Blocking in that sense is usually used in relation to some event. E.g. sleep() blocks on a timer, read() blocks on IO, etc. In the general sense, it means that

Blocking in that sense is usually used in relation to some event. E.g. sleep() blocks on a timer, read() blocks on IO, etc. In the general sense, it means that the call has an indefinite run time. E.g. “this call blocks” = “this call could take an arbitrarily long amount of time” getpid() is blocking but it likely does not block on IO (though it could as that is allowed by the spec).