See https://en.cppreference.com/w/cpp/header/cerrno for additional details. Some of used constants are defined in this header.
Inclusion is necessary to avoid these errors:
```
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:189:30: error: use of undeclared identifier 'EAFNOSUPPORT'
default: errno = EAFNOSUPPORT; return 0;
^
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:191:17: error: use of undeclared identifier 'ENOSPC'
errno = ENOSPC;
^
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:175:25: warning: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long long') to 'int' [-Wshorten-64-to-32]
j = strspn(buf + i, ":0");
~ ^~~~~~~~~~~~~~~~~~~~~
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:234:21: error: use of undeclared identifier 'EAFNOSUPPORT'
errno = EAFNOSUPPORT;
^
2 warnings and 3 errors generated.
```
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