1errno(3) Library Functions Manual errno(3)
2
6 errno - number of last error
7
9 Standard C library (libc, -lc)
10
12 #include <errno.h>
13
15 The <errno.h> header file defines the integer variable errno, which is
16 set by system calls and some library functions in the event of an error
17 to indicate what went wrong.
18 errno
20 The value in errno is significant only when the return value of the
21 call indicated an error (i.e., -1 from most system calls; -1 or NULL
22 from most library functions); a function that succeeds is allowed to
23 change errno. The value of errno is never set to zero by any system
24 call or library function.
25 For some system calls and library functions (e.g., getpriority(2)), -1
27 is a valid return on success. In such cases, a successful return can
28 be distinguished from an error return by setting errno to zero before
29 the call, and then, if the call returns a status that indicates that an
30 error may have occurred, checking to see if errno has a nonzero value.
31 errno is defined by the ISO C standard to be a modifiable lvalue of
33 type int, and must not be explicitly declared; errno may be a macro.
34 errno is thread-local; setting it in one thread does not affect its
35 value in any other thread.
36 Error numbers and names
38 Valid error numbers are all positive numbers. The <errno.h> header
39 file defines symbolic names for each of the possible error numbers that
40 may appear in errno.
41 All the error names specified by POSIX.1 must have distinct values,
43 with the exception of EAGAIN and EWOULDBLOCK, which may be the same.
44 On Linux, these two have the same value on all architectures.
45 The error numbers that correspond to each symbolic name vary across
47 UNIX systems, and even across different architectures on Linux. There‐
48 fore, numeric values are not included as part of the list of error
49 names below. The perror(3) and strerror(3) functions can be used to
50 convert these names to corresponding textual error messages.
51 On any particular Linux system, one can obtain a list of all symbolic
53 error names and the corresponding error numbers using the errno(1) com‐
54 mand (part of the moreutils package):
55 $ errno -l
57 EPERM 1 Operation not permitted
58 ENOENT 2 No such file or directory
59 ESRCH 3 No such process
60 EINTR 4 Interrupted system call
61 EIO 5 Input/output error
62 ...
63 The errno(1) command can also be used to look up individual error num‐
65 bers and names, and to search for errors using strings from the error
66 description, as in the following examples:
67 $ errno 2
69 ENOENT 2 No such file or directory
70 $ errno ESRCH
71 ESRCH 3 No such process
72 $ errno -s permission
73 EACCES 13 Permission denied
74 List of error names
76 In the list of the symbolic error names below, various names are marked
77 as follows:
78 POSIX.1-2001
80 The name is defined by POSIX.1-2001, and is defined in later
81 POSIX.1 versions, unless otherwise indicated.
82 POSIX.1-2008
84 The name is defined in POSIX.1-2008, but was not present in ear‐
85 lier POSIX.1 standards.
86 C99 The name is defined by C99.
88 Below is a list of the symbolic error names that are defined on Linux:
90 E2BIG Argument list too long (POSIX.1-2001).
92 EACCES Permission denied (POSIX.1-2001).
94 EADDRINUSE Address already in use (POSIX.1-2001).
96 EADDRNOTAVAIL Address not available (POSIX.1-2001).
98 EAFNOSUPPORT Address family not supported (POSIX.1-2001).
100 EAGAIN Resource temporarily unavailable (may be the same value
102 as EWOULDBLOCK) (POSIX.1-2001).
103 EALREADY Connection already in progress (POSIX.1-2001).
105 EBADE Invalid exchange.
107 EBADF Bad file descriptor (POSIX.1-2001).
109 EBADFD File descriptor in bad state.
111 EBADMSG Bad message (POSIX.1-2001).
113 EBADR Invalid request descriptor.
115 EBADRQC Invalid request code.
117 EBADSLT Invalid slot.
119 EBUSY Device or resource busy (POSIX.1-2001).
121 ECANCELED Operation canceled (POSIX.1-2001).
123 ECHILD No child processes (POSIX.1-2001).
125 ECHRNG Channel number out of range.
127 ECOMM Communication error on send.
129 ECONNABORTED Connection aborted (POSIX.1-2001).
131 ECONNREFUSED Connection refused (POSIX.1-2001).
133 ECONNRESET Connection reset (POSIX.1-2001).
135 EDEADLK Resource deadlock avoided (POSIX.1-2001).
137 EDEADLOCK On most architectures, a synonym for EDEADLK. On some
139 architectures (e.g., Linux MIPS, PowerPC, SPARC), it is
140 a separate error code "File locking deadlock error".
141 EDESTADDRREQ Destination address required (POSIX.1-2001).
143 EDOM Mathematics argument out of domain of function
145 (POSIX.1, C99).
146 EDQUOT Disk quota exceeded (POSIX.1-2001).
148 EEXIST File exists (POSIX.1-2001).
150 EFAULT Bad address (POSIX.1-2001).
152 EFBIG File too large (POSIX.1-2001).
154 EHOSTDOWN Host is down.
156 EHOSTUNREACH Host is unreachable (POSIX.1-2001).
158 EHWPOISON Memory page has hardware error.
160 EIDRM Identifier removed (POSIX.1-2001).
162 EILSEQ Invalid or incomplete multibyte or wide character
164 (POSIX.1, C99).
165 The text shown here is the glibc error description; in
167 POSIX.1, this error is described as "Illegal byte se‐
168 quence".
169 EINPROGRESS Operation in progress (POSIX.1-2001).
171 EINTR Interrupted function call (POSIX.1-2001); see sig‐
173 nal(7).
174 EINVAL Invalid argument (POSIX.1-2001).
176 EIO Input/output error (POSIX.1-2001).
178 EISCONN Socket is connected (POSIX.1-2001).
180 EISDIR Is a directory (POSIX.1-2001).
182 EISNAM Is a named type file.
184 EKEYEXPIRED Key has expired.
186 EKEYREJECTED Key was rejected by service.
188 EKEYREVOKED Key has been revoked.
190 EL2HLT Level 2 halted.
192 EL2NSYNC Level 2 not synchronized.
194 EL3HLT Level 3 halted.
196 EL3RST Level 3 reset.
198 ELIBACC Cannot access a needed shared library.
200 ELIBBAD Accessing a corrupted shared library.
202 ELIBMAX Attempting to link in too many shared libraries.
204 ELIBSCN .lib section in a.out corrupted
206 ELIBEXEC Cannot exec a shared library directly.
208 ELNRNG Link number out of range.
210 ELOOP Too many levels of symbolic links (POSIX.1-2001).
212 EMEDIUMTYPE Wrong medium type.
214 EMFILE Too many open files (POSIX.1-2001). Commonly caused by
216 exceeding the RLIMIT_NOFILE resource limit described in
217 getrlimit(2). Can also be caused by exceeding the
218 limit specified in /proc/sys/fs/nr_open.
219 EMLINK Too many links (POSIX.1-2001).
221 EMSGSIZE Message too long (POSIX.1-2001).
223 EMULTIHOP Multihop attempted (POSIX.1-2001).
225 ENAMETOOLONG Filename too long (POSIX.1-2001).
227 ENETDOWN Network is down (POSIX.1-2001).
229 ENETRESET Connection aborted by network (POSIX.1-2001).
231 ENETUNREACH Network unreachable (POSIX.1-2001).
233 ENFILE Too many open files in system (POSIX.1-2001). On
235 Linux, this is probably a result of encountering the
236 /proc/sys/fs/file-max limit (see proc(5)).
237 ENOANO No anode.
239 ENOBUFS No buffer space available (POSIX.1 (XSI STREAMS op‐
241 tion)).
242 ENODATA The named attribute does not exist, or the process has
244 no access to this attribute; see xattr(7).
245 In POSIX.1-2001 (XSI STREAMS option), this error was
247 described as "No message is available on the STREAM
248 head read queue".
249 ENODEV No such device (POSIX.1-2001).
251 ENOENT No such file or directory (POSIX.1-2001).
253 Typically, this error results when a specified pathname
255 does not exist, or one of the components in the direc‐
256 tory prefix of a pathname does not exist, or the speci‐
257 fied pathname is a dangling symbolic link.
258 ENOEXEC Exec format error (POSIX.1-2001).
260 ENOKEY Required key not available.
262 ENOLCK No locks available (POSIX.1-2001).
264 ENOLINK Link has been severed (POSIX.1-2001).
266 ENOMEDIUM No medium found.
268 ENOMEM Not enough space/cannot allocate memory (POSIX.1-2001).
270 ENOMSG No message of the desired type (POSIX.1-2001).
272 ENONET Machine is not on the network.
274 ENOPKG Package not installed.
276 ENOPROTOOPT Protocol not available (POSIX.1-2001).
278 ENOSPC No space left on device (POSIX.1-2001).
280 ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option)).
282 ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option)).
284 ENOSYS Function not implemented (POSIX.1-2001).
286 ENOTBLK Block device required.
288 ENOTCONN The socket is not connected (POSIX.1-2001).
290 ENOTDIR Not a directory (POSIX.1-2001).
292 ENOTEMPTY Directory not empty (POSIX.1-2001).
294 ENOTRECOVERABLE State not recoverable (POSIX.1-2008).
296 ENOTSOCK Not a socket (POSIX.1-2001).
298 ENOTSUP Operation not supported (POSIX.1-2001).
300 ENOTTY Inappropriate I/O control operation (POSIX.1-2001).
302 ENOTUNIQ Name not unique on network.
304 ENXIO No such device or address (POSIX.1-2001).
306 EOPNOTSUPP Operation not supported on socket (POSIX.1-2001).
308 (ENOTSUP and EOPNOTSUPP have the same value on Linux,
310 but according to POSIX.1 these error values should be
311 distinct.)
312 EOVERFLOW Value too large to be stored in data type
314 (POSIX.1-2001).
315 EOWNERDEAD Owner died (POSIX.1-2008).
317 EPERM Operation not permitted (POSIX.1-2001).
319 EPFNOSUPPORT Protocol family not supported.
321 EPIPE Broken pipe (POSIX.1-2001).
323 EPROTO Protocol error (POSIX.1-2001).
325 EPROTONOSUPPORT Protocol not supported (POSIX.1-2001).
327 EPROTOTYPE Protocol wrong type for socket (POSIX.1-2001).
329 ERANGE Result too large (POSIX.1, C99).
331 EREMCHG Remote address changed.
333 EREMOTE Object is remote.
335 EREMOTEIO Remote I/O error.
337 ERESTART Interrupted system call should be restarted.
339 ERFKILL Operation not possible due to RF-kill.
341 EROFS Read-only filesystem (POSIX.1-2001).
343 ESHUTDOWN Cannot send after transport endpoint shutdown.
345 ESPIPE Invalid seek (POSIX.1-2001).
347 ESOCKTNOSUPPORT Socket type not supported.
349 ESRCH No such process (POSIX.1-2001).
351 ESTALE Stale file handle (POSIX.1-2001).
353 This error can occur for NFS and for other filesystems.
355 ESTRPIPE Streams pipe error.
357 ETIME Timer expired (POSIX.1 (XSI STREAMS option)).
359 (POSIX.1 says "STREAM ioctl(2) timeout".)
361 ETIMEDOUT Connection timed out (POSIX.1-2001).
363 ETOOMANYREFS Too many references: cannot splice.
365 ETXTBSY Text file busy (POSIX.1-2001).
367 EUCLEAN Structure needs cleaning.
369 EUNATCH Protocol driver not attached.
371 EUSERS Too many users.
373 EWOULDBLOCK Operation would block (may be same value as EAGAIN)
375 (POSIX.1-2001).
376 EXDEV Invalid cross-device link (POSIX.1-2001).
378 EXFULL Exchange full.
380
382 A common mistake is to do
383 if (somecall() == -1) {
385 printf("somecall() failed\n");
386 if (errno == ...) { ... }
387 }
388 where errno no longer needs to have the value it had upon return from
390 somecall() (i.e., it may have been changed by the printf(3)). If the
391 value of errno should be preserved across a library call, it must be
392 saved:
393 if (somecall() == -1) {
395 int errsv = errno;
396 printf("somecall() failed\n");
397 if (errsv == ...) { ... }
398 }
399 Note that the POSIX threads APIs do not set errno on error. Instead,
401 on failure they return an error number as the function result. These
402 error numbers have the same meanings as the error numbers returned in
403 errno by other APIs.
404 On some ancient systems, <errno.h> was not present or did not declare
406 errno, so that it was necessary to declare errno manually (i.e., extern
407 int errno). Do not do this. It long ago ceased to be necessary, and
408 it will cause problems with modern versions of the C library.
409
411 errno(1), err(3), error(3), perror(3), strerror(3)
412Linux man-pages 6.05 2022-12-04 errno(3)