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FormatMessage function

Formats a message string. The function requires a message definition as input. The message definition can come from a buffer passed into the function. It can come from a message table resource in an already-loaded module. Or the caller can ask the function to search the system's message table resource(s) for the message definition. The function finds the message definition in a message table resource based on a message identifier and a language identifier. The function copies the formatted message text to an output buffer, processing any embedded insert sequences if requested.

Syntax


DWORD WINAPI FormatMessage(
  _In_      DWORD dwFlags,
  _In_opt_  LPCVOID lpSource,
  _In_      DWORD dwMessageId,
  _In_      DWORD dwLanguageId,
  _Out_     LPTSTR lpBuffer,
  _In_      DWORD nSize,
  _In_opt_  va_list *Arguments
);

Parameters

dwFlags [in]

The formatting options, and how to interpret the lpSource parameter. The low-order byte of dwFlags specifies how the function handles line breaks in the output buffer. The low-order byte can also specify the maximum width of a formatted output line.

This parameter can be one or more of the following values.

ValueMeaning
FORMAT_MESSAGE_ALLOCATE_BUFFER
0x00000100

The function allocates a buffer large enough to hold the formatted message, and places a pointer to the allocated buffer at the address specified by lpBuffer. The lpBuffer parameter is a pointer to an LPTSTR; you must cast the pointer to an LPTSTR (for example, (LPTSTR)&lpBuffer). The nSize parameter specifies the minimum number of TCHARs to allocate for an output message buffer. The caller should use the LocalFree function to free the buffer when it is no longer needed.

If the length of the formatted message exceeds 128K bytes, then FormatMessage will fail and a subsequent call to GetLastError will return ERROR_MORE_DATA.

This value is not available for use when compiling Windows Store apps.

Windows Server 2003 and Windows XP:  

If the length of the formatted message exceeds 128K bytes, then FormatMessage will not automatically fail with an error of ERROR_MORE_DATA.

FORMAT_MESSAGE_ARGUMENT_ARRAY
0x00002000

The Arguments parameter is not a va_list structure, but is a pointer to an array of values that represent the arguments.

This flag cannot be used with 64-bit integer values. If you are using a 64-bit integer, you must use the va_list structure.

FORMAT_MESSAGE_FROM_HMODULE
0x00000800

The lpSource parameter is a module handle containing the message-table resource(s) to search. If this lpSource handle is NULL, the current process's application image file will be searched. This flag cannot be used with FORMAT_MESSAGE_FROM_STRING.

If the module has no message table resource, the function fails with ERROR_RESOURCE_TYPE_NOT_FOUND.

FORMAT_MESSAGE_FROM_STRING
0x00000400

The lpSource parameter is a pointer to a null-terminated string that contains a message definition. The message definition may contain insert sequences, just as the message text in a message table resource may. This flag cannot be used with FORMAT_MESSAGE_FROM_HMODULE or FORMAT_MESSAGE_FROM_SYSTEM.

FORMAT_MESSAGE_FROM_SYSTEM
0x00001000

The function should search the system message-table resource(s) for the requested message. If this flag is specified with FORMAT_MESSAGE_FROM_HMODULE, the function searches the system message table if the message is not found in the module specified by lpSource. This flag cannot be used with FORMAT_MESSAGE_FROM_STRING.

If this flag is specified, an application can pass the result of the GetLastError function to retrieve the message text for a system-defined error.

FORMAT_MESSAGE_IGNORE_INSERTS
0x00000200

Insert sequences in the message definition are to be ignored and passed through to the output buffer unchanged. This flag is useful for fetching a message for later formatting. If this flag is set, the Arguments parameter is ignored.

 

The low-order byte of dwFlags can specify the maximum width of a formatted output line. The following are possible values of the low-order byte.

ValueMeaning
0

There are no output line width restrictions. The function stores line breaks that are in the message definition text into the output buffer.

FORMAT_MESSAGE_MAX_WIDTH_MASK
0x000000FF

The function ignores regular line breaks in the message definition text. The function stores hard-coded line breaks in the message definition text into the output buffer. The function generates no new line breaks.

 

If the low-order byte is a nonzero value other than FORMAT_MESSAGE_MAX_WIDTH_MASK, it specifies the maximum number of characters in an output line. The function ignores regular line breaks in the message definition text. The function never splits a string delimited by white space across a line break. The function stores hard-coded line breaks in the message definition text into the output buffer. Hard-coded line breaks are coded with the %n escape sequence.

lpSource [in, optional]

The location of the message definition. The type of this parameter depends upon the settings in the dwFlags parameter.

dwFlags SettingMeaning
FORMAT_MESSAGE_FROM_HMODULE
0x00000800

A handle to the module that contains the message table to search.

FORMAT_MESSAGE_FROM_STRING
0x00000400

Pointer to a string that consists of unformatted message text. It will be scanned for inserts and formatted accordingly.

 

If neither of these flags is set in dwFlags, then lpSource is ignored.

dwMessageId [in]

The message identifier for the requested message. This parameter is ignored if dwFlags includes FORMAT_MESSAGE_FROM_STRING.

dwLanguageId [in]

The language identifier for the requested message. This parameter is ignored if dwFlags includes FORMAT_MESSAGE_FROM_STRING.

If you pass a specific LANGID in this parameter, FormatMessage will return a message for that LANGID only. If the function cannot find a message for that LANGID, it sets Last-Error to ERROR_RESOURCE_LANG_NOT_FOUND. If you pass in zero, FormatMessage looks for a message for LANGIDs in the following order:

  1. Language neutral
  2. Thread LANGID, based on the thread's locale value
  3. User default LANGID, based on the user's default locale value
  4. System default LANGID, based on the system default locale value
  5. US English

If FormatMessage does not locate a message for any of the preceding LANGIDs, it returns any language message string that is present. If that fails, it returns ERROR_RESOURCE_LANG_NOT_FOUND.

lpBuffer [out]

A pointer to a buffer that receives the null-terminated string that specifies the formatted message. If dwFlags includes FORMAT_MESSAGE_ALLOCATE_BUFFER, the function allocates a buffer using the LocalAlloc function, and places the pointer to the buffer at the address specified in lpBuffer.

This buffer cannot be larger than 64K bytes.

nSize [in]

If the FORMAT_MESSAGE_ALLOCATE_BUFFER flag is not set, this parameter specifies the size of the output buffer, in TCHARs. If FORMAT_MESSAGE_ALLOCATE_BUFFER is set, this parameter specifies the minimum number of TCHARs to allocate for an output buffer.

The output buffer cannot be larger than 64K bytes.

Arguments [in, optional]

An array of values that are used as insert values in the formatted message. A %1 in the format string indicates the first value in the Arguments array; a %2 indicates the second argument; and so on.

The interpretation of each value depends on the formatting information associated with the insert in the message definition. The default is to treat each value as a pointer to a null-terminated string.

By default, the Arguments parameter is of type va_list*, which is a language- and implementation-specific data type for describing a variable number of arguments. The state of the va_list argument is undefined upon return from the function. To use the va_list again, destroy the variable argument list pointer using va_end and reinitialize it with va_start.

If you do not have a pointer of type va_list*, then specify the FORMAT_MESSAGE_ARGUMENT_ARRAY flag and pass a pointer to an array of DWORD_PTR values; those values are input to the message formatted as the insert values. Each insert must have a corresponding element in the array.

Return value

If the function succeeds, the return value is the number of TCHARs stored in the output buffer, excluding the terminating null character.

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

Remarks

Within the message text, several escape sequences are supported for dynamically formatting the message. These escape sequences and their meanings are shown in the following tables. All escape sequences start with the percent character (%).

Escape sequenceMeaning
%0Terminates a message text line without a trailing new line character. This escape sequence can be used to build up long lines or to terminate the message itself without a trailing new line character. It is useful for prompt messages.
%n!format string!

Identifies an insert. The value of n can be in the range from 1 through 99. The format string (which must be surrounded by exclamation marks) is optional and defaults to !s! if not specified. For more information, see Format Specification Fields.

The format string can include a width and precision specifier for strings and a width specifier for integers. Use an asterisk (*) to specify the width and precision. For example, %1!*.*s! or %1!*u!.

If you do not use the width and precision specifiers, the insert numbers correspond directly to the input arguments. For example, if the source string is "%1 %2 %1" and the input arguments are "Bill" and "Bob", the formatted output string is "Bill Bob Bill".

However, if you use a width and precision specifier, the insert numbers do not correspond directly to the input arguments. For example, the insert numbers for the previous example could change to "%1!*.*s! %4 %5!*s!".

The insert numbers depend on whether you use an arguments array (FORMAT_MESSAGE_ARGUMENT_ARRAY) or a va_list. For an arguments array, the next insert number is n+2 if the previous format string contained one asterisk and is n+3 if two asterisks were specified. For a va_list, the next insert number is n+1 if the previous format string contained one asterisk and is n+2 if two asterisks were specified.

If you want to repeat "Bill", as in the previous example, the arguments must include "Bill" twice. For example, if the source string is "%1!*.*s! %4 %5!*s!", the arguments could be, 4, 2, Bill, Bob, 6, Bill (if using the FORMAT_MESSAGE_ARGUMENT_ARRAY flag). The formatted string would then be "  Bi Bob   Bill".

Repeating insert numbers when the source string contains width and precision specifiers may not yield the intended results. If you replaced %5 with %1, the function would try to print a string at address 6 (likely resulting in an access violation).

Floating-point format specifiers—e, E, f, and g—are not supported. The workaround is to use the StringCchPrintf function to format the floating-point number into a temporary buffer, then use that buffer as the insert string.

Inserts that use the I64 prefix are treated as two 32-bit arguments. They must be used before subsequent arguments are used. Note that it may be easier for you to use StringCchPrintf instead of this prefix.

 

Any other nondigit character following a percent character is formatted in the output message without the percent character. Following are some examples.

Format stringResulting output
%%A single percent sign.
%spaceA single space. This format string can be used to ensure the appropriate number of trailing spaces in a message text line.
%.A single period. This format string can be used to include a single period at the beginning of a line without terminating the message text definition.
%!A single exclamation point. This format string can be used to include an exclamation point immediately after an insert without its being mistaken for the beginning of a format string.
%nA hard line break when the format string occurs at the end of a line. This format string is useful when FormatMessage is supplying regular line breaks so the message fits in a certain width.
%rA hard carriage return without a trailing newline character.
%tA single tab.

 

Security Remarks

If this function is called without FORMAT_MESSAGE_IGNORE_INSERTS, the Arguments parameter must contain enough parameters to satisfy all insertion sequences in the message string, and they must be of the correct type. Therefore, do not use untrusted or unknown message strings with inserts enabled because they can contain more insertion sequences than Arguments provides, or those that may be of the wrong type. In particular, it is unsafe to take an arbitrary system error code returned from an API and use FORMAT_MESSAGE_FROM_SYSTEM without FORMAT_MESSAGE_IGNORE_INSERTS.

Windows Phone 8: This API is supported.

Windows Phone 8.1: This API is supported.

Examples

The FormatMessage function can be used to obtain error message strings for the system error codes returned by GetLastError. For an example, see Retrieving the Last-Error Code.

The following example shows how to use an argument array and the width and precision specifiers.


#ifndef UNICODE
#define UNICODE
#endif

#include <windows.h>
#include <stdio.h>

void main(void)
{
    LPWSTR pMessage = L"%1!*.*s! %4 %5!*s!";
    DWORD_PTR pArgs[] = { (DWORD_PTR)4, (DWORD_PTR)2, (DWORD_PTR)L"Bill",  // %1!*.*s! refers back to the first insertion string in pMessage
         (DWORD_PTR)L"Bob",                                                // %4 refers back to the second insertion string in pMessage
         (DWORD_PTR)6, (DWORD_PTR)L"Bill" };                               // %5!*s! refers back to the third insertion string in pMessage
    const DWORD size = 100+1;
    WCHAR buffer[size];


    if (!FormatMessage(FORMAT_MESSAGE_FROM_STRING | FORMAT_MESSAGE_ARGUMENT_ARRAY,
                       pMessage, 
                       0,
                       0,
                       buffer, 
                       size, 
                       (va_list*)pArgs))
    {
        wprintf(L"Format message failed with 0x%x\n", GetLastError());
        return;
    }

    // Buffer contains "  Bi Bob   Bill".
    wprintf(L"Formatted message: %s\n", buffer);
}



The following example shows how to implement the previous example using va_list.


#ifndef UNICODE
#define UNICODE
#endif

#include <windows.h>
#include <stdio.h>

LPWSTR GetFormattedMessage(LPWSTR pMessage, ...);

void main(void)
{
    LPWSTR pBuffer = NULL;
    LPWSTR pMessage = L"%1!*.*s! %3 %4!*s!";

    // The variable length arguments correspond directly to the format
    // strings in pMessage.
    pBuffer = GetFormattedMessage(pMessage, 4, 2, L"Bill", L"Bob", 6, L"Bill");
    if (pBuffer)
    {
        // Buffer contains "  Bi Bob   Bill".
        wprintf(L"Formatted message: %s\n", pBuffer);
        LocalFree(pBuffer);
    }
    else
    {
        wprintf(L"Format message failed with 0x%x\n", GetLastError());
    }
}

// Formats a message string using the specified message and variable
// list of arguments.
LPWSTR GetFormattedMessage(LPWSTR pMessage, ...)
{
    LPWSTR pBuffer = NULL;

    va_list args = NULL;
    va_start(args, pMessage);

    FormatMessage(FORMAT_MESSAGE_FROM_STRING |
                  FORMAT_MESSAGE_ALLOCATE_BUFFER,
                  pMessage, 
                  0,
                  0,
                  (LPWSTR)&pBuffer, 
                  0, 
                  &args);

    va_end(args);

    return pBuffer;
}


Requirements

Minimum supported client

Windows XP [desktop apps | Windows Store apps]

Minimum supported server

Windows Server 2003 [desktop apps | Windows Store apps]

Header

WinBase.h (include Windows.h)

Library

Kernel32.lib

DLL

Kernel32.dll

Unicode and ANSI names

FormatMessageW (Unicode) and FormatMessageA (ANSI)

See also

Error Handling Functions
Message Tables
Message Compiler
Message Tables

 

 

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