Errno(3) number of last error

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. Its value 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.

Valid error numbers are all nonzero; 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.

All the error names specified by POSIX.1 must have distinct values, with the exception of EAGAIN and EWOULDBLOCK, which may be the same.

Below is a list of the symbolic error names that are defined on Linux. Some of these are marked POSIX.1, indicating that the name is defined by POSIX.1-2001, or C99, indicating that the name is defined by C99.

E2BIG
Argument list too long (POSIX.1)
EACCES
Permission denied (POSIX.1)
EADDRINUSE
Address already in use (POSIX.1)
EADDRNOTAVAIL
Address not available (POSIX.1)
EAFNOSUPPORT
Address family not supported (POSIX.1)
EAGAIN
Resource temporarily unavailable (may be the same value as EWOULDBLOCK) (POSIX.1)
EALREADY
Connection already in progress (POSIX.1)
EBADE
Invalid exchange
EBADF
Bad file descriptor (POSIX.1)
EBADFD
File descriptor in bad state
EBADMSG
Bad message (POSIX.1)
EBADR
Invalid request descriptor
EBADRQC
Invalid request code
EBADSLT
Invalid slot
EBUSY
Device or resource busy (POSIX.1)
ECANCELED
Operation canceled (POSIX.1)
ECHILD
No child processes (POSIX.1)
ECHRNG
Channel number out of range
ECOMM
Communication error on send
ECONNABORTED
Connection aborted (POSIX.1)
ECONNREFUSED
Connection refused (POSIX.1)
ECONNRESET
Connection reset (POSIX.1)
EDEADLK
Resource deadlock avoided (POSIX.1)
EDEADLOCK
Synonym for EDEADLK
EDESTADDRREQ
Destination address required (POSIX.1)
EDOM
Mathematics argument out of domain of function (POSIX.1, C99)
EDQUOT
Disk quota exceeded (POSIX.1)
EEXIST
File exists (POSIX.1)
EFAULT
Bad address (POSIX.1)
EFBIG
File too large (POSIX.1)
EHOSTDOWN
Host is down
EHOSTUNREACH
Host is unreachable (POSIX.1)
EIDRM
Identifier removed (POSIX.1)
EILSEQ
Illegal byte sequence (POSIX.1, C99)
EINPROGRESS
Operation in progress (POSIX.1)
EINTR
Interrupted function call (POSIX.1); see signal(7).
EINVAL
Invalid argument (POSIX.1)
EIO
Input/output error (POSIX.1)
EISCONN
Socket is connected (POSIX.1)
EISDIR
Is a directory (POSIX.1)
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 halted
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
ELOOP
Too many levels of symbolic links (POSIX.1)
EMEDIUMTYPE
Wrong medium type
EMFILE
Too many open files (POSIX.1); commonly caused by exceeding the RLIMIT_NOFILE resource limit described in getrlimit(2)
EMLINK
Too many links (POSIX.1)
EMSGSIZE
Message too long (POSIX.1)
EMULTIHOP
Multihop attempted (POSIX.1)
ENAMETOOLONG
Filename too long (POSIX.1)
ENETDOWN
Network is down (POSIX.1)
ENETRESET
Connection aborted by network (POSIX.1)
ENETUNREACH
Network unreachable (POSIX.1)
ENFILE
Too many open files in system (POSIX.1); on Linux, this is probably a result of encountering the /proc/sys/fs/file-max limit (see proc(5)).
ENOBUFS
No buffer space available (POSIX.1 (XSI STREAMS option))
ENODATA
No message is available on the STREAM head read queue (POSIX.1)
ENODEV
No such device (POSIX.1)
ENOENT
No such file or directory (POSIX.1)

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)
ENOKEY
Required key not available
ENOLCK
No locks available (POSIX.1)
ENOLINK
Link has been severed (POSIX.1)
ENOMEDIUM
No medium found
ENOMEM
Not enough space (POSIX.1)
ENOMSG
No message of the desired type (POSIX.1)
ENONET
Machine is not on the network
ENOPKG
Package not installed
ENOPROTOOPT
Protocol not available (POSIX.1)
ENOSPC
No space left on device (POSIX.1)
ENOSR
No STREAM resources (POSIX.1 (XSI STREAMS option))
ENOSTR
Not a STREAM (POSIX.1 (XSI STREAMS option))
ENOSYS
Function not implemented (POSIX.1)
ENOTBLK
Block device required
ENOTCONN
The socket is not connected (POSIX.1)
ENOTDIR
Not a directory (POSIX.1)
ENOTEMPTY
Directory not empty (POSIX.1)
ENOTSOCK
Not a socket (POSIX.1)
ENOTSUP
Operation not supported (POSIX.1)
ENOTTY
Inappropriate I/O control operation (POSIX.1)
ENOTUNIQ
Name not unique on network
ENXIO
No such device or address (POSIX.1)
EOPNOTSUPP
Operation not supported on socket (POSIX.1)

(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)
EPERM
Operation not permitted (POSIX.1)
EPFNOSUPPORT
Protocol family not supported
EPIPE
Broken pipe (POSIX.1)
EPROTO
Protocol error (POSIX.1)
EPROTONOSUPPORT
Protocol not supported (POSIX.1)
EPROTOTYPE
Protocol wrong type for socket (POSIX.1)
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
EROFS
Read-only filesystem (POSIX.1)
ESHUTDOWN
Cannot send after transport endpoint shutdown
ESPIPE
Invalid seek (POSIX.1)
ESOCKTNOSUPPORT
Socket type not supported
ESRCH
No such process (POSIX.1)
ESTALE
Stale file handle (POSIX.1)

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)
ETXTBSY
Text file busy (POSIX.1)
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)
EXDEV
Improper link (POSIX.1)
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 == ...) { ... }
}

It was common in traditional C to declare errno manually (i.e., extern int errno) instead of including <errno.h>. Do not do this. It will not work with modern versions of the C library. However, on (very) old UNIX systems, there may be no <errno.h> and the declaration is needed.

COLOPHON

This page is part of release 4.06 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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