change the exit status of a C/C++ program during atexit callback

Question

I'm looking for a way to change the exit status of a C/C++ program during an atexit callback sequence.

I'm develping some C++ code that registers a book keeping function with atexit. If the program terminates as a result of an exit() call, this book keeping function prints a message if the data was left in an incomplete state. If this is the case, I want to make sure that the exit status of the program is nonzero. This is probably going to be true anyway, but it's also possible to call exit(0).

One solution is to call exit(-1) within the book keeping function that I've registered with atexit. This seems to work, but I think it's also undefined behavior. Does anyone know if this is correct? It's also unclear if this would terminate the atexit callback chain, which would be bad if there's another critical function registered with atexit.

Answer 1

On POSIX systems, it is allowed to call _exit from within an atexit handler, however doing so means that any other atexit handler is not called.

Since atexit handlers are called in the reverse order of registration, have the first registered handler read a global variable and, if it's not set to some initial sentinel value, call _exit with that value. Then any other handlers you register can modify that global if they want to override the exit value passed to exit.

For example:

#define EXIT_DEFAULT 0xffff
int exit_override = EXIT_DEFAULT;

void override_exit(void)
{
    if (exit_override != EXIT_DEFAULT) _exit(exit_override);
}

void handler1(void)
{
    if (some_error_condition) exit_override = 1;
}

int main()
{
    atexit(override_exit);
    atexit(handler1);
    // do something that might call exit
    return 0;
}