_malloca

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

Syntax

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 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.

If size is greater than _ALLOCA_S_THRESHOLD, then _malloca attempts to allocate on the heap, and returns a null pointer if the space can't be allocated. If size is less than or equal to _ALLOCA_S_THRESHOLD, then _malloca attempts to allocate on the stack, and a stack overflow exception is generated if the space can't be allocated. The stack overflow exception isn't a C++ exception; it's a structured exception. Instead of using C++ exception handling, you must use Structured exception handling (SEH) to catch this exception.

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 doesn't 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 isn't 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 SEH exception filter expression: __except (_malloca () )

• Windows 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.

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

Requirements

Example: _malloca

// 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();
}

Example: _malloca exception

// 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

1000

Sample output

Enter the number of bytes to allocate using _malloca: 1000

See also

Memory allocation
calloc
malloc
realloc
_resetstkoflw