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. 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 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). 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.
SEE ALSO
errno(1), err(3), error(3), perror(3), strerror(3)
COLOPHON
This page is part of release 4.09 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/.
2016-12-12 ERRNO(3)
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