errno is always left unchanged by successful operations.
errno may be a macro. In a multithreaded process it is almost invariably a macro. However, it is always an lvalue, that is, it may be assigned to.
Each numeric code has a symbolic name and a textual expansion. The symbolic names are used in source code; the textual expansions are printed out when errors are reported to a user.
The textual expansions can be retrieved with strerror or printed with err or warn.
ENOSYS No such system call: the call made was not defined, or perhaps not yet implemented. EUNIMP Unimplemented feature: functionality not yet implemented would have been needed to perform the requested operation. This error code is not found in production systems. ENOMEM Out of memory: a memory allocation failed. This normally means that a process has used up all the memory available to it (either because of limits or because it has used up all the memory available to the system.) It may also mean that memory allocation within the kernel has failed. EAGAIN Operation would block: some resource is temporarily unavailable, or a non-blocking I/O operation (if such things exist) could not be completed without waiting. Historically, the message was "Try again later"; in 4.4BSD EAGAIN and the old EWOULDBLOCK code were folded together. EINTR Interrupted system call: handling of a system call was interrupted by the delivery of a signal. (If you have signals.) EFAULT Bad memory reference: a pointer passed as an argument was not valid. ENAMETOOLONG String too long: a string passed as an argument was too long to process. EINVAL Invalid argument: an argument passed to a command or system call was badly formed, invalid, or nonsensical, in a way for which some more specific error code is not available. EPERM Operation not permitted: the requested operation is restricted to privileged users. EACCES Permission denied: the current process's credentials do not allow the desired form of access to the target object according to its permission settings. EMPROC Too many processes: the current user ID has reached its limit of simultaneous running processes. In Unix, tihs is EPROCLIM. ENPROC Too many processes on system: the system process table is full. (Void where impossible or prohibited by law.) ENOEXEC File is not executable: an execv operation was attempted but the kernel was unable to run the requested program. E2BIG Argument list too long: the space taken up by the argv[] strings passed to a newly started program is larger than the system allows. ESRCH No such process: the supplied process ID does not name any of the currently running processes. ECHILD No child processes: the current process has no exited child processes whose exit status has not yet been collected with waitpid. ENOTDIR Not a directory: a directory was expected and a non-directory filesystem object was found. EISDIR Is a directory: a non-directory was expected and a directory was found. ENOENT No such file or directory: the requested filesystem object does/did not exist. ELOOP Too many levels of symbolic links: pathname lookup crossed more than the maximum allowed number of symbolic links. Usually means a link points to itself, or a family of links has been arranged into a loop. (If you have symbolic links.) ENOTEMPTY Directory not empty: a directory must be empty of everything (except . and ..) before it may be removed. EEXIST File exists: a filesystem object that was expected not to exist did in fact already exist. EMLINK Too many hard links: the maximum number of hard links to the target file already exist. EXDEV Cross-device link: an attempt was made to instruct one filesystem to handle files on another filesystem. ENODEV No such device: the requested device does not exist. ENXIO Device not available: the requested device exists but is not available (is not mounted, is powered off, etc.) EBUSY Device busy: the requested object cannot be used (or, perhaps, released) because something else is using it. EMFILE Too many open files: the process file table is full, so the process cannot open more files. ENFILE Too many open files in system: a system-wide limit of some sort, if any exists, on the number of open files has been reached. EBADF Bad file number: a file operation was requested on an illegal file handle, or a file handle that was not open. Or, a write operation was attempted on a file handle that was open only for read or vice-versa. EIOCTL Invalid or inappropriate ioctl: an operation requested via the ioctl system call was not defined or could not be performed on the indicated object. In Unix, for historical reasons, this is ENOTTY, with the historic message "Not a typewriter". EIO Input/output error: a hardware error occured on a device. Media errors on disks fall into this category. ESPIPE Illegal seek: a seek operation was attempted on a sequential object where seeking makes no sense, like a terminal. EROFS Read-only file system: an attempt was made to modify a filesystem that was mounted read-only. (If you have read-only mounts.) ENOSPC No space left on device: the target filesystem is full. EDQUOT Disc(sic) quota exceeded: the current user ID's quota (of space or number of files) on the target filesystem has been used up. (If you have disk quotas.) EFBIG File too large: an attempt was made to exceed the target filesystem's maximum file size, or a per-user limit on maximum file size was reached, if such a thing exists. EFTYPE Invalid file type or format: the file provided was the wrong kind of file or contained invalid syntax. EDOM Argument out of range: the (numeric) argument provided was outside the values upon which the operation is defined. For example, attempting to evaluate the logarithm of zero. ERANGE Result out of range: the result of an operation did not fit in the space provided or could not be represented. EILSEQ Invalid multibyte character sequence: the input string contained a byte sequence whose value is undefined or whose use is restricted. Only applicable when a multibyte character set is in use, and if someone has added locale support. ENOTSOCK Not a socket: the file handle in question does not refer to a socket, but a socket was expected. EISSOCK Is a socket: the file handle in question refers to a socket, but a socket was not expected. In Unix this is EOPNOTSUPP, "Operation not supported on socket". ENOTCONN Socket is not connected: given the protocol in use, the operation requires a connected socket, but no connection has yet been made. ESHUTDOWN Socket has been shut down: the operation requires a running socket, but the socket provided has been closed down. EPFNOSUPPORT Protocol family not supported: the requested protocol family (PF_INET, PF_LOCAL, etc.) is not supported by the system. ESOCKTNOSUPPORT Socket type not supported: the requested socket type (SOCK_STREAM, SOCK_DGRAM, etc.) is not supported by the system. EPROTONOSUPPORT Protocol not supported: the protocol requested for a socket was not one supported by the system. EAFNOSUPPORT Address family not supported by protocol family: the address family named in a struct sockaddr (AF_INET, AF_LOCAL, etc.) is not supported by the protocol family used to create the socket (PF_INET, PF_LOCAL, etc.). In practice each protocol family has exactly one address family and the values of AF_* and PF_* are often, if incorrectly, used interchangeably. ENOPROTOOPT Protocol option not available: the protocol option that was requested is not supported or cannot be activated. EADDRINUSE Address already in use: the requested socket address is already in use by another socket somewhere on the system. EADDRNOTAVAIL Cannot assign requested address: the requested socket address is unavailable. Usually caused by attempting to bind a socket to the IP address of another machine. ENETDOWN Network is down: the network or subnet needed is offline. ENETUNREACH Network is unreachable: the network or subnet needed cannot be reached from here, possibly due to routing problems on the network, possibly due to local configuration trouble. EHOSTDOWN Host is down: the specific machine requested is offline. EHOSTUNREACH Host is unreachable: the specific machine requested cannot be reached from here. ECONNREFUSED Connection refused: the remote machine is not listening for connections on the requested port. ETIMEDOUT Connection timed out: there was no response from the remote machine; it may be down, it may not be listening, or it may not be receiving our packets at all. ECONNRESET Connection reset by peer: the connection was abandoned by the remote host. Usually seen on already-open connections after the remote machine reboots and thereby loses its network state. Sometimes also caused by defective network devices between the local and remote hosts. EMSGSIZE Message too large: an internal protocol length limit was exceeded. ENOTSUP Threads operation not supported: a special error code defined by the POSIX threads standard, which is a "special" interface.