1ERRNO(3) Linux Programmer's Manual ERRNO(3)
2
6 errno - number of last error
7
9 #include <errno.h>
10
12 The <errno.h> header file defines the integer variable errno, which is
13 set by system calls and some library functions in the event of an error
14 to indicate what went wrong.
15 errno
17 The value in errno is significant only when the return value of the
18 call indicated an error (i.e., -1 from most system calls; -1 or NULL
19 from most library functions); a function that succeeds is allowed to
20 change errno. The value of errno is never set to zero by any system
21 call or library function.
22 For some system calls and library functions (e.g., getpriority(2)), -1
24 is a valid return on success. In such cases, a successful return can
25 be distinguished from an error return by setting errno to zero before
26 the call, and then, if the call returns a status that indicates that an
27 error may have occurred, checking to see if errno has a nonzero value.
28 errno is defined by the ISO C standard to be a modifiable lvalue of
30 type int, and must not be explicitly declared; errno may be a macro.
31 errno is thread-local; setting it in one thread does not affect its
32 value in any other thread.
33 Error numbers and names
35 Valid error numbers are all positive numbers. The <errno.h> header
36 file defines symbolic names for each of the possible error numbers that
37 may appear in errno.
38 All the error names specified by POSIX.1 must have distinct values,
40 with the exception of EAGAIN and EWOULDBLOCK, which may be the same.
41 On Linux, these two have the same value on all architectures.
42 The error numbers that correspond to each symbolic name vary across
44 UNIX systems, and even across different architectures on Linux. There‐
45 fore, numeric values are not included as part of the list of error
46 names below. The perror(3) and strerror(3) functions can be used to
47 convert these names to corresponding textual error messages.
48 On any particular Linux system, one can obtain a list of all symbolic
50 error names and the corresponding error numbers using the errno(1) com‐
51 mand (part of the moreutils package):
52 $ errno -l
54 EPERM 1 Operation not permitted
55 ENOENT 2 No such file or directory
56 ESRCH 3 No such process
57 EINTR 4 Interrupted system call
58 EIO 5 Input/output error
59 ...
60 The errno(1) command can also be used to look up individual error num‐
62 bers and names, and to search for errors using strings from the error
63 description, as in the following examples:
64 $ errno 2
66 ENOENT 2 No such file or directory
67 $ errno ESRCH
68 ESRCH 3 No such process
69 $ errno -s permission
70 EACCES 13 Permission denied
71 List of error names
73 In the list of the symbolic error names below, various names are marked
74 as follows:
75 * POSIX.1-2001: The name is defined by POSIX.1-2001, and is defined in
77 later POSIX.1 versions, unless otherwise indicated.
78 * POSIX.1-2008: The name is defined in POSIX.1-2008, but was not
80 present in earlier POSIX.1 standards.
81 * C99: The name is defined by C99. Below is a list of the symbolic
83 error names that are defined on Linux:
84 E2BIG Argument list too long (POSIX.1-2001).
86 EACCES Permission denied (POSIX.1-2001).
88 EADDRINUSE Address already in use (POSIX.1-2001).
90 EADDRNOTAVAIL Address not available (POSIX.1-2001).
92 EAFNOSUPPORT Address family not supported (POSIX.1-2001).
94 EAGAIN Resource temporarily unavailable (may be the same value
96 as EWOULDBLOCK) (POSIX.1-2001).
97 EALREADY Connection already in progress (POSIX.1-2001).
99 EBADE Invalid exchange.
101 EBADF Bad file descriptor (POSIX.1-2001).
103 EBADFD File descriptor in bad state.
105 EBADMSG Bad message (POSIX.1-2001).
107 EBADR Invalid request descriptor.
109 EBADRQC Invalid request code.
111 EBADSLT Invalid slot.
113 EBUSY Device or resource busy (POSIX.1-2001).
115 ECANCELED Operation canceled (POSIX.1-2001).
117 ECHILD No child processes (POSIX.1-2001).
119 ECHRNG Channel number out of range.
121 ECOMM Communication error on send.
123 ECONNABORTED Connection aborted (POSIX.1-2001).
125 ECONNREFUSED Connection refused (POSIX.1-2001).
127 ECONNRESET Connection reset (POSIX.1-2001).
129 EDEADLK Resource deadlock avoided (POSIX.1-2001).
131 EDEADLOCK On most architectures, a synonym for EDEADLK. On some
133 architectures (e.g., Linux MIPS, PowerPC, SPARC), it is
134 a separate error code "File locking deadlock error".
135 EDESTADDRREQ Destination address required (POSIX.1-2001).
137 EDOM Mathematics argument out of domain of function
139 (POSIX.1, C99).
140 EDQUOT Disk quota exceeded (POSIX.1-2001).
142 EEXIST File exists (POSIX.1-2001).
144 EFAULT Bad address (POSIX.1-2001).
146 EFBIG File too large (POSIX.1-2001).
148 EHOSTDOWN Host is down.
150 EHOSTUNREACH Host is unreachable (POSIX.1-2001).
152 EHWPOISON Memory page has hardware error.
154 EIDRM Identifier removed (POSIX.1-2001).
156 EILSEQ Invalid or incomplete multibyte or wide character
158 (POSIX.1, C99).
159 The text shown here is the glibc error description; in
161 POSIX.1, this error is described as "Illegal byte
162 sequence".
163 EINPROGRESS Operation in progress (POSIX.1-2001).
165 EINTR Interrupted function call (POSIX.1-2001); see sig‐
167 nal(7).
168 EINVAL Invalid argument (POSIX.1-2001).
170 EIO Input/output error (POSIX.1-2001).
172 EISCONN Socket is connected (POSIX.1-2001).
174 EISDIR Is a directory (POSIX.1-2001).
176 EISNAM Is a named type file.
178 EKEYEXPIRED Key has expired.
180 EKEYREJECTED Key was rejected by service.
182 EKEYREVOKED Key has been revoked.
184 EL2HLT Level 2 halted.
186 EL2NSYNC Level 2 not synchronized.
188 EL3HLT Level 3 halted.
190 EL3RST Level 3 reset.
192 ELIBACC Cannot access a needed shared library.
194 ELIBBAD Accessing a corrupted shared library.
196 ELIBMAX Attempting to link in too many shared libraries.
198 ELIBSCN .lib section in a.out corrupted
200 ELIBEXEC Cannot exec a shared library directly.
202 ELNRANGE Link number out of range.
204 ELOOP Too many levels of symbolic links (POSIX.1-2001).
206 EMEDIUMTYPE Wrong medium type.
208 EMFILE Too many open files (POSIX.1-2001). Commonly caused by
210 exceeding the RLIMIT_NOFILE resource limit described in
211 getrlimit(2).
212 EMLINK Too many links (POSIX.1-2001).
214 EMSGSIZE Message too long (POSIX.1-2001).
216 EMULTIHOP Multihop attempted (POSIX.1-2001).
218 ENAMETOOLONG Filename too long (POSIX.1-2001).
220 ENETDOWN Network is down (POSIX.1-2001).
222 ENETRESET Connection aborted by network (POSIX.1-2001).
224 ENETUNREACH Network unreachable (POSIX.1-2001).
226 ENFILE Too many open files in system (POSIX.1-2001). On
228 Linux, this is probably a result of encountering the
229 /proc/sys/fs/file-max limit (see proc(5)).
230 ENOANO No anode.
232 ENOBUFS No buffer space available (POSIX.1 (XSI STREAMS
234 option)).
235 ENODATA No message is available on the STREAM head read queue
237 (POSIX.1-2001).
238 ENODEV No such device (POSIX.1-2001).
240 ENOENT No such file or directory (POSIX.1-2001).
242 Typically, this error results when a specified pathname
244 does not exist, or one of the components in the direc‐
245 tory prefix of a pathname does not exist, or the speci‐
246 fied pathname is a dangling symbolic link.
247 ENOEXEC Exec format error (POSIX.1-2001).
249 ENOKEY Required key not available.
251 ENOLCK No locks available (POSIX.1-2001).
253 ENOLINK Link has been severed (POSIX.1-2001).
255 ENOMEDIUM No medium found.
257 ENOMEM Not enough space/cannot allocate memory (POSIX.1-2001).
259 ENOMSG No message of the desired type (POSIX.1-2001).
261 ENONET Machine is not on the network.
263 ENOPKG Package not installed.
265 ENOPROTOOPT Protocol not available (POSIX.1-2001).
267 ENOSPC No space left on device (POSIX.1-2001).
269 ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option)).
271 ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option)).
273 ENOSYS Function not implemented (POSIX.1-2001).
275 ENOTBLK Block device required.
277 ENOTCONN The socket is not connected (POSIX.1-2001).
279 ENOTDIR Not a directory (POSIX.1-2001).
281 ENOTEMPTY Directory not empty (POSIX.1-2001).
283 ENOTRECOVERABLE State not recoverable (POSIX.1-2008).
285 ENOTSOCK Not a socket (POSIX.1-2001).
287 ENOTSUP Operation not supported (POSIX.1-2001).
289 ENOTTY Inappropriate I/O control operation (POSIX.1-2001).
291 ENOTUNIQ Name not unique on network.
293 ENXIO No such device or address (POSIX.1-2001).
295 EOPNOTSUPP Operation not supported on socket (POSIX.1-2001).
297 (ENOTSUP and EOPNOTSUPP have the same value on Linux,
299 but according to POSIX.1 these error values should be
300 distinct.)
301 EOVERFLOW Value too large to be stored in data type
303 (POSIX.1-2001).
304 EOWNERDEAD Owner died (POSIX.1-2008).
306 EPERM Operation not permitted (POSIX.1-2001).
308 EPFNOSUPPORT Protocol family not supported.
310 EPIPE Broken pipe (POSIX.1-2001).
312 EPROTO Protocol error (POSIX.1-2001).
314 EPROTONOSUPPORT Protocol not supported (POSIX.1-2001).
316 EPROTOTYPE Protocol wrong type for socket (POSIX.1-2001).
318 ERANGE Result too large (POSIX.1, C99).
320 EREMCHG Remote address changed.
322 EREMOTE Object is remote.
324 EREMOTEIO Remote I/O error.
326 ERESTART Interrupted system call should be restarted.
328 ERFKILL Operation not possible due to RF-kill.
330 EROFS Read-only filesystem (POSIX.1-2001).
332 ESHUTDOWN Cannot send after transport endpoint shutdown.
334 ESPIPE Invalid seek (POSIX.1-2001).
336 ESOCKTNOSUPPORT Socket type not supported.
338 ESRCH No such process (POSIX.1-2001).
340 ESTALE Stale file handle (POSIX.1-2001).
342 This error can occur for NFS and for other filesystems.
344 ESTRPIPE Streams pipe error.
346 ETIME Timer expired (POSIX.1 (XSI STREAMS option)).
348 (POSIX.1 says "STREAM ioctl(2) timeout".)
350 ETIMEDOUT Connection timed out (POSIX.1-2001).
352 ETOOMANYREFS Too many references: cannot splice.
354 ETXTBSY Text file busy (POSIX.1-2001).
356 EUCLEAN Structure needs cleaning.
358 EUNATCH Protocol driver not attached.
360 EUSERS Too many users.
362 EWOULDBLOCK Operation would block (may be same value as EAGAIN)
364 (POSIX.1-2001).
365 EXDEV Improper link (POSIX.1-2001).
367 EXFULL Exchange full.
369
371 A common mistake is to do
372 if (somecall() == -1) {
374 printf("somecall() failed\n");
375 if (errno == ...) { ... }
376 }
377 where errno no longer needs to have the value it had upon return from
379 somecall() (i.e., it may have been changed by the printf(3)). If the
380 value of errno should be preserved across a library call, it must be
381 saved:
382 if (somecall() == -1) {
384 int errsv = errno;
385 printf("somecall() failed\n");
386 if (errsv == ...) { ... }
387 }
388 On some ancient systems, <errno.h> was not present or did not declare
390 errno, so that it was necessary to declare errno manually (i.e., extern
391 int errno). Do not do this. It long ago ceased to be necessary, and
392 it will cause problems with modern versions of the C library.
393
395 errno(1), err(3), error(3), perror(3), strerror(3)
396
398 This page is part of release 5.04 of the Linux man-pages project. A
399 description of the project, information about reporting bugs, and the
400 latest version of this page, can be found at
401 https://www.kernel.org/doc/man-pages/.
402 2019-10-10 ERRNO(3)