When assert() fails, what is the program exit code?

It's implementation-specific. You could do this:

int main()
{
    assert(0);
}

Then run it:

./a.out
echo $?

1 (<- or whatever)

This'll at least tell you what to expect for your setup. I'm getting 134 on a couple of Linux boxes with both GCC and g++.

The C99 (unchanged in C11) standard states that assert calls abort and the abort stuff states this about the return code:

An implementation-defined form of the status unsuccessful termination is returned to the host environment by means of the function call raise(SIGABRT).

It's documented in section 7.2.1.1 (assert) and 7.20.4.1 (abort) of the C99 standard here.

Many UNIX systems will return 128 plus the signal number (SIGABRT is signal number 6) so you may get 134. Whatever you get, it should be documented by the C implementation.

For example, see here for gcc. Although it's quite silent on what gets returned to the calling environment. From the specific sections here:

Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.

And here:

The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

So is the glibc doco here on program termination (specifically the exit status bit). It mentions conventions but no firm rules.