Assert: To Abort Or Not To Abort, That's The Question

Everyone agrees that internal state checking using assert(), Q_ASSERT(), assert are good. Programmer can declare expected input (asserting parameters), internal state (invariants) and verify return values (postconditions) and runtime will verify such expectations. There are languages with direct support for assertions in those three variants (Eiffel with his Design By Contract philosophy).

Those assertions typically will show filename/line number/message information and abort program / raise an exception if the condition is not met. Runtime environment then can collect current stacktrace to give developer more information on failed expectation.

One can disable assertions entirely (C++, Java) or select subset of assertions (Eiffel) for production releases. Resulting code will be faster, but failed expectations will not be verified by software – problem reports may be harder to interpret.

On the other hand if an assertion is too strict (the assumption may be invalid) it may abort program giving user negative impression about software stability.

What to do then? How can we keep problem-diagnosing power of enabled assertions and prevent minor of invalid failed assertions from aborting whole program?

The answer is: weak assertions (assertion without abort).

OK, you may ask, if we are not calling abort() then we can loose details about failed assertion. Nobody will send a bug report if program is not crashing.

But why do we need to rely on end user? ;-) Let's send failed assertion reports automatically under-the-covers and leave system running!

I'm attaching solution below for Q_ASSERT (C++, QT), but you should get the idea:

void qt_assert_x(const char *where, const char *what, const char *file, int line) {
    char buf[BUFSIZE];
    snprintf(buf, BUFSIZE, "%s:%d: Q_ASSERT(%s, %s) FAILED", file, line, where, what);
    if (getenv("ASSERT_FATAL")) {

If you place this code in LD_PRELOAD-ed library you will overload symbol from QT library and catch any reference to qt_assert_x() symbol. Supplied function will save important information into log file, send it using network connection (of possible) then return. I described in this post how can we collect such crash reports on a server and this post will tell you about implementation of stacktraces for C++.

Optionally you can ask for abort() call (typical Q_ASSERT behavior) using some kind of configuration (environment variable in my case). It may be useful in development environment to die early is something goes wrong to have better diagnostics.

Using custom error handlers you can have both benefits:

  • "robust" program that will not die after minor expectation is not met (end user will not care)
  • automatic diagnostics about failed expectation
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