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 The error numbers that correspond to each symbolic name vary across
43 UNIX systems, and even across different architectures on Linux. There‐
44 fore, numeric values are not included as part of the list of error
45 names below. The perror(3) and strerror(3) functions can be used to
46 convert these names to corresponding textual error messages.
47 On any particular Linux system, one can obtain a list of all symbolic
49 error names and the corresponding error numbers using the errno(1) com‐
50 mand:
51 $ errno -l
53 EPERM 1 Operation not permitted
54 ENOENT 2 No such file or directory
55 ESRCH 3 No such process
56 EINTR 4 Interrupted system call
57 EIO 5 Input/output error
58 ...
59 The errno(1) command can also be used to look up individual error num‐
61 bers and names, and to search for errors using strings from the error
62 description, as in the following examples:
63 $ errno 2
65 ENOENT 2 No such file or directory
66 $ errno ESRCH
67 ESRCH 3 No such process
68 $ errno -s permission
69 EACCES 13 Permission denied
70 List of error names
72 In the list of the symbolic error names below, various names are marked
73 as follows:
74 * POSIX.1-2001: The name is defined by POSIX.1-2001, and is defined in
76 later POSIX.1 versions, unless otherwise indicated.
77 * POSIX.1-2008: The name is defined in POSIX.1-2008, but was not
79 present in earlier POSIX.1 standards.
80 * C99: The name is defined by C99. Below is a list of the symbolic
82 error names that are defined on Linux:
83 E2BIG Argument list too long (POSIX.1-2001).
85 EACCES Permission denied (POSIX.1-2001).
87 EADDRINUSE Address already in use (POSIX.1-2001).
89 EADDRNOTAVAIL Address not available (POSIX.1-2001).
91 EAFNOSUPPORT Address family not supported (POSIX.1-2001).
93 EAGAIN Resource temporarily unavailable (may be the same value
95 as EWOULDBLOCK) (POSIX.1-2001).
96 EALREADY Connection already in progress (POSIX.1-2001).
98 EBADE Invalid exchange.
100 EBADF Bad file descriptor (POSIX.1-2001).
102 EBADFD File descriptor in bad state.
104 EBADMSG Bad message (POSIX.1-2001).
106 EBADR Invalid request descriptor.
108 EBADRQC Invalid request code.
110 EBADSLT Invalid slot.
112 EBUSY Device or resource busy (POSIX.1-2001).
114 ECANCELED Operation canceled (POSIX.1-2001).
116 ECHILD No child processes (POSIX.1-2001).
118 ECHRNG Channel number out of range.
120 ECOMM Communication error on send.
122 ECONNABORTED Connection aborted (POSIX.1-2001).
124 ECONNREFUSED Connection refused (POSIX.1-2001).
126 ECONNRESET Connection reset (POSIX.1-2001).
128 EDEADLK Resource deadlock avoided (POSIX.1-2001).
130 EDEADLOCK Synonym for EDEADLK.
132 EDESTADDRREQ Destination address required (POSIX.1-2001).
134 EDOM Mathematics argument out of domain of function
136 (POSIX.1, C99).
137 EDQUOT Disk quota exceeded (POSIX.1-2001).
139 EEXIST File exists (POSIX.1-2001).
141 EFAULT Bad address (POSIX.1-2001).
143 EFBIG File too large (POSIX.1-2001).
145 EHOSTDOWN Host is down.
147 EHOSTUNREACH Host is unreachable (POSIX.1-2001).
149 EHWPOISON Memory page has hardware error.
151 EIDRM Identifier removed (POSIX.1-2001).
153 EILSEQ Invalid or incomplete multibyte or wide character
155 (POSIX.1, C99).
156 The text shown here is the glibc error description; in
158 POSIX.1, this error is described as "Illegal byte
159 sequence".
160 EINPROGRESS Operation in progress (POSIX.1-2001).
162 EINTR Interrupted function call (POSIX.1-2001); see sig‐
164 nal(7).
165 EINVAL Invalid argument (POSIX.1-2001).
167 EIO Input/output error (POSIX.1-2001).
169 EISCONN Socket is connected (POSIX.1-2001).
171 EISDIR Is a directory (POSIX.1-2001).
173 EISNAM Is a named type file.
175 EKEYEXPIRED Key has expired.
177 EKEYREJECTED Key was rejected by service.
179 EKEYREVOKED Key has been revoked.
181 EL2HLT Level 2 halted.
183 EL2NSYNC Level 2 not synchronized.
185 EL3HLT Level 3 halted.
187 EL3RST Level 3 reset.
189 ELIBACC Cannot access a needed shared library.
191 ELIBBAD Accessing a corrupted shared library.
193 ELIBMAX Attempting to link in too many shared libraries.
195 ELIBSCN .lib section in a.out corrupted
197 ELIBEXEC Cannot exec a shared library directly.
199 ELNRANGE Link number out of range.
201 ELOOP Too many levels of symbolic links (POSIX.1-2001).
203 EMEDIUMTYPE Wrong medium type.
205 EMFILE Too many open files (POSIX.1-2001). Commonly caused by
207 exceeding the RLIMIT_NOFILE resource limit described in
208 getrlimit(2).
209 EMLINK Too many links (POSIX.1-2001).
211 EMSGSIZE Message too long (POSIX.1-2001).
213 EMULTIHOP Multihop attempted (POSIX.1-2001).
215 ENAMETOOLONG Filename too long (POSIX.1-2001).
217 ENETDOWN Network is down (POSIX.1-2001).
219 ENETRESET Connection aborted by network (POSIX.1-2001).
221 ENETUNREACH Network unreachable (POSIX.1-2001).
223 ENFILE Too many open files in system (POSIX.1-2001). On
225 Linux, this is probably a result of encountering the
226 /proc/sys/fs/file-max limit (see proc(5)).
227 ENOANO No anode.
229 ENOBUFS No buffer space available (POSIX.1 (XSI STREAMS
231 option)).
232 ENODATA No message is available on the STREAM head read queue
234 (POSIX.1-2001).
235 ENODEV No such device (POSIX.1-2001).
237 ENOENT No such file or directory (POSIX.1-2001).
239 Typically, this error results when a specified pathname
241 does not exist, or one of the components in the direc‐
242 tory prefix of a pathname does not exist, or the speci‐
243 fied pathname is a dangling symbolic link.
244 ENOEXEC Exec format error (POSIX.1-2001).
246 ENOKEY Required key not available.
248 ENOLCK No locks available (POSIX.1-2001).
250 ENOLINK Link has been severed (POSIX.1-2001).
252 ENOMEDIUM No medium found.
254 ENOMEM Not enough space/cannot allocate memory (POSIX.1-2001).
256 ENOMSG No message of the desired type (POSIX.1-2001).
258 ENONET Machine is not on the network.
260 ENOPKG Package not installed.
262 ENOPROTOOPT Protocol not available (POSIX.1-2001).
264 ENOSPC No space left on device (POSIX.1-2001).
266 ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option)).
268 ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option)).
270 ENOSYS Function not implemented (POSIX.1-2001).
272 ENOTBLK Block device required.
274 ENOTCONN The socket is not connected (POSIX.1-2001).
276 ENOTDIR Not a directory (POSIX.1-2001).
278 ENOTEMPTY Directory not empty (POSIX.1-2001).
280 ENOTRECOVERABLE State not recoverable (POSIX.1-2008).
282 ENOTSOCK Not a socket (POSIX.1-2001).
284 ENOTSUP Operation not supported (POSIX.1-2001).
286 ENOTTY Inappropriate I/O control operation (POSIX.1-2001).
288 ENOTUNIQ Name not unique on network.
290 ENXIO No such device or address (POSIX.1-2001).
292 EOPNOTSUPP Operation not supported on socket (POSIX.1-2001).
294 (ENOTSUP and EOPNOTSUPP have the same value on Linux,
296 but according to POSIX.1 these error values should be
297 distinct.)
298 EOVERFLOW Value too large to be stored in data type
300 (POSIX.1-2001).
301 EOWNERDEAD Owner died (POSIX.1-2008).
303 EPERM Operation not permitted (POSIX.1-2001).
305 EPFNOSUPPORT Protocol family not supported.
307 EPIPE Broken pipe (POSIX.1-2001).
309 EPROTO Protocol error (POSIX.1-2001).
311 EPROTONOSUPPORT Protocol not supported (POSIX.1-2001).
313 EPROTOTYPE Protocol wrong type for socket (POSIX.1-2001).
315 ERANGE Result too large (POSIX.1, C99).
317 EREMCHG Remote address changed.
319 EREMOTE Object is remote.
321 EREMOTEIO Remote I/O error.
323 ERESTART Interrupted system call should be restarted.
325 ERFKILL Operation not possible due to RF-kill.
327 EROFS Read-only filesystem (POSIX.1-2001).
329 ESHUTDOWN Cannot send after transport endpoint shutdown.
331 ESPIPE Invalid seek (POSIX.1-2001).
333 ESOCKTNOSUPPORT Socket type not supported.
335 ESRCH No such process (POSIX.1-2001).
337 ESTALE Stale file handle (POSIX.1-2001).
339 This error can occur for NFS and for other filesystems.
341 ESTRPIPE Streams pipe error.
343 ETIME Timer expired (POSIX.1 (XSI STREAMS option)).
345 (POSIX.1 says "STREAM ioctl(2) timeout".)
347 ETIMEDOUT Connection timed out (POSIX.1-2001).
349 ETOOMANYREFS Too many references: cannot splice.
351 ETXTBSY Text file busy (POSIX.1-2001).
353 EUCLEAN Structure needs cleaning.
355 EUNATCH Protocol driver not attached.
357 EUSERS Too many users.
359 EWOULDBLOCK Operation would block (may be same value as EAGAIN)
361 (POSIX.1-2001).
362 EXDEV Improper link (POSIX.1-2001).
364 EXFULL Exchange full.
366
368 A common mistake is to do
369 if (somecall() == -1) {
371 printf("somecall() failed\n");
372 if (errno == ...) { ... }
373 }
374 where errno no longer needs to have the value it had upon return from
376 somecall() (i.e., it may have been changed by the printf(3)). If the
377 value of errno should be preserved across a library call, it must be
378 saved:
379 if (somecall() == -1) {
381 int errsv = errno;
382 printf("somecall() failed\n");
383 if (errsv == ...) { ... }
384 }
385 On some ancient systems, <errno.h> was not present or did not declare
387 errno, so that it was necessary to declare errno manually (i.e., extern
388 int errno). Do not do this. It long ago ceased to be necessary, and
389 it will cause problems with modern versions of the C library.
390
392 errno(1), err(3), error(3), perror(3), strerror(3)
393
395 This page is part of release 4.15 of the Linux man-pages project. A
396 description of the project, information about reporting bugs, and the
397 latest version of this page, can be found at
398 https://www.kernel.org/doc/man-pages/.
399 2018-02-02 ERRNO(3)