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ERRNO(3) Linux Programmer's Manual ERRNO(3)

NAME

errno - 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.

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.

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:


$ 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
Synonym for EDEADLK.
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.
ELNRANGE
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).
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
No message is available on the STREAM head read queue (POSIX.1-2001).
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
Improper 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 == ...) { ... }
}

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)

COLOPHON

This page is part of release 4.16 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/.
2018-02-02