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_alloca

Allocates memory on the stack. This function is deprecated because a more secure version is available; see _malloca.

void *_alloca( 
   size_t size 
);
[in] size

Bytes to be allocated from the stack.

The _alloca 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, _alloca 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).

_alloca allocates size bytes from the program stack. The allocated space is automatically freed when the calling function exits (not when the allocation merely passes out of scope). Therefore, do not pass the pointer value returned by _alloca as an argument to free.

There are restrictions to explicitly calling _alloca 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 _alloca 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 (_alloca () )

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

  • C++ EH catch clause expression

However, _alloca 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 noteSecurity Note:

In Windows XP, if _alloca 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, _alloca cannot be used in __except blocks. For more information, see /clr Restrictions.

Routine

Required header

_alloca

<malloc.h>

// crt_alloca.c
// This program demonstrates the use of
// _alloca and trapping any exceptions
// that may occur.

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

int main()
{
    int     size = 1000;
    int     errcode = 0;
    void    *pData = NULL;

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

    __try {
        // An unbounded _alloca can easily result in a 
        // stack overflow.
        // Checking for a size < 1024 bytes is recommended.
        if (size > 0 && size < 1024)
        {
            pData = _alloca( size );
            printf_s( "Allocated %d bytes of stack at 0x%p",
                      size, pData);
        }
        else
        {
            printf_s("Tried to allocate too many bytes.\n");
        }
    }

    // If an exception occured with the _alloca function
    __except( GetExceptionCode() == STATUS_STACK_OVERFLOW )
    {
        printf_s("_alloca failed!\n");

        // If the stack overflows, use this function to restore.
        errcode = _resetstkoflw();
        if (errcode)
        {
            printf_s("Could not reset the stack!\n");
            _exit(1);
        }
    };
}
Allocated 1000 bytes of stack at 0x0012FB50

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

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