errno(3)

NAME
errno - number of last error

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <errno.h>

DESCRIPTION
The <errno.h> header file defines the integer variable errno, which is set by system calls and some library functions in the event of an error to indicate what went wrong.

  errno
The value in errno is significant only when the return value of the call indicated an error (i.e., -1 from most system calls; -1 or NULL from most library functions); a function that succeeds is allowed to change errno. The value of errno is never set to zero by any system call or library function.

For some system calls and library functions (e.g., getpriority(2)), -1 is a valid return on success. In such cases, a successful return can be distinguished from an error return by setting errno to zero before the call, and then, if the call returns a status that indicates that an error may have occurred, checking to see if errno has a nonzero value.

errno is defined by the ISO C standard to be a modifiable lvalue of type int, and must not be explicitly declared; errno may be a macro. errno is thread-local; setting it in one thread does not affect its value in any other thread.

  Error numbers and names
Valid error numbers are all positive numbers. The <errno.h> header file defines symbolic names for each of the possible error numbers that may appear in errno.

All the error names specified by POSIX.1 must have distinct values, with the exception of EAGAIN and EWOULDBLOCK, which may be the same. On Linux, these two have the same value on all architectures.

The error numbers that correspond to each symbolic name vary across UNIX systems, and even across different architectures on Linux. Therefore, numeric values are not included as part of the list of error names below. The perror(3) and strerror(3) functions can be used to convert these names to corresponding textual error messages.

On any particular Linux system, one can obtain a list of all symbolic error names and the corresponding error numbers using the errno(1) command (part of the moreutils package):

    $ errno -l
    EPERM 1 Operation not permitted
    ENOENT 2 No such file or directory
    ESRCH 3 No such process
    EINTR 4 Interrupted system call
    EIO 5 Input/output error
    ...

The errno(1) command can also be used to look up individual error numbers and names, and to search for errors using strings from the error description, as in the following examples:

    $ errno 2
    ENOENT 2 No such file or directory
    $ errno ESRCH
    ESRCH 3 No such process
    $ errno -s permission
    EACCES 13 Permission denied

  List of error names
In the list of the symbolic error names below, various names are marked as follows:

POSIX.1-2001
    The name is defined by POSIX.1-2001, and is defined in later POSIX.1 versions, unless otherwise indicated.

POSIX.1-2008
    The name is defined in POSIX.1-2008, but was not present in earlier POSIX.1 standards.

C99 The name is defined by C99.

Below is a list of the symbolic error names that are defined on Linux:

E2BIG
    Argument list too long (POSIX.1-2001).

EACCES
    Permission denied (POSIX.1-2001).

EADDRINUSE
    Address already in use (POSIX.1-2001).

EADDRNOTAVAIL
    Address not available (POSIX.1-2001).

EAFNOSUPPORT
    Address family not supported (POSIX.1-2001).

EAGAIN
    Resource temporarily unavailable (may be the same value as EWOULDBLOCK) (POSIX.1-2001).

EALREADY
    Connection already in progress (POSIX.1-2001).

EBADE
    Invalid exchange.

EBADF
    Bad file descriptor (POSIX.1-2001).

EBADFD
    File descriptor in bad state.

EBADMSG
    Bad message (POSIX.1-2001).

EBADR
    Invalid request descriptor.

EBADRQC
    Invalid request code.

EBADSLT
    Invalid slot.

EBUSY
    Device or resource busy (POSIX.1-2001).

ECANCELED
    Operation canceled (POSIX.1-2001).

ECHILD
    No child processes (POSIX.1-2001).

ECHRNG
    Channel number out of range.

ECOMM
    Communication error on send.

ECONNABORTED
    Connection aborted (POSIX.1-2001).

ECONNREFUSED
    Connection refused (POSIX.1-2001).

ECONNRESET
    Connection reset (POSIX.1-2001).

EDEADLK
    Resource deadlock avoided (POSIX.1-2001).

EDEADLOCK
    On most architectures, a synonym for EDEADLK. On some architectures (e.g., Linux MIPS, PowerPC, SPARC), it is a separate error code "File locking deadlock error".

EDESTADDRREQ
    Destination address required (POSIX.1-2001).

EDOM
    Mathematics argument out of domain of function (POSIX.1, C99).

EDQUOT
    Disk quota exceeded (POSIX.1-2001).

EEXIST
    File exists (POSIX.1-2001).

EFAULT
    Bad address (POSIX.1-2001).

EFBIG
    File too large (POSIX.1-2001).

EHOSTDOWN
    Host is down.

EHOSTUNREACH
    Host is unreachable (POSIX.1-2001).

EHWPOISON
    Memory page has hardware error.

EIDRM
    Identifier removed (POSIX.1-2001).

EILSEQ
    Invalid or incomplete multibyte or wide character (POSIX.1, C99).

    The text shown here is the glibc error description; in POSIX.1, this error is described as "Illegal byte sequence".

EINPROGRESS
    Operation in progress (POSIX.1-2001).

EINTR
    Interrupted function call (POSIX.1-2001); see signal(7).

EINVAL
    Invalid argument (POSIX.1-2001).

EIO Input/output error (POSIX.1-2001).

EISCONN
    Socket is connected (POSIX.1-2001).

EISDIR
    Is a directory (POSIX.1-2001).

EISNAM
    Is a named type file.

EKEYEXPIRED
    Key has expired.

EKEYREJECTED
    Key was rejected by service.

EKEYREVOKED
    Key has been revoked.

EL2HLT
    Level 2 halted.

EL2NSYNC
    Level 2 not synchronized.

EL3HLT
    Level 3 halted.

EL3RST
    Level 3 reset.

ELIBACC
    Cannot access a needed shared library.

ELIBBAD
    Accessing a corrupted shared library.

ELIBMAX
    Attempting to link in too many shared libraries.

ELIBSCN
    .lib section in a.out corrupted

ELIBEXEC
    Cannot exec a shared library directly.

ELNRNG
    Link number out of range.

ELOOP
    Too many levels of symbolic links (POSIX.1-2001).

EMEDIUMTYPE
    Wrong medium type.

EMFILE
    Too many open files (POSIX.1-2001). Commonly caused by exceeding the RLIMIT_NOFILE resource limit described in getrlimit(2). Can also be caused by exceeding the limit specified in /proc/sys/fs/nr_open.

EMLINK
    Too many links (POSIX.1-2001).

EMSGSIZE
    Message too long (POSIX.1-2001).

EMULTIHOP
    Multihop attempted (POSIX.1-2001).

ENAMETOOLONG
    Filename too long (POSIX.1-2001).

ENETDOWN
    Network is down (POSIX.1-2001).

ENETRESET
    Connection aborted by network (POSIX.1-2001).

ENETUNREACH
    Network unreachable (POSIX.1-2001).

ENFILE
    Too many open files in system (POSIX.1-2001). On Linux, this is probably a result of encountering the /proc/sys/fs/file-max limit (see proc(5)).

ENOANO
    No anode.

ENOBUFS
    No buffer space available (POSIX.1 (XSI STREAMS option)).

ENODATA
    The named attribute does not exist, or the process has no access to this attribute; see xattr(7).

    In POSIX.1-2001 (XSI STREAMS option), this error was described as "No message is available on the STREAM head read queue".

ENODEV
    No such device (POSIX.1-2001).

ENOENT
    No such file or directory (POSIX.1-2001).

    Typically, this error results when a specified pathname does not exist, or one of the components in the directory prefix of a pathname does not exist, or the specified pathname is a dangling symbolic link.

ENOEXEC
    Exec format error (POSIX.1-2001).

ENOKEY
    Required key not available.

ENOLCK
    No locks available (POSIX.1-2001).

ENOLINK
    Link has been severed (POSIX.1-2001).

ENOMEDIUM
    No medium found.

ENOMEM
    Not enough space/cannot allocate memory (POSIX.1-2001).

ENOMSG
    No message of the desired type (POSIX.1-2001).

ENONET
    Machine is not on the network.

ENOPKG
    Package not installed.

ENOPROTOOPT
    Protocol not available (POSIX.1-2001).

ENOSPC
    No space left on device (POSIX.1-2001).

ENOSR
    No STREAM resources (POSIX.1 (XSI STREAMS option)).

ENOSTR
    Not a STREAM (POSIX.1 (XSI STREAMS option)).

ENOSYS
    Function not implemented (POSIX.1-2001).

ENOTBLK
    Block device required.

ENOTCONN
    The socket is not connected (POSIX.1-2001).

ENOTDIR
    Not a directory (POSIX.1-2001).

ENOTEMPTY
    Directory not empty (POSIX.1-2001).

ENOTRECOVERABLE
    State not recoverable (POSIX.1-2008).

ENOTSOCK
    Not a socket (POSIX.1-2001).

ENOTSUP
    Operation not supported (POSIX.1-2001).

ENOTTY
    Inappropriate I/O control operation (POSIX.1-2001).

ENOTUNIQ
    Name not unique on network.

ENXIO
    No such device or address (POSIX.1-2001).

EOPNOTSUPP
    Operation not supported on socket (POSIX.1-2001).

    (ENOTSUP and EOPNOTSUPP have the same value on Linux, but according to POSIX.1 these error values should be distinct.)

EOVERFLOW
    Value too large to be stored in data type (POSIX.1-2001).

EOWNERDEAD
    Owner died (POSIX.1-2008).

EPERM
    Operation not permitted (POSIX.1-2001).

EPFNOSUPPORT
    Protocol family not supported.

EPIPE
    Broken pipe (POSIX.1-2001).

EPROTO
    Protocol error (POSIX.1-2001).

EPROTONOSUPPORT
    Protocol not supported (POSIX.1-2001).

EPROTOTYPE
    Protocol wrong type for socket (POSIX.1-2001).

ERANGE
    Result too large (POSIX.1, C99).

EREMCHG
    Remote address changed.

EREMOTE
    Object is remote.

EREMOTEIO
    Remote I/O error.

ERESTART
    Interrupted system call should be restarted.

ERFKILL
    Operation not possible due to RF-kill.

EROFS
    Read-only filesystem (POSIX.1-2001).

ESHUTDOWN
    Cannot send after transport endpoint shutdown.

ESPIPE
    Invalid seek (POSIX.1-2001).

ESOCKTNOSUPPORT
    Socket type not supported.

ESRCH
    No such process (POSIX.1-2001).

ESTALE
    Stale file handle (POSIX.1-2001).

    This error can occur for NFS and for other filesystems.

ESTRPIPE
    Streams pipe error.

ETIME
    Timer expired (POSIX.1 (XSI STREAMS option)).

    (POSIX.1 says "STREAM ioctl(2) timeout".)

ETIMEDOUT
    Connection timed out (POSIX.1-2001).

ETOOMANYREFS
    Too many references: cannot splice.

ETXTBSY
    Text file busy (POSIX.1-2001).

EUCLEAN
    Structure needs cleaning.

EUNATCH
    Protocol driver not attached.

EUSERS
    Too many users.

EWOULDBLOCK
    Operation would block (may be same value as EAGAIN) (POSIX.1-2001).

EXDEV
    Invalid cross-device link (POSIX.1-2001).

EXFULL
    Exchange full.

NOTES
A common mistake is to do

    if (somecall() == -1) {
        printf("somecall() failed\n");
        if (errno == ...) { ... }
    }

where errno no longer needs to have the value it had upon return from somecall() (i.e., it may have been changed by the printf(3)). If the value of errno should be preserved across a library call, it must be saved:

    if (somecall() == -1) {
        int errsv = errno;
        printf("somecall() failed\n");
        if (errsv == ...) { ... }
    }

Note that the POSIX threads APIs do not set errno on error. Instead, on failure they return an error number as the function result. These error numbers have the same meanings as the error numbers returned in errno by other APIs.

On some ancient systems, <errno.h> was not present or did not declare errno, so that it was necessary to declare errno manually (i.e., extern int errno). Do not do this. It long ago ceased to be necessary, and it will cause problems with modern versions of the C library.

SEE ALSO
errno(1), err(3), error(3), perror(3), strerror(3)