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/RTC (Run-Time Error Checks)


Run-time error checks are a way for you to find problems in your running code; see Using Native Run-Time Checks for a full description of this feature. The run-time error checks feature is enabled and disabled by the /RTC group of compiler options and the runtime_checks pragma.

Option Description
/RTC1 Equivalent of /RTCsu.
/RTCc Reports when a value is assigned to a smaller data type that results in a data loss. For example, if a value of type short 0x101 is assigned to a variable of type char.

This option will report situations where you intend to truncate, as in a situation where you want the first eight bits of an int returned as a char. Since /RTCc will cause a run-time error if any information will be 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()
/RTCs Enables stack frame run-time error checking:
  • 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, 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.
/RTCu 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 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.

Note   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.

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 Setting Visual C++ Project Properties.
  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 BasicRuntimeChecks and SmallerTypeCheck properties.

See Also

Compiler Options | Setting Compiler Options | RTC sample