...
There are a couple of problems with this code. The first problem is that this code assumes an implementation where unsigned long long has a least twice the number of bits as four more bits than size_t. The second problem, assuming an implementation where size_t is a 32-bit value and unsigned long long is represented by a 64-bit value, is that to be compliant with C99, multiplying two 32-bit numbers in this context must yield a 32-bit result. Any integer overflow resulting from this multiplication will remain undetected by this code, and the expression alloc < UINT_MAX will always be true.
...
Note that this code does not prevent overflows unless the unsigned long long type is at least twice the length of four bits larger than size_t.
Risk Assessment
Failure to cast integers before comparing or assigning to a larger integer size can result in software vulnerabilities that can allow the execution of arbitrary code by an attacker with the permissions of the vulnerable process.
...