The docs at http://golang.org/cmd/cgo/ say:

---
Any C function that returns a value may be called in a multiple assignment context to
retrieve both the return value and the C errno variable as an error. For example:

n, err := C.atoi("abc")

---

This has two errors:
1. Need to wrap "abc" in C.CString
2. atoi does not set errno (at least on Linux), strtol does

I suggest to change the example to
    // Will generate error since base 100 is too large
    n, err := C.strtol(C.CString("10"), nil, 100)