CBMC malloc should be able to non-determinstically fail

[danielsn]

#include <assert.h>
#include <stdlib.h>
#include <stdint.h>

int main() {
    char*p = malloc(100);
    assert(p);
}

the assertion never triggers, tho it ought to be able to, given that real malloc can fail.

[hannes-steffenhagen-diffblue]

This has been implemented with the --malloc-may-fail and the --malloc-fail-null flags.