Some of the functionality described on this reference page
extends the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The ISO C standard only requires the symbols [EDOM], [EILSEQ],
and [ERANGE] to be defined.
The <errno.h> header shall provide a declaration or definition
for errno. The symbol errno shall expand to a modifiable lvalue
of type int. It is unspecified whether errno is a macro or an
identifier declared with external linkage. If a macro definition
is suppressed in order to access an actual object, or a program
defines an identifier with the name errno, the behavior is
undefined.
The <errno.h> header shall define the following macros which
shall expand to integer constant expressions with type int,
distinct positive values (except as noted below), and which shall
be suitable for use in #if preprocessing directives:
E2BIG Argument list too long.
EACCES Permission denied.
EADDRINUSE
Address in use.
EADDRNOTAVAIL
Address not available.
EAFNOSUPPORT
Address family not supported.
EAGAIN Resource unavailable, try again (may be the same value as
[EWOULDBLOCK]).
EALREADY
Connection already in progress.
EBADF Bad file descriptor.
EBADMSG
Bad message.
EBUSY Device or resource busy.
ECANCELED
Operation canceled.
ECHILD No child processes.
ECONNABORTED
Connection aborted.
ECONNREFUSED
Connection refused.
ECONNRESET
Connection reset.
EDEADLK
Resource deadlock would occur.
EDESTADDRREQ
Destination address required.
EDOM Mathematics argument out of domain of function.
EDQUOT Reserved.
EEXIST File exists.
EFAULT Bad address.
EFBIG File too large.
EHOSTUNREACH
Host is unreachable.
EIDRM Identifier removed.
EILSEQ Illegal byte sequence.
EINPROGRESS
Operation in progress.
EINTR Interrupted function.
EINVAL Invalid argument.
EIO I/O error.
EISCONN
Socket is connected.
EISDIR Is a directory.
ELOOP Too many levels of symbolic links.
EMFILE File descriptor value too large.
EMLINK Too many links.
EMSGSIZE
Message too large.
EMULTIHOP
Reserved.
ENAMETOOLONG
Filename too long.
ENETDOWN
Network is down.
ENETRESET
Connection aborted by network.
ENETUNREACH
Network unreachable.
ENFILE Too many files open in system.
ENOBUFS
No buffer space available.
ENODATA
No message is available on the STREAM head read queue.
ENODEV No such device.
ENOENT No such file or directory.
ENOEXEC
Executable file format error.
ENOLCK No locks available.
ENOLINK
Reserved.
ENOMEM Not enough space.
ENOMSG No message of the desired type.
ENOPROTOOPT
Protocol not available.
ENOSPC No space left on device.
ENOSR No STREAM resources.
ENOSTR Not a STREAM.
ENOSYS Functionality not supported.
ENOTCONN
The socket is not connected.
ENOTDIR
Not a directory or a symbolic link to a directory.
ENOTEMPTY
Directory not empty.
ENOTRECOVERABLE
State not recoverable.
ENOTSOCK
Not a socket.
ENOTSUP
Not supported (may be the same value as [EOPNOTSUPP]).
ENOTTY Inappropriate I/O control operation.
ENXIO No such device or address.
EOPNOTSUPP
Operation not supported on socket (may be the same value
as [ENOTSUP]).
EOVERFLOW
Value too large to be stored in data type.
EOWNERDEAD
Previous owner died.
EPERM Operation not permitted.
EPIPE Broken pipe.
EPROTO Protocol error.
EPROTONOSUPPORT
Protocol not supported.
EPROTOTYPE
Protocol wrong type for socket.
ERANGE Result too large.
EROFS Read-only file system.
ESPIPE Invalid seek.
ESRCH No such process.
ESTALE Reserved.
ETIME Stream ioctl() timeout.
ETIMEDOUT
Connection timed out.
ETXTBSY
Text file busy.
EWOULDBLOCK
Operation would block (may be the same value as [EAGAIN]).
EXDEV Cross-device link.
The following sections are informative.
