Double-Free exercise doesn't act as expected

Question

I am reading the book "Effective C" by Robert C. Seacord. In this book, it has an exercise where you intentionally double-free a pointer so you can test using dmalloc to debug the cause. However, it doesn't fail as expected.

#include <string.h>
#include <stdlib.h>

#ifdef DMALLOC
#include "dmalloc.h"
#endif

void usage(char *msg) {
    fprintf(stderr, "%s", msg);
    free(msg);
    return;
}

int main(int argc, char *argv[]) {
    if (argc != 3 && argc !=4) {
        /* The error message won't be more than 80 chars */
        char *errmsg = (char *)malloc(80);
        sprintf(
            errmsg,
            "Sorry %s,\nUsage: caesar secret_file keys_file [output_file]\n",
            getenv("USER")
        );
        usage(errmsg);
        free(errmsg);
        exit(EXIT_FAILURE);
    }
    exit(EXIT_SUCCESS);
}

Its clear here that *errmsg should get freed twice: First by the usage function when its passed to it, and then right after in main. Why doesn't this fail when ran with no arguments? I am using linux (POP!_OS 20.04) with GCC 9.3.0.

EDIT: For more context the book suggests I should see an output like this:

% ./caesar
Sorry student,
Usage: caesar secret_file keys_file [output_file]
debug-malloc library: dumping program, fatal error
  Error: tried to free previously freed pointer (err 61)
Aborted (core dumped)

Adding more calls to free doesn't do anything either. I get the usage portion but not a core dump.

Answer 1

I am sorry for taking up people's time with this. I figured it out. That crash behavior is supposed to be provided by dmalloc, however its usage has changed a little since the writing of the book I am reading. I needed to add -DDMALLOC_FUNC_CHECK to the compiler options in order for it to produce the expected result.

I learned its dmalloc, not the OS that causes the program to crash when you double-free the pointer.