errno - Online Linux Manual PageSection : 3
Updated : 2022-12-04
Source : Linux man-pages 6.03
NAMEerrno − number of last error
LIBRARYStandard C library (libc, −lc)
SYNOPSIS#include <errno.h>
DESCRIPTIONThe <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.
errnoThe 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 namesValid 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 namesIn 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.
NOTESA 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 ALSOerrno(1), err(3), error(3), perror(3), strerror(3) 0
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