In this section, synchronous socket operations are those member functions specified as two overloads, with and without an argument of type error_code&:

R f(A1 a1, A2 a2, ..., AN aN);
R f(A1 a1, A2 a2, ..., AN aN, error_code& ec);

For an object s, the conditions under which its synchronous socket operations may block the calling thread (C++Std [defns.block]) are determined as follows.

If:

— s.non_blocking() == true,

— the synchronous socket operation is specified in terms of a POSIX function other than poll(),

— that POSIX function lists EWOULDBLOCK or EAGAIN in its failure conditions, and

— the effects of the operation cannot be established immediately

then the synchronous socket operation shall not block the calling thread. [Note: And the effects of the operation are not established. —end note]

Otherwise, the synchronous socket operation shall block the calling thread until the effects are established.