/RTC (Run-Time Error Checks)

Used to enable and disable the run-time error checks feature, in conjunction with the runtime_checks pragma.




Equivalent of /RTCsu.

Reports when a value is assigned to a smaller data type and results in a data loss. For example, if a value of type short 0x101 is assigned to a variable of type char.

This option reports situations in which you intend to truncate, for example, if you want the first eight bits of an int returned as a char. Because /RTCc causes a run-time error if any information is lost as a result of the assignment, you can mask off the information you need to avoid a run-time error as a result of /RTCc. For example:

#include <crtdbg.h>

char get8bits(int value, int position) {
   _ASSERT(position < 32);
   return (char)(value >> position);
   // Try the following line instead:
   // return (char)((value >> position) & 0xff);

int main() {

Enables stack frame run-time error checking, as follows:

  • Initialization of local variables to a nonzero value. This helps identify bugs that do not appear when running in debug mode. There is a greater chance that stack variables will still be zero in a debug build compared to a release build because of compiler optimizations of stack variables in a release build. Once a program has used an area of its stack, it is never reset to 0 by the compiler. Therefore, subsequent, uninitialized stack variables that happen to use the same stack area can return values left over from the prior use of this stack memory.

  • Detection of overruns and underruns of local variables such as arrays. /RTCs will not detect overruns when accessing memory that results from compiler padding within a structure. Padding could occur by using align, /Zp (Struct Member Alignment), or pack, or if you order structure elements in such a way as to require the compiler to add padding.

  • Stack pointer verification, which detects stack pointer corruption. Stack pointer corruption can be caused by a calling convention mismatch. For example, using a function pointer, you call a function in a DLL that is exported as __stdcall but you declare the pointer to the function as __cdecl.

Reports when a variable is used without having been initialized. For example, an instruction that generates C4701 may also generate a run-time error under /RTCu. Any instruction that generates Compiler Warning (level 1 and level 4) C4700 will generate a run-time error under /RTCu.

However, consider the following code fragment:

int a, *b, c;
if ( 1 )
b = &a;
c = a;  // No run-time error with /RTCu

If a variable could have been initialized, it will not be reported at run time by /RTCu. For example, after a variable is aliased through a pointer, the compiler will not track the variable and report uninitialized uses. In effect, you can initialize a variable by taking its address. The & operator works like an assignment operator in this situation.


Run-time error checks are a way for you to find problems in your running code; for more information, see How to: Use Native Run-Time Checks.

If you compile your program at the command line using any of the /RTC compiler options, any pragma optimize instructions in your code will silently fail. This is because run-time error checks are not valid in a release (optimized) build.

You should use /RTC for development builds; /RTC should not be used for a retail build. /RTC cannot be used with compiler optimizations (/O Options (Optimize Code)). A program image built with /RTC will be slightly larger and slightly slower than an image built with /Od (up to 5 percent slower than an /Od build).

The __MSVC_RUNTIME_CHECKS preprocessor directive will be defined when you use any /RTC option or /GZ.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Click the C/C++ folder.

  3. Click the Code Generation property page.

  4. Modify one or both of the following properties: Basic Runtime Checks or Smaller Type Check.

To set this compiler option programmatically

See also

MSVC Compiler Options
MSVC Compiler Command-Line Syntax
How to: Use Native Run-Time Checks