该库定义了一个变量 errno，储存最后一次错误的错误代码，需搭配 string.h 中的 strerror 函数获取 错误代码对应的描述信息。
此外，还预定义了许多宏，对应不同的错误代码：
| 宏 | 对应的描述文本 |
|---|---|
| PERM | Operation not permitted |
| ENOENT | No such file or directory |
| ESRCH | No such process |
| EINTR | Interrupted function call |
| EIO | Input/output error |
| ENXIO | No such device or address |
| E2BIG | Arg list too long |
| ENOEXEC | Exec format error |
| EBADF | Bad file descriptor |
| ECHILD | No child processes |
| EAGAIN | Resource temporarily unavailable |
| ENOMEM | Not enough space |
| EACCES | Permission denied |
| EFAULT | Bad address |
| EBUSY | Resource device |
| EEXIST | File exists |
| EXDEV | Improper link |
| ENODEV | No such device |
| ENOTDIR | Not a directory |
| EISDIR | Is a directory |
| ENFILE | Too many open files in system |
| EMFILE | Too many open files |
| ENOTTY | Inappropriate I/O control operation |
| EFBIG | File too large |
| ENOSPC | No space left on device |
| ESPIPE | Invalid seek |
| EROFS | Read-only file system |
| EMLINK | Too many links |
| EPIPE | Broken pipe |
| EDOM | Domain error |
| EDEADLK | Resource deadlock avoided |
| ENAMETOOLONG | Filename too long |
| ENOLCK | No locks available |
| ENOSYS | Function not implemented |
| ENOTEMPTY | Directory not empty |
| EINVAL | Invalid argument |
| ERANGE | Result too large |
| EILSEQ | Illegal byte sequence |
| STRUNCATE | Unknown error |
| ENOTSUP | not supported |
| EAFNOSUPPORT | address family not supported |
| EADDRINUSE | address in use |
| EADDRNOTAVAIL | address not available |
| EISCONN | already connected |
| ENOBUFS | no buffer space |
| ECONNABORTED | connection aborted |
| EALREADY | connection already in progress |
| ECONNREFUSED | connection refused |
| ECONNRESET | connection reset |
| EDESTADDRREQ | destination address required |
| EHOSTUNREACH | host unreachable |
| EMSGSIZE | message size |
| ENETDOWN | network down |
| ENETRESET | network reset |
| ENETUNREACH | network unreachable |
| ENOPROTOOPT | no protocol option |
| ENOTSOCK | not a socket |
| ENOTCONN | not connected |
| ECANCELED | operation canceled |
| EINPROGRESS | operation in progress |
| EOPNOTSUPP | operation not supported |
| EWOULDBLOCK | operation would block |
| EOWNERDEAD | owner dead |
| EPROTO | protocol error |
| EPROTONOSUPPORT | protocol not supported |
| ETIMEDOUT | timed out |
| ELOOP | too many symbolic link levels |
| EPROTOTYPE | wrong protocol type |
| EOVERFLOW | value too large |