per thread c++ guard to prevent re-entrant function calls - c++

Question

I've got function that call the registry that can fail and print the failure reason.
This function can also be called directly or indirectly from the context of a dedicated built-in printing function, and I wish to avoid printing the reason in this case to avoid endless recursion.
I can use thread_local to define per thread flag to avoid calling the print function from this function, but I guess it's rather widespread problem, so I'm looking for std implementation for this guard or any other well debugged code.
Here's an example that just made to express the problem.
Each print function comes with log level, and it's being compared with the current log level threshold that reside in registry. if lower than threshold, the function returns without print. However, in order to get the threshold, additional print can be made, so I wanted to create a guard that will prevent the print from getPrintLevelFromRegistry if it's called from print
int getPrintLevelFromRegistry() {
int value = 0;
DWORD res = RegGetValueW("//Software//...//logLevel" , &value);
if (res != ERROR_SUCCESS) {
print("couldn't find registry key");
return 0;
}
return value;
}
void print(const char * text, int printLoglevel) {
if (printLogLevel < getPrintLevelFromRegistry()) {
return;
}
// do the print itself
...
}
Thanks !

Answer 1

The root of the problem is that you are attempting to have your logging code log itself. Rather than some complicated guard, consider the fact that you really don't need to log a registry read. Just have it return a default value and just log the error to the console.
int getPrintLevelFromRegistry() {
int value = 0;
DWORD res = RegGetValueW("//Software//...//logLevel" , &value);
if (res != ERROR_SUCCESS) {
OutputDebugStringA("getPrintLevelFromRegistry: Can't read from registry\r\n");
}
return value;
}
Further, it's OK to read from the registry on each log statement, but it's redundant and unnecessary.
Better:
int getPrintLevelFromRegistry() {
static std::atomic<int> cachedValue(-1);
int value = cachedValue;
if (value == -1) {
DWORD res = RegGetValueW("//Software//...//logLevel" , &value);
if (res == ERROR_SUCCESS) {
cachedValue = value;
}
}
return value;
}