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2.2 Error Codes

The error code macros are defined in the header file errno.h. All of them expand into integer constant values. Some of these error codes can’t occur on GNU systems, but they can occur using the GNU C Library on other systems.

Macro: int EPERM

Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

Macro: int ENOENT

No such file or directory. This is a “file doesn’t exist” error for ordinary files that are referenced in contexts where they are expected to already exist.

Macro: int ESRCH

No process matches the specified process ID.

Macro: int EINTR

Interrupted function call; an asynchronous signal occurred and prevented completion of the call. When this happens, you should try the call again.

You can choose to have functions resume after a signal that is handled, rather than failing with EINTR; see Primitives Interrupted by Signals.

Macro: int EIO

Input/output error; usually used for physical read or write errors.

Macro: int ENXIO

No such device or address. The system tried to use the device represented by a file you specified, and it couldn’t find the device. This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer.

Macro: int E2BIG

Argument list too long; used when the arguments passed to a new program being executed with one of the exec functions (see Executing a File) occupy too much memory space. This condition never arises on GNU/Hurd systems.

Macro: int ENOEXEC

Invalid executable file format. This condition is detected by the exec functions; see Executing a File.

Macro: int EBADF

Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).

Macro: int ECHILD

There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren’t any processes to manipulate.

Macro: int EDEADLK

Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. The system does not guarantee that it will notice all such situations. This error means you got lucky and the system noticed; it might just hang. See File Locks, for an example.

Macro: int ENOMEM

No memory available. The system cannot allocate more virtual memory because its capacity is full.

Macro: int EACCES

Permission denied; the file permissions do not allow the attempted operation.

Macro: int EFAULT

Bad address; an invalid pointer was detected. On GNU/Hurd systems, this error never happens; you get a signal instead.

Macro: int ENOTBLK

A file that isn’t a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error.

Macro: int EBUSY

Resource busy; a system resource that can’t be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error.

Macro: int EEXIST

File exists; an existing file was specified in a context where it only makes sense to specify a new file.

Macro: int EXDEV

An attempt to make an improper link across file systems was detected. This happens not only when you use link (see Hard Links) but also when you rename a file with rename (see Renaming Files).

Macro: int ENODEV

The wrong type of device was given to a function that expects a particular sort of device.

Macro: int ENOTDIR

A file that isn’t a directory was specified when a directory is required.

Macro: int EISDIR

File is a directory; you cannot open a directory for writing, or create or remove hard links to it.

Macro: int EINVAL

Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.

Macro: int EMFILE

The current process has too many files open and can’t open any more. Duplicate descriptors do count toward this limit.

In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased. If you get this error, you might want to increase the RLIMIT_NOFILE limit or make it unlimited; see Limiting Resource Usage.

Macro: int ENFILE

There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see Linked Channels. This error never occurs on GNU/Hurd systems.

Macro: int ENOTTY

Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

Macro: int ETXTBSY

An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. Often using a debugger to run a program is considered having it open for writing and will cause this error. (The name stands for “text file busy”.) This is not an error on GNU/Hurd systems; the text is copied as necessary.

Macro: int EFBIG

File too big; the size of a file would be larger than allowed by the system.

Macro: int ENOSPC

No space left on device; write operation on a file failed because the disk is full.

Macro: int ESPIPE

Invalid seek operation (such as on a pipe).

Macro: int EROFS

An attempt was made to modify something on a read-only file system.

Too many links; the link count of a single file would become too large. rename can cause this error if the file being renamed already has as many links as it can take (see Renaming Files).

Macro: int EPIPE

Broken pipe; there is no process reading from the other end of a pipe. Every library function that returns this error code also generates a SIGPIPE signal; this signal terminates the program if not handled or blocked. Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

Macro: int EDOM

Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

Macro: int ERANGE

Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

Macro: int EAGAIN

Resource temporarily unavailable; the call might work if you try again later. The macro EWOULDBLOCK is another name for EAGAIN; they are always the same in the GNU C Library.

This error can happen in a few different situations:

  • An operation that would block was attempted on an object that has non-blocking mode selected. Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation). You can use select to find out when the operation will be possible; see Waiting for Input or Output.

    Portability Note: In many older Unix systems, this condition was indicated by EWOULDBLOCK, which was a distinct error code different from EAGAIN. To make your program portable, you should check for both codes and treat them the same.

  • A temporary resource shortage made an operation impossible. fork can return this error. It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop.

In the GNU C Library, this is another name for EAGAIN (above). The values are always the same, on every operating system.

C libraries in many older Unix systems have EWOULDBLOCK as a separate error code.


An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected. Some functions that must always block (such as connect; see Making a Connection) never return EAGAIN. Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time. Attempts to manipulate the object before the call completes return EALREADY. You can use the select function to find out when the pending operation has completed; see Waiting for Input or Output.

Macro: int EALREADY

An operation is already in progress on an object that has non-blocking mode selected.

Macro: int ENOTSOCK

A file that isn’t a socket was specified when a socket is required.

Macro: int EMSGSIZE

The size of a message sent on a socket was larger than the supported maximum size.


The socket type does not support the requested communications protocol.


You specified a socket option that doesn’t make sense for the particular protocol being used by the socket. See Socket Options.


The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See Creating a Socket.


The socket type is not supported.


The operation you requested is not supported. Some socket functions don’t make sense for all types of sockets, and others may not be implemented for all communications protocols. On GNU/Hurd systems, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call.


The socket communications protocol family you requested is not supported.


The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. See Sockets.


The requested socket address is already in use. See Socket Addresses.


The requested socket address is not available; for example, you tried to give a socket a name that doesn’t match the local host name. See Socket Addresses.

Macro: int ENETDOWN

A socket operation failed because the network was down.


A socket operation failed because the subnet containing the remote host was unreachable.

Macro: int ENETRESET

A network connection was reset because the remote host crashed.


A network connection was aborted locally.


A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

Macro: int ENOBUFS

The kernel’s buffers for I/O operations are all in use. In GNU, this error is always synonymous with ENOMEM; you may get one or the other from network operations.

Macro: int EISCONN

You tried to connect a socket that is already connected. See Making a Connection.

Macro: int ENOTCONN

The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data. For a connectionless socket (for datagram protocols, such as UDP), you get EDESTADDRREQ instead.


No default destination address was set for the socket. You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with connect.

Macro: int ESHUTDOWN

The socket has already been shut down.



Macro: int ETIMEDOUT

A socket operation with a specified timeout received no response during the timeout period.


A remote host refused to allow the network connection (typically because it is not running the requested service).

Macro: int ELOOP

Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.


Filename too long (longer than PATH_MAX; see Limits on File System Capacity) or host name too long (in gethostname or sethostname; see Host Identification).

Macro: int EHOSTDOWN

The remote host for a requested network connection is down.


The remote host for a requested network connection is not reachable.

Macro: int ENOTEMPTY

Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.

Macro: int EPROCLIM

This means that the per-user limit on new process would be exceeded by an attempted fork. See Limiting Resource Usage, for details on the RLIMIT_NPROC limit.

Macro: int EUSERS

The file quota system is confused because there are too many users.

Macro: int EDQUOT

The user’s disk quota was exceeded.

Macro: int ESTALE

Stale file handle. This indicates an internal confusion in the file system which is due to file system rearrangements on the server host for NFS file systems or corruption in other file systems. Repairing this condition usually requires unmounting, possibly repairing and remounting the file system.

Macro: int EREMOTE

An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on GNU/Hurd systems, making this error code impossible.)

Macro: int EBADRPC










Macro: int ENOLCK

No locks available. This is used by the file locking facilities; see File Locks. This error is never generated by GNU/Hurd systems, but it can result from an operation to an NFS server running another operating system.

Macro: int EFTYPE

Inappropriate file type or format. The file was the wrong type for the operation, or a data file had the wrong format.

On some systems chmod returns this error if you try to set the sticky bit on a non-directory file; see Assigning File Permissions.

Macro: int EAUTH


Macro: int ENEEDAUTH


Macro: int ENOSYS

Function not implemented. This indicates that the function called is not implemented at all, either in the C library itself or in the operating system. When you get this error, you can be sure that this particular function will always fail with ENOSYS unless you install a new version of the C library or the operating system.

Macro: int ENOTSUP

Not supported. A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all. For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only that specific object (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values.

If the entire function is not available at all in the implementation, it returns ENOSYS instead.

Macro: int EILSEQ

While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid.


On GNU/Hurd systems, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal. Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal. See Job Control, for information on process groups and these signals.

Macro: int EDIED

On GNU/Hurd systems, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file.

Macro: int ED

The experienced user will know what is wrong.

Macro: int EGREGIOUS

You did what?

Macro: int EIEIO

Go home and have a glass of warm, dairy-fresh milk.


This error code has no purpose.

Macro: int EBADMSG
Macro: int EIDRM
Macro: int EMULTIHOP
Macro: int ENODATA
Macro: int ENOMSG
Macro: int ENOSR
Macro: int ENOSTR
Macro: int EOVERFLOW
Macro: int EPROTO
Macro: int ETIME
Macro: int ECANCELED

Operation canceled; an asynchronous operation was canceled before it completed. See Perform I/O Operations in Parallel. When you call aio_cancel, the normal result is for the operations affected to complete with this error; see Cancellation of AIO Operations.

The following error codes are defined by the Linux/i386 kernel. They are not yet documented.

Macro: int ERESTART
Macro: int ECHRNG
Macro: int EL2NSYNC
Macro: int EL3HLT
Macro: int EL3RST
Macro: int ELNRNG
Macro: int EUNATCH
Macro: int ENOCSI
Macro: int EL2HLT
Macro: int EBADE
Macro: int EBADR
Macro: int EXFULL
Macro: int ENOANO
Macro: int EBADRQC
Macro: int EBADSLT
Macro: int EDEADLOCK
Macro: int EBFONT
Macro: int ENONET
Macro: int ENOPKG
Macro: int EADV
Macro: int ESRMNT
Macro: int ECOMM
Macro: int EDOTDOT
Macro: int ENOTUNIQ
Macro: int EBADFD
Macro: int EREMCHG
Macro: int ELIBACC
Macro: int ELIBBAD
Macro: int ELIBSCN
Macro: int ELIBMAX
Macro: int ELIBEXEC
Macro: int ESTRPIPE
Macro: int EUCLEAN
Macro: int ENOTNAM
Macro: int ENAVAIL
Macro: int EISNAM
Macro: int EREMOTEIO
Macro: int ENOMEDIUM
Macro: int ENOKEY
Macro: int ERFKILL
Macro: int EHWPOISON

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