ERRNO
Section: Linux Programmer's Manual (3)
Updated: 2020-11-01
Page Index
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.
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.
- 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).
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
-
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 == ...) { ... }
}
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)
COLOPHON
This page is part of release 5.10 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/.