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_malloca

Allocates memory on the stack. This is a version of _alloca with security enhancements as described in Security Features in the CRT.

void *_malloca( 
   size_t size 
);
size

Bytes to be allocated from the stack.

The _malloca routine returns a void pointer to the allocated space, which is guaranteed to be suitably aligned for storage of any type of object. If size is 0, _malloca allocates a zero-length item and returns a valid pointer to that item.

A stack overflow exception is generated if the space cannot be allocated. The stack overflow exception is not a C++ exception; it is a structured exception. Instead of using C++ exception handling, you must use Structured Exception Handling (SEH).

_malloca allocates size bytes from the program stack or the heap if the request exceeds a certain size in bytes given by _ALLOCA_S_THRESHOLD. The difference between _malloca and _alloca is that _alloca always allocates on the stack, regardless of the size. Unlike _alloca, which does not require or permit a call to free to free the memory so allocated, _malloca requires the use of _freea to free memory. In debug mode, _malloca always allocates memory from the heap.

There are restrictions to explicitly calling _malloca in an exception handler (EH). EH routines that run on x86-class processors operate in their own memory frame: They perform their tasks in memory space that is not based on the current location of the stack pointer of the enclosing function. The most common implementations include Windows NT structured exception handling (SEH) and C++ catch clause expressions. Therefore, explicitly calling _malloca in any of the following scenarios results in program failure during the return to the calling EH routine:

  • Windows NT SEH exception filter expression: __except (_malloca () )

  • Windows NT SEH final exception handler: __finally {_malloca () }

  • C++ EH catch clause expression

However, _malloca can be called directly from within an EH routine or from an application-supplied callback that gets invoked by one of the EH scenarios previously listed.

Security note Security Note

In Windows XP, if _malloca is called inside a try/catch block, you must call _resetstkoflw in the catch block.

In addition to the above restrictions, when using the /clr (Common Language Runtime Compilation) option, _malloca cannot be used in __except blocks. For more information, see /clr Restrictions.

Routine

Required header

_malloca

<malloc.h>

// crt_malloca_simple.c
#include <stdio.h>
#include <malloc.h>

void Fn()
{
   char * buf = (char *)_malloca( 100 );
   // do something with buf
   _freea( buf );
}

int main()
{
   Fn();
}
// crt_malloca_exception.c
// This program demonstrates the use of
// _malloca and trapping any exceptions
// that may occur.

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

int main()
{
    int     size;
    int     numberRead = 0;
    int     errcode = 0;
    void    *p = NULL;
    void    *pMarker = NULL;

    while (numberRead == 0)
    {
        printf_s("Enter the number of bytes to allocate "
                 "using _malloca: ");
        numberRead = scanf_s("%d", &size);
    }

    // Do not use try/catch for _malloca,
    // use __try/__except, since _malloca throws
    // Structured Exceptions, not C++ exceptions.

    __try
    {
        if (size > 0)
        {
            p =  _malloca( size );
        }
        else
        {
            printf_s("Size must be a positive number.");
        }
        _freea( p );
    }

    // Catch any exceptions that may occur.
    __except( GetExceptionCode() == STATUS_STACK_OVERFLOW )
    {
        printf_s("_malloca failed!\n");

        // If the stack overflows, use this function to restore.
        errcode = _resetstkoflw();
        if (errcode)
        {
            printf("Could not reset the stack!");
            _exit(1);
        }
    };
}
Enter the number of bytes to allocate using _malloca: 1000

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

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