Allocates memory on the stack.
void*_alloca(size_tsize**);**
| Routine | Required Header | Compatibility |
| _alloca | <malloc.h> | Win 95, Win NT |
For additional compatibility information, see Compatibility in the Introduction.
Libraries
| LIBC.LIB | Single thread static library, retail version |
| LIBCMT.LIB | Multithread static library, retail version |
| MSVCRT.LIB | Import library for MSVCRT.DLL, retail version |
Return Value
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. To get a pointer to a type other than char, use a type cast on the return value. A stack overflow exception is generated if the space cannot be allocated.
Parameter
size
Bytes to be allocated from stack
Remarks
_alloca allocates size bytes from the program stack. The allocated space is automatically freed when the calling function exits. 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 listed above.