module Errno

When an operating system encounters an error, it typically reports the error as an integer error code:

$ ls nosuch.txt
ls: cannot access 'nosuch.txt': No such file or directory
$ echo $? # Code for last error.
2

When the Ruby interpreter interacts with the operating system and receives such an error code (e.g., 2), it maps the code to a particular Ruby exception class (e.g., Errno::ENOENT):

File.open('nosuch.txt')
# => No such file or directory @ rb_sysopen - nosuch.txt (Errno::ENOENT)

Each such class is:

A nested class in this module, Errno.
A subclass of class SystemCallError.
Associated with an error code.

Thus:

Errno::ENOENT.superclass # => SystemCallError
Errno::ENOENT::Errno     # => 2

The names of nested classes are returned by method Errno.constants:

Errno.constants.size         # => 158
Errno.constants.sort.take(5) # => [:E2BIG, :EACCES, :EADDRINUSE, :EADDRNOTAVAIL, :EADV]

As seen above, the error code associated with each class is available as the value of a constant; the value for a particular class may vary among operating systems. If the class is not needed for the particular operating system, the value is zero:

Errno::ENOENT::Errno      # => 2
Errno::ENOTCAPABLE::Errno # => 0

Each class in Errno can be created with optional messages:

Errno::EPIPE.new                  # => #<Errno::EPIPE: Broken pipe>
Errno::EPIPE.new("foo")           # => #<Errno::EPIPE: Broken pipe - foo>
Errno::EPIPE.new("foo", "here")   # => #<Errno::EPIPE: Broken pipe @ here - foo>

See SystemCallError.new.

Constants

E2BIG: “Argument list too long” error
EACCES: “Permission denied” error
EADDRINUSE: “Address already in use” error
EADDRNOTAVAIL: “Address not available” error
EADV: “Advertise error” error
EAFNOSUPPORT: “Address family not supported” error
EAGAIN: “Resource temporarily unavailable, try again (may be the same value as EWOULDBLOCK)” error
EALREADY: “Connection already in progress” error
EAUTH: “Authentication error” error
EBADARCH: “Bad CPU type in executable” error
EBADE: “Bad exchange” error
EBADEXEC: “Bad executable” error
EBADF: “Bad file descriptor” error
EBADFD: “File descriptor in bad state” error
EBADMACHO: “Malformed Macho file” error
EBADMSG: “Bad message” error
EBADR: “Invalid request descriptor” error
EBADRPC: “RPC struct is bad” error
EBADRQC: “Invalid request code” error
EBADSLT: “Invalid slot” error
EBFONT: “Bad font file format” error
EBUSY: “Device or resource busy” error
ECANCELED: “Operation canceled” error
ECAPMODE: “Not permitted in capability mode” error
ECHILD: “No child processes” error
ECHRNG: “Channel number out of range” error
ECOMM: “Communication error on send” error
ECONNABORTED: “Connection aborted” error
ECONNREFUSED: “Connection refused” error
ECONNRESET: “Connection reset” error
EDEADLK: “Resource deadlock avoided” error
EDEADLOCK: “File locking deadlock error” error
EDESTADDRREQ: “Destination address required” error
EDEVERR: “Device error; e.g., printer paper out” error
EDOM: “Mathematics argument out of domain of function” error
EDOOFUS: “Improper function use” error
EDOTDOT: “RFS specific error” error
EDQUOT: “Disk quota exceeded” error
EEXIST: “File exists” error
EFAULT: “Bad address” error
EFBIG: “File too large” error
EFTYPE: “Invalid file type or format” error
EHOSTDOWN: “Host is down” error
EHOSTUNREACH: “Host is unreachable” error
EHWPOISON: “Memory page has hardware error” error
EIDRM: “Identifier removed” error
EILSEQ: “Invalid or incomplete multibyte or wide character” error
EINPROGRESS: “Operation in progress” error
EINTR: “Interrupted function call” error
EINVAL: “Invalid argument” error
EIO: “Input/output error” error
EIPSEC: “IPsec processing failure” error
EISCONN: “Socket is connected” error
EISDIR: “Is a directory” error
EISNAM: “Is a named file type” error
EKEYEXPIRED: “Key has expired” error
EKEYREJECTED: “Key was rejected by service” error
EKEYREVOKED: “Key has been revoked” error
EL2HLT: “Level 2 halted” error
EL2NSYNC: “Level 2 not synchronized” error
EL3HLT: “Level 3 halted” error
EL3RST: “Level 3 reset” error
ELAST: “Largest errno value” error
ELIBACC: “Cannot access a needed shared library” error
ELIBBAD: “Accessing a corrupted shared library” error
ELIBEXEC: “Cannot exec a shared library directly” error
ELIBMAX: “Attempting to link in too many shared libraries” error
ELIBSCN: “.lib section in a.out corrupted” error
ELNRNG: “Link number out of range” error
ELOOP: “Too many levels of symbolic links” error
EMEDIUMTYPE: “Wrong medium type” error
EMFILE: “Too many open files” error
EMLINK: “Too many links” error
EMSGSIZE: “Message too long” error
EMULTIHOP: “Multihop attempted” error
ENAMETOOLONG: “Filename too long” error
ENAVAIL: “No XENIX semaphores available” error
ENEEDAUTH: “Need authenticator” error
ENETDOWN: “Network is down” error
ENETRESET: “Connection aborted by network” error
ENETUNREACH: “Network unreachable” error
ENFILE: “Too many open files in system” error
ENOANO: “No anode” error
ENOATTR: “Attribute not found” error
ENOBUFS: “No buffer space available” error
ENOCSI: “No CSI structure available” error
ENODATA: “No data available” error
ENODEV: “No such device” error
ENOENT: “No such file or directory” error
ENOEXEC: “Exec format error” error
ENOKEY: “Required key not available” error
ENOLCK: “No locks available” error
ENOLINK: “Link has been severed” error
ENOMEDIUM: “No medium found” error
ENOMEM: “Not enough space/cannot allocate memory” error
ENOMSG: “No message of the desired type” error
ENONET: “Machine is not on the network” error
ENOPKG: “Package not installed” error
ENOPOLICY: “No such policy” error
ENOPROTOOPT: “Protocol not available” error
ENOSPC: “No space left on device” error
ENOSR: “No STREAM resources” error
ENOSTR: “Not a STREAM” error
ENOSYS: “Functionality not implemented” error
ENOTBLK: “Block device required” error
ENOTCAPABLE: “Capabilities insufficient” error
ENOTCONN: “The socket is not connected” error
ENOTDIR: “Not a directory” error
ENOTEMPTY: “Directory not empty” error
ENOTNAM: “Not a XENIX named type file” error
ENOTRECOVERABLE: “State not recoverable” error
ENOTSOCK: “Not a socket” error
ENOTSUP: “Operation not supported” error
ENOTTY: “Inappropriate I/O control operation” error
ENOTUNIQ: “Name not unique on network” error
ENXIO: “No such device or address” error
EOPNOTSUPP: “Operation not supported on socket” error
EOVERFLOW: “Value too large to be stored in data type” error
EOWNERDEAD: “Owner died” error
EPERM: “Operation not permitted” error
EPFNOSUPPORT: “Protocol family not supported” error
EPIPE: “Broken pipe” error
EPROCLIM: “Too many processes” error
EPROCUNAVAIL: “Bad procedure for program” error
EPROGMISMATCH: “Program version wrong” error
EPROGUNAVAIL: “RPC program isn’t available” error
EPROTO: “Protocol error” error
EPROTONOSUPPORT: “Protocol not supported” error
EPROTOTYPE: “Protocol wrong type for socket” error
EPWROFF: “Device power is off” error
EQFULL: “Interface output queue is full” error
ERANGE: “Result too large” error
EREMCHG: “Remote address changed” error
EREMOTE: “Object is remote” error
EREMOTEIO: “Remote I/O error” error
ERESTART: “Interrupted system call should be restarted” error
ERFKILL: “Operation not possible due to RF-kill” error
EROFS: “Read-only file system” error
ERPCMISMATCH: “RPC version wrong” error
ESHLIBVERS: “Shared library version mismatch” error
ESHUTDOWN: “Cannot send after transport endpoint shutdown” error
ESOCKTNOSUPPORT: “Socket type not supported” error
ESPIPE: “Illegal seek” error
ESRCH: “No such process” error
ESRMNT: “Server mount error” error
ESTALE: “Stale file handle” error
ESTRPIPE: “Streams pipe error” error
ETIME: “Timer expired” error
ETIMEDOUT: “Connection timed out” error
ETOOMANYREFS: cannot splice“ error
ETXTBSY: “Text file busy” error
EUCLEAN: “Structure needs cleaning” error
EUNATCH: “Protocol driver not attached” error
EUSERS: “Too many users” error
EWOULDBLOCK: “Operation would block” error
EXDEV: “Invalid cross-device link” error
EXFULL: “Exchange full” error
NOERROR: No error