[TR24731-1] provides a consistent mechanism to handle constraints violations that are discerned at runtime. Most functions defined by [TR24731-1] include as part of their specification a list of runtime - constraints. Library implementations must verify that the runtime - constraints for a function are not violated by the program. If a runtime - constraint is violated, the runtime - constraint handler currently registered with set_constraint_handler_s() is called.
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- A pointer to a character string describing the runtime - constraint violation.
- A null pointer or a pointer to an implementation-defined object.
- If the function calling the handler has a return type declared as
errno_t, the return value of the function is passed. Otherwise, a positive value of typeerrno_tis passed.
The implementation has a default constraint handler that is used if no calls to the set_constraint_handler_s() function have been made or the handler argument to set_constraint_handler_s() is a NULL null pointer. The behavior of the default handler is implementation-defined, and it may cause the program to exit or abort.
Section 6.1.4 states:
These runtime - constraints are requirements on the program using the library.
and
The runtime - constraint handler might not return. If the handler does return, the library function whose runtime - constraint was violated shall return some indication of failure as given by the returns section in the function's specification.
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These runtime constraint handlers mitigate some of the potential insecurity caused by in-band error indicators (see \[[ERR02-A. Avoid in-band error indicators]\]).
Non-Compliant Code Example
In this non-compliant example, the strcpy_s() function is called, but no runtime - constraint handler has been explicitly registered. As a result, the implementation-defined default handler will be called on a run-time runtime error.
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|---|---|---|
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errno_t function(char *dst1, size_t size){
char src1[100] = "hello";
if (strcpy_s(dst1, size, src1) != 0) {
return -1;
}
/* ... */
return 0;
}
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This will result in inconsistent behavior across implementations and possible termination of the program instead of a graceful exit. The implementation-defined default handler performs a default action consistent with a particular implementation. However, this may not be the desired action, and because the behavior is implementation-defined, it is not guaranteed to be the same on all implementations.
As a result, in it is generally prudent to explicitly install a run-time runtime constraint handler to ensure consistent behavior across implementations.
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This compliant solution explicitly installs a runtime - constraint handler by invoking the set_constraint_handler() function. This would typically be performed during system initialization , and before any functions that used the mechanism were invoked.
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|---|---|---|
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constraint_handler_t handle_errors() {
/* handle runtime- constraint error */
}
/*...*/
set_constraint_handler(handle_errors);
/*...*/
/* Returns zero on success */
errno_t function(char *dst1, size_t size){
char src1[100] = "hello";
if (strcpy_s(dst1, size, src1) != 0) {
return -1;
}
/* ... */
return 0;
}
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Unfortunately, although the ISO/IEC TR 24731-1 functions were created by Microsoft, currently available versions of Microsoft Visual Studio do not support the same interface defined by the TR for installing run-time runtime constraint handlers. Visual Studio calls these functions "invalid parameter handlers," and they are installed by calling the _set_invalid_parameter_handler() function. The signature of the handler is also significantly different.
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The TR24731-1 standard indicates that if no constraint handler is set, a default one executes when errors arise. The default handler is implementation-defined and "may cause the program to exit or abort." . It is important to understand the behavior of the default handler for all implementations being used , and replace it if the behavior is inappropriate for the application.
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