_malloca

2011-07-25

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 
);

Parameters

size
Bytes to be allocated from the stack.

Return Value

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).

Remarks

_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
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.

Requirements

Routine	Required header
_malloca	<malloc.h>

Example

// 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);
        }
    };
}

Input

Sample Output

Enter the number of bytes to allocate using _malloca: 1000

.NET Framework Equivalent

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

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