In CPython, almost all memory allocations are protected against integer overflow with code looking like that:

    if (length > ((PY_SSIZE_T_MAX - struct_size) / char_size - 1)) {
        PyErr_NoMemory();
        return NULL;
    }
    new_size = (struct_size + (length + 1) * char_size);

For performances, GCC 5 introduces __builtin_mul_overflow() which is an integer multiplication with overflow check. On x86/x86_64, it is implemented in hardware (assembler instruction JO, jump if overflow, if I remember correctly).

The function already exists in Clang: "... which existed in Clang/LLVM for a while" says http://lwn.net/Articles/623368/ According to this mail sent to the Linux kernel mailing list, the Linux kernel has functions like "check_mul_overflow(X, Y, C)".

For other compilers, it should be easy to reimplement it, but I don't know what is the most efficient implementation (Py_LOCAL_INLINE function in an header?)

GCC 5 changelog:
https://gcc.gnu.org/gcc-5/changes.html

Note: GCC 5 is not released yet.