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assert macro, _assert, _wassert

Evaluates an expression and, when the result is false, prints a diagnostic message and aborts the program.


void _assert(
   char const* message,
   char const* filename,
   unsigned line
void _wassert(
   wchar_t const* message,
   wchar_t const* filename,
   unsigned line


A scalar expression (including pointer expressions) that evaluates to nonzero (true) or 0 (false).

The message to display.

The name of the source file the assertion failed in.

The line number in the source file of the failed assertion.


The assert macro is typically used to identify logic errors during program development. Use it to stop program execution when unexpected conditions occur by implementing the expression argument to evaluate to false only when the program is operating incorrectly. Assertion checks can be turned off at compile time by defining the macro NDEBUG. You can turn off the assert macro without modifying your source files by using a /DNDEBUG command-line option. You can turn off the assert macro in your source code by using a #define NDEBUG directive before <assert.h> is included.

The assert macro prints a diagnostic message when expression evaluates to false (0) and calls abort to stop program execution. No action is taken if expression is true (nonzero). The diagnostic message includes the failed expression, the name of the source file and line number where the assertion failed.

The diagnostic message is printed in wide (wchar_t) characters. Therefore, it will work as expected even if there are Unicode characters in the expression.

The destination of the diagnostic message depends on the type of application that called the routine. Console applications receive the message through stderr. In a Windows-based application, assert calls the Windows MessageBox function to create a message box to display the message with three buttons: Abort, Retry, and Ignore. If the user chooses Abort, the program aborts immediately. If the user chooses Retry, the debugger is called, and the user can debug the program if just-in-time (JIT) debugging is enabled. If the user chooses Ignore, the program will continue with normal execution. Clicking Ignore when an error condition exists can result in undefined behavior since preconditions of the calling code weren't met.

To override the default output behavior regardless of the app type, call _set_error_mode to select between the output-to-stderr and display-dialog-box behavior.

After assert displays its message, it calls abort, which displays a dialog box with Abort, Retry, and Ignore buttons. abort exits the program, so the Retry and Ignore button won't resume program execution following the assert call. If assert displayed a dialog box, the abort dialog box isn't shown. The only time the abort dialog box is shown, is when assert sends its output to stderr.

As a consequence of the above behavior, a dialog box is always displayed following an assert call in debug mode. The behavior of each button is captured in the below table.

Error mode Output to stderr (Console/_OUT_TO_STDERR) Display Dialog Box (Windows/_OUT_TO_MSGBOX)
Abort Exit immediately with exit code 3 Exit immediately with exit code 3
Retry Break into debugger during abort Break into debugger during assert
Ignore Finish exiting via abort Continue program as though assert didn't fire (may result in undefined behavior since preconditions of the calling code weren't met)

For more information about CRT debugging, see CRT debugging techniques.

The _assert and _wassert functions are internal CRT functions. They help minimize the code required in your object files to support assertions. We don't recommend that you call these functions directly.

The assert macro is enabled in both the release and debug versions of the C run-time libraries when NDEBUG isn't defined. When NDEBUG is defined, the macro is available, but doesn't evaluate its argument and has no effect. When it's enabled, the assert macro calls _wassert for its implementation. Other assertion macros, _ASSERT, _ASSERTE and _ASSERT_EXPR, are also available, but they only evaluate the expressions passed to them when the _DEBUG macro has been defined and when they are in code linked with the debug version of the C run-time libraries.


Routine Required header
assert, _wassert <assert.h>

The signature of the _assert function isn't available in a header file. The signature of the _wassert function is only available when the NDEBUG macro isn't defined.


In this program, the analyze_string function uses the assert macro to test several conditions related to string and length. If any of the conditions fails, the program prints a message indicating what caused the failure.

// crt_assert.c
// compile by using: cl /W4 crt_assert.c
#include <stdio.h>
#include <assert.h>
#include <string.h>

void analyze_string( char *string );   // Prototype

int main( void )
   char  test1[] = "abc", *test2 = NULL, test3[] = "";

   printf ( "Analyzing string '%s'\n", test1 ); fflush( stdout );
   analyze_string( test1 );
   printf ( "Analyzing string '%s'\n", test2 ); fflush( stdout );
   analyze_string( test2 );
   printf ( "Analyzing string '%s'\n", test3 ); fflush( stdout );
   analyze_string( test3 );

// Tests a string to see if it is NULL,
// empty, or longer than 0 characters.
void analyze_string( char * string )
   assert( string != NULL );        // Cannot be NULL
   assert( *string != '\0' );       // Cannot be empty
   assert( strlen( string ) > 2 );  // Length must exceed 2

The program generates this output:

Analyzing string 'abc'
Analyzing string '(null)'
Assertion failed: string != NULL, file crt_assert.c, line 25

After the assertion failure, depending on the version of the operating system and run-time library, you may see a message box that contains something similar to:

A problem caused the program to stop working correctly. Windows will close the program and notify you if a solution is available.

If a debugger is installed, choose the Debug button to start the debugger, or Close program to exit.

See also

Error handling
Process and environment control