bindresvport() is used to bind the socket referred to by the file
descriptor sockfd to a privileged anonymous IP port, that is, a
port number arbitrarily selected from the range 512 to 1023.
If the bind(2) performed by bindresvport() is successful, and sin
is not NULL, then sin->sin_port returns the port number actually
sin can be NULL, in which case sin->sin_family is implicitly
taken to be AF_INET. However, in this case, bindresvport() has
no way to return the port number actually allocated. (This
information can later be obtained using getsockname(2).)
bindresvport() can fail for any of the same reasons as bind(2).
In addition, the following errors may occur:
EACCES The calling process was not privileged (on Linux: the
calling process did not have the CAP_NET_BIND_SERVICE
capability in the user namespace governing its network
All privileged ports are in use.
EAFNOSUPPORT (EPFNOSUPPORT in glibc 2.7 and earlier)
sin is not NULL and sin->sin_family is not AF_INET.
For an explanation of the terms used in this section, see
│Interface │ Attribute │ Value │
│bindresvport() │ Thread safety │ glibc >= 2.17: MT-Safe; │
│ │ │ glibc < 2.17: MT-Unsafe │
The bindresvport() function uses a static variable that was not
protected by a lock before glibc 2.17, rendering the function MT-