No More Errors

If you don't like errors in your code, you will have to fix them. This handy list of Ruby's errors will hopefully help you do so! (And welcome back for the second season of Idiosyncratic Ruby!)

Built-in Exceptions

Exception Thrown by core Ruby when Remarks
NoMemoryError The Ruby interpreter could not allocate the required memory "Idiosyncratic" * 100_000_000_000
ScriptError - Not thrown directly
ScriptError → LoadError A source file could not be loaded, required, or required relatively It is meant for source code the current program is running and therefore should not be used for other kinds of files
ScriptError → NotImplementedError A feature of Ruby does not work on this platform/OS, for example, fork Although it is often used as "I have not implemented it (yet)", its original purpose is low-level platform support errors
ScriptError → SyntaxError Invalid Ruby syntax was found Do not raise when parsing some other file format fails. It is meant for source code that the current program is running.
SecurityError Mostly raised when attempting a forbidden $SAFE level 1 operation Use as base class for critical security issues, which should intentionally not be rescued by default
SignalException A signal is received The common way to catch a signal is the trap method
SignalException → Interrupt An interrupt signal (SIGINT) is received The common way to catch a signal is the trap method
StandardError - Base class for normal exceptions. See next table.
SystemExit Thrown by Kernel#exit Not thrown by Kernel#exit!
SystemStackError Raised for stack overflows, for example, in case of infinite recursion Cannot always be caught by the interpreter
fatal When a deadlock is discovered or this special vm exception is triggered Never rescuable. You can use this hack to get a reference to the error object: ObjectSpace.each_object(Class){|c| break c if c.ancestors.include?(Exception) && == 'fatal' }


These will be caught by the rescue statement without defining an explicit error class.

Exception Thrown by core Ruby when Remarks
ArgumentError Raised when a method is not given the proper arguments, for example, a wrong argument count Best class to inherit from for custom errors describing that the method usage was wrong
ArgumentError → UncaughtThrowError When throw is used without the proper catch around -
EncodingError Sometimes raised directly in unicode edge cases or when symbols are broken -
EncodingError → Encoding:: CompatibilityError Raised when trying to combine strings of two incompatible encodings Example: "Idiosyn" +"crätic".encode("UTF-16LE") under UTF-8 leads to Encoding::CompatibilityError: incompatible character encodings: UTF-8 and UTF-16LE
EncodingError → Encoding:: ConverterNotFoundError Raised when no encoding converter is available Example: "Idiosyncrätic".encode("ABC") under UTF-8 leads to Encoding::ConverterNotFoundError: code converter not found (UTF-8 to ABC)
EncodingError → Encoding:: InvalidByteSequenceError String contains byte sequences not valid in the current encoding Example: "Idiosyncr\u{D800}tic".encode("ASCII") under UTF-8 leads to Encoding::InvalidByteSequenceError: "\xED" followed by "\xA0" on UTF-8
EncodingError → Encoding:: UndefinedConversionError Raised when an encoding converter is available, but it cannot convert a specific codepoint Example: "\u{00A0}".encode("EUC-JP") under UTF-8 leads to Encoding::UndefinedConversionError: U+00A0 from UTF-8 to EUC-JP
FiberError Raised when an invalid operation is attempted on a Fiber Such as: Attempting to call/resume a dead Fiber, yield from the root fiber, calling a Fiber across threads
IOError Raised when an IO operation fails, like opening a file Pay attention that there are some failures will raise a SystemCallError, which is not a sub-class of IOError!
IOError → EOFError Raised by many IO operations when reaching the end of file -
IndexError Raised when an element or deducted value cannot be retrieved, for example, when using Array#fetch Thrown by many core classes like Array, Regex, String, Struct or some standard libraries like strscan
IndexError → KeyError Raised when an element cannot be retrieved (by a key), for example, when using Hash#fetch Mainly used by hashes
IndexError → StopIteration Raised when an external iterator has reached its end You can use the Kernel#loop method to automatically catch StopIteration
IndexError → StopIteration → ClosedQueueError Raised when trying to modify a closed Queue -
LocalJumpError Thrown when an invalid control flow operation is attempted, like calling yield from a method that was not passed a block to LocalJumpErrors contains a reason and an exit_value property from the last valid context
Math::DomainError An invalid mathematical operation was attempted, for example, trying to calculate the arctan of a value greater than 1 -
NameError Raised when referencing things that do not exist, like unknown constants -
NameError → NoMethodError Raised if trying to call a method that does not exist, and no method_missing is defined Trying to call non-existent private methods of self (like Kernel#puts) will result in NameError instead of NoMethodError, because it could also have been a local variable
RangeError A numerical number is off, most often it is too small or big For example, [].pack("C100000000000000000000") does not work, because pack requires the number to fit into long
RangeError → FloatDomainError Raised when trying to convert a special Float value (NaN, Float::INFINITY) to another number format that does not support special values -
RegexpError Raised when trying to dynamically create an invalid regex -
RuntimeError Unspecific occurrences Default error class that will be used when raising a string: raise "something"
RuntimeError → FrozenError Raised when trying to modify a frozen object Introduced with Ruby 2.5
SystemCallError - Lower level system call errors. See next table.
ThreadError Raised for invalid Thread operations -
TypeError Violations against Ruby's type system For example, "" + nil will lead to "TypeError: no implicit conversion of nil into String"
ZeroDivisionError An integer, complex, or rational number was tried to divide by another integer, complex, or rational number Floats and BigDecimal return Infinity, negative Infinity, or NaN


These errors are thrown by the operating system under certain circumstances. The exact list depends on your platform and can be found in a file called errno.h. You get such errors when working with processes, sockets, files, and other functionality that relies on low level system operations.

What follows is the list of errors that are known by Ruby. The error numbers and messages were generated on a recent ubuntu Linux machine. All system call errors are namespaced under Errno:: and can be dynamically created using

Exception No Message Example One-Liner¹
Errno::E2BIG 7 Argument list too long -
Errno::EACCES 13 Permission denied As non-root:'/root')
Errno::EADDRINUSE 98 Address already in use require "socket"; 2.times{"", 3003) }
Errno::EADDRNOTAVAIL 99 Cannot assign requested address -
Errno::EADV 68 Advertise error -
Errno::EAFNOSUPPORT 97 Address family not supported by protocol -
Errno::EAGAIN 11 Resource temporarily unavailable -
Errno::EAGAIN → IO::EAGAINWaitReadable² - - STDIN.read_nonblock(1) (if blocking)
Errno::EAGAIN → IO::EAGAINWaitWritable² - - -
Errno::EALREADY 114 Operation already in progress -
Errno::EBADE 52 Invalid exchange -
Errno::EBADF 9 Bad file descriptor
Errno::EBADFD 77 File descriptor in bad state -
Errno::EBADMSG 74 Bad message -
Errno::EBADR 53 Invalid request descriptor -
Errno::EBADRQC 56 Invalid request code -
Errno::EBADSLT 57 Invalid slot -
Errno::EBFONT 59 Bad font file format -
Errno::EBUSY 16 Device or resource busy -
Errno::ECANCELED 125 Operation canceled -
Errno::ECHILD 10 No child processes Process.wait (with no child processes)
Errno::ECHRNG 44 Channel number out of range -
Errno::ECOMM 70 Communication error on send -
Errno::ECONNABORTED 103 Software caused connection abort -
Errno::ECONNREFUSED 111 Connection refused require "socket";"", rand(987654321))
Errno::ECONNRESET 104 Connection reset by peer -
Errno::EDEADLK 35 Resource deadlock avoided -
Errno::EDEADLOCK 35 Resource deadlock avoided -
Errno::EDESTADDRREQ 89 Destination address required -
Errno::EDOM 33 Numerical argument out of domain -
Errno::EDOTDOT 73 RFS specific error -
Errno::EDQUOT 122 Disk quota exceeded -
Errno::EEXIST 17 File exists Dir.mkdir("/")
Errno::EFAULT 14 Bad address -
Errno::EFBIG 27 File too large -
Errno::EHOSTDOWN 112 Host is down -
Errno::EHOSTUNREACH 113 No route to host -
Errno::EHWPOISON 133 Memory page has hardware error -
Errno::EIDRM 43 Identifier removed -
Errno::EILSEQ 84 Invalid or incomplete multibyte or wide character -
Errno::EINPROGRESS 115 Operation now in progress -
Errno::EINPROGRESS → IO::EINPROGRESSWaitReadable² - - -
Errno::EINPROGRESS → IO::EINPROGRESSWaitWritable² - - -
Errno::EINTR 4 Interrupted system call -
Errno::EINVAL 22 Invalid argument Process.clock_gettime(42)
Errno::EIO 5 Input/output error -
Errno::EISCONN 106 Transport endpoint is already connected -
Errno::EISDIR 21 Is a directory"/")
Errno::EISNAM 120 Is a named type file -
Errno::EKEYEXPIRED 127 Key has expired -
Errno::EKEYREJECTED 129 Key was rejected by service -
Errno::EKEYREVOKED 128 Key has been revoked -
Errno::EL2HLT 51 Level 2 halted -
Errno::EL2NSYNC 45 Level 2 not synchronized -
Errno::EL3HLT 46 Level 3 halted -
Errno::EL3RST 47 Level 3 reset -
Errno::ELIBACC 79 Can not access a needed shared library -
Errno::ELIBBAD 80 Accessing a corrupted shared library -
Errno::ELIBEXEC 83 Cannot exec a shared library directly -
Errno::ELIBMAX 82 Attempting to link in too many shared libraries -
Errno::ELIBSCN 81 .lib section in a.out corrupted -
Errno::ELNRNG 48 Link number out of range -
Errno::ELOOP 40 Too many levels of symbolic links -
Errno::EMEDIUMTYPE 124 Wrong medium type -
Errno::EMFILE 24 Too many open files -
Errno::EMLINK 31 Too many links -
Errno::EMSGSIZE 90 Message too long -
Errno::EMULTIHOP 72 Multihop attempted -
Errno::ENAMETOOLONG 36 File name too long"Ruby"*1000)
Errno::ENAVAIL 119 No XENIX semaphores available -
Errno::ENETDOWN 100 Network is down -
Errno::ENETRESET 102 Network dropped connection on reset -
Errno::ENETUNREACH 101 Network is unreachable -
Errno::ENFILE 23 Too many open files in system -
Errno::ENOANO 55 No anode -
Errno::ENOBUFS 105 No buffer space available -
Errno::ENOCSI 50 No CSI structure available -
Errno::ENODATA 61 No data available -
Errno::ENODEV 19 No such device -
Errno::ENOENT 2 No such file or directory"does not exist")
Errno::ENOEXEC 8 Exec format error -
Errno::ENOKEY 126 Required key not available -
Errno::ENOLCK 37 No locks available -
Errno::ENOLINK 67 Link has been severed -
Errno::ENOMEDIUM 123 No medium found -
Errno::ENOMEM 12 Cannot allocate memory -
Errno::ENOMSG 42 No message of desired type -
Errno::ENONET 64 Machine is not on the network -
Errno::ENOPKG 65 Package not installed -
Errno::ENOPROTOOPT 92 Protocol not available -
Errno::ENOSPC 28 No space left on device -
Errno::ENOSR 63 Out of streams resources -
Errno::ENOSTR 60 Device not a stream -
Errno::ENOSYS 38 Function not implemented -
Errno::ENOTBLK 15 Block device required -
Errno::ENOTCONN 107 Transport endpoint is not connected -
Errno::ENOTDIR 20 Not a directory -
Errno::ENOTEMPTY 39 Directory not empty -
Errno::ENOTNAM 118 Not a XENIX named type file -
Errno::ENOTRECOVERABLE 131 State not recoverable -
Errno::ENOTSOCK 88 Socket operation on non-socket -
Errno::ENOTSUP 95 Operation not supported -
Errno::ENOTTY 25 Inappropriate ioctl for device -
Errno::ENOTUNIQ 76 Name not unique on network -
Errno::ENXIO 6 No such device or address -
Errno::EOPNOTSUPP 95 Operation not supported -
Errno::EOVERFLOW 75 Value too large for defined data type -
Errno::EOWNERDEAD 130 Owner died -
Errno::EPERM 1 Operation not permitted -
Errno::EPFNOSUPPORT 96 Protocol family not supported -
Errno::EPIPE 32 Broken pipe -
Errno::EPROTO 71 Protocol error -
Errno::EPROTONOSUPPORT 93 Protocol not supported -
Errno::EPROTOTYPE 91 Protocol wrong type for socket -
Errno::ERANGE 34 Numerical result out of range -
Errno::EREMCHG 78 Remote address changed -
Errno::EREMOTE 66 Object is remote -
Errno::EREMOTEIO 121 Remote I/O error -
Errno::ERESTART 85 Interrupted system call should be restarted -
Errno::ERFKILL 132 Operation not possible due to RF-kill -
Errno::EROFS 30 Read-only file system -
Errno::ESHUTDOWN 108 Cannot send after transport endpoint shutdown -
Errno::ESOCKTNOSUPPORT 94 Socket type not supported -
Errno::ESPIPE 29 Illegal seek -
Errno::ESRCH 3 No such process Process.kill(:KILL, 987654321)
Errno::ESRMNT 69 Surmount error -
Errno::ESTALE 116 Stale file handle -
Errno::ESTRPIPE 86 Streams pipe error -
Errno::ETIME 62 Timer expired -
Errno::ETIMEDOUT 110 Connection timed out -
Errno::ETOOMANYREFS 109 Too many references: cannot splice -
Errno::ETXTBSY 26 Text file busy -
Errno::EUCLEAN 117 Structure needs cleaning -
Errno::EUNATCH 49 Protocol driver not attached -
Errno::EUSERS 87 Too many users -
Errno::EWOULDBLOCK³ 11 Resource temporarily unavailable -
Errno::EXDEV 18 Invalid cross-device link -
Errno::EXFULL 54 Exchange full -
Errno::NOERROR 0 Success -

¹ Feel free to suggest more one-liners
² These include IO::WaitReadable/IO::WaitWritable. See IO#read_nonblock.
³ EAGAIN == EWOULDBLOCK on most systems


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