INFO: Stack Checking for Windows NT-based Applications

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

Stack checking in an application developed for Windows NT differs from stack checking in an MS-DOS application.

MORE INFORMATION

In an MS-DOS application, stack overflow is detected only by software. By default, the compiler inserts a call to the __chkstk() function in the prologue code for each function. The __chkstk() function compares the amount of stack space a function requires with the amount of stack space available. The function issues an overflow error if the current stack pointer and requested stack allocation exceeds the maximum stack size specified in the EXE header.

In the Microsoft Windows NT operating system, stack overflow is detected by hardware and software working together, using the page protection mechanisms. Each new Windows NT process has a maximum reserved stack size and an initial committed stack allocation. Committed memory is physically allocated to the process and is backed by the page file; it is a relatively "expensive" resource. Reserved memory is address space that is not mapped to real memory; it is a relatively "cheap" resource.

As the stack grows, it moves from the committed portion of stack memory into the reserved or uncommitted memory. When this happens, a page fault occurs and the operating system commits another page of memory to the stack. If a page fault occurs when the stack has already grown to its maximum specified size, the system reports a stack overflow exception.

This automatic growth method uses a guard page, a reserved, uncommitted, memory page that is contiguous with the committed portion of memory. When the application touches the guard page, the operating system commits that page and the next uncommitted page becomes the new guard page. Automatic stack growth works only for the guard page and stack memory must grow in 4K, or one page, increments. If the application touches another reserved but uncommitted page of stack memory before it touches the guard page, a normal page fault exception occurs and unpredictable behavior can result.

If a function included the following statements in its prologue code, this problem could occur:

   PUSH   EBP
   MOV    EBP,ESP
   SUB    ESP,10000
   PUSH   ESI

To prevent the fault, the compiler calls the __chkstk() function each time the local allocation exceeds 4K. The Windows NT __chkstk() function does not explicitly check for stack overflow as the MS-DOS version does. It simply touches memory addresses every 4K from the current stack pointer location to the requested allocation. This triggers the guard pages in the proper sequence and commits additional memory to the stack as required.

Therefore, when the compiler command line includes the /Ge option switch and the prologue code always calls the __chkstk() function, the application is not running as efficiently as it might because the operating system supports an automatic mechanism to perform stack overflow detection.

The /Gs compiler option switch does not disable all __chkstk() calls. Instead, it disables __chkstk() calls for those functions that require less than 4K of local storage. The /Gs option is the default behavior of the compiler.

The /Gs option accepts an optional parameter, the threshold value. If a function's local stack allocation exceeds the specified threshold, the compiler inserts a __chkstk() call into the function prologue. For a user- mode application to run correctly in Windows NT, the default threshold 4096 is required. To suppress all __chkstk() calls, specify an artificially high threshold value such as /Gs999999. The /Gs0 option has the same function as the /Ge option and instructs the compiler to call __chkstk() in every function. It might be advantageous to change the default value if the code executes in an environment where the stack is fully committed or if the stack growth mechanism is otherwise not available.

For more information, refer to the /Gs compilation option and the check_stack preprocessor pragma in the Visual C++ Books Online.