MessageBox(NULL, "You chose No", "Result", MB_OK | MB_ICONINFORMATION);
    }
    else if (result == IDCANCEL)
    {
        MessageBox(NULL, "You chose Cancel", "Result", MB_OK | MB_ICONWARNING);
    }
 
    // Error message example
    MessageBox(GetActiveWindow(),
               "File not found. Please check the path.",
               "Error",
               MB_OK | MB_ICONSTOP | MB_SYSTEMMODAL);
 
    return 0;
}

; Example for MASM (Microsoft Macro Assembler)
; Assemble with: MASM /MX program.asm
; Link with: LINK program.obj,,,libw.lib,,
 
.MODEL LARGE
.386
 
INCLUDE windows.inc
 
.DATA
    msg_hello    DB "Hello, world!", 0
    cap_hello    DB "Greeting", 0
 
    msg_question DB "Continue operation?", 0
    cap_question DB "Question", 0
 
    msg_yes      DB "You pressed Yes", 0
    msg_no       DB "You pressed No", 0
    msg_cancel   DB "Operation cancelled", 0
    cap_result   DB "Result", 0
 
.CODE
START PROC FAR
    ; Initialize
    push    ds
    pop     ax
    xor     ax, ax
 
    ; Simple message box
    push    MB_OK OR MB_ICONINFORMATION    ; uType
    push    OFFSET cap_hello                 ; lpCaption
    push    OFFSET msg_hello                  ; lpText
    push    0                                  ; hWnd = NULL
    call    MessageBox
 
    ; Question message box
    push    MB_YESNOCANCEL OR MB_ICONQUESTION ; uType
    push    OFFSET cap_question                 ; lpCaption
    push    OFFSET msg_question                  ; lpText
    push    0                                      ; hWnd = NULL
    call    MessageBox
 
    ; Check result (result is in AX)
    cmp     ax, IDYES
    je      show_yes
    cmp     ax, IDNO
    je      show_no
    cmp     ax, IDCANCEL
    je      show_cancel
    jmp     exit_program
 
show_yes:
    push    MB_OK OR MB_ICONINFORMATION
    push    OFFSET cap_result
    push    OFFSET msg_yes
    push    0
    call    MessageBox
    jmp     exit_program
 
show_no:
    push    MB_OK OR MB_ICONINFORMATION
    push    OFFSET cap_result
    push    OFFSET msg_no
    push    0
    call    MessageBox
    jmp     exit_program
 
show_cancel:
    push    MB_OK OR MB_ICONWARNING
    push    OFFSET cap_result
    push    OFFSET msg_cancel
    push    0
    call    MessageBox
 
exit_program:
    ; Exit program
    mov     ax, 4C00h
    int     21h
START ENDP
 
END START

• MessageBoxIndirect
• MessageBeep
• DialogBox
• DialogBoxParam
• CreateDialog
• FlashWindow
• GetActiveWindow
• SetActiveWindow