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.
83 Below is a list of the symbolic error names that are defined on Linux:
85 E2BIG Argument list too long (POSIX.1-2001).
87 EACCES Permission denied (POSIX.1-2001).
89 EADDRINUSE Address already in use (POSIX.1-2001).
91 EADDRNOTAVAIL Address not available (POSIX.1-2001).
93 EAFNOSUPPORT Address family not supported (POSIX.1-2001).
95 EAGAIN Resource temporarily unavailable (may be the same value
97 as EWOULDBLOCK) (POSIX.1-2001).
98 EALREADY Connection already in progress (POSIX.1-2001).
100 EBADE Invalid exchange.
102 EBADF Bad file descriptor (POSIX.1-2001).
104 EBADFD File descriptor in bad state.
106 EBADMSG Bad message (POSIX.1-2001).
108 EBADR Invalid request descriptor.
110 EBADRQC Invalid request code.
112 EBADSLT Invalid slot.
114 EBUSY Device or resource busy (POSIX.1-2001).
116 ECANCELED Operation canceled (POSIX.1-2001).
118 ECHILD No child processes (POSIX.1-2001).
120 ECHRNG Channel number out of range.
122 ECOMM Communication error on send.
124 ECONNABORTED Connection aborted (POSIX.1-2001).
126 ECONNREFUSED Connection refused (POSIX.1-2001).
128 ECONNRESET Connection reset (POSIX.1-2001).
130 EDEADLK Resource deadlock avoided (POSIX.1-2001).
132 EDEADLOCK On most architectures, a synonym for EDEADLK. On some
134 architectures (e.g., Linux MIPS, PowerPC, SPARC), it is
135 a separate error code "File locking deadlock error".
136 EDESTADDRREQ Destination address required (POSIX.1-2001).
138 EDOM Mathematics argument out of domain of function
140 (POSIX.1, C99).
141 EDQUOT Disk quota exceeded (POSIX.1-2001).
143 EEXIST File exists (POSIX.1-2001).
145 EFAULT Bad address (POSIX.1-2001).
147 EFBIG File too large (POSIX.1-2001).
149 EHOSTDOWN Host is down.
151 EHOSTUNREACH Host is unreachable (POSIX.1-2001).
153 EHWPOISON Memory page has hardware error.
155 EIDRM Identifier removed (POSIX.1-2001).
157 EILSEQ Invalid or incomplete multibyte or wide character
159 (POSIX.1, C99).
160 The text shown here is the glibc error description; in
162 POSIX.1, this error is described as "Illegal byte se‐
163 quence".
164 EINPROGRESS Operation in progress (POSIX.1-2001).
166 EINTR Interrupted function call (POSIX.1-2001); see sig‐
168 nal(7).
169 EINVAL Invalid argument (POSIX.1-2001).
171 EIO Input/output error (POSIX.1-2001).
173 EISCONN Socket is connected (POSIX.1-2001).
175 EISDIR Is a directory (POSIX.1-2001).
177 EISNAM Is a named type file.
179 EKEYEXPIRED Key has expired.
181 EKEYREJECTED Key was rejected by service.
183 EKEYREVOKED Key has been revoked.
185 EL2HLT Level 2 halted.
187 EL2NSYNC Level 2 not synchronized.
189 EL3HLT Level 3 halted.
191 EL3RST Level 3 reset.
193 ELIBACC Cannot access a needed shared library.
195 ELIBBAD Accessing a corrupted shared library.
197 ELIBMAX Attempting to link in too many shared libraries.
199 ELIBSCN .lib section in a.out corrupted
201 ELIBEXEC Cannot exec a shared library directly.
203 ELNRANGE Link number out of range.
205 ELOOP Too many levels of symbolic links (POSIX.1-2001).
207 EMEDIUMTYPE Wrong medium type.
209 EMFILE Too many open files (POSIX.1-2001). Commonly caused by
211 exceeding the RLIMIT_NOFILE resource limit described in
212 getrlimit(2). Can also be caused by exceeding the
213 limit specified in /proc/sys/fs/nr_open.
214 EMLINK Too many links (POSIX.1-2001).
216 EMSGSIZE Message too long (POSIX.1-2001).
218 EMULTIHOP Multihop attempted (POSIX.1-2001).
220 ENAMETOOLONG Filename too long (POSIX.1-2001).
222 ENETDOWN Network is down (POSIX.1-2001).
224 ENETRESET Connection aborted by network (POSIX.1-2001).
226 ENETUNREACH Network unreachable (POSIX.1-2001).
228 ENFILE Too many open files in system (POSIX.1-2001). On
230 Linux, this is probably a result of encountering the
231 /proc/sys/fs/file-max limit (see proc(5)).
232 ENOANO No anode.
234 ENOBUFS No buffer space available (POSIX.1 (XSI STREAMS op‐
236 tion)).
237 ENODATA No message is available on the STREAM head read queue
239 (POSIX.1-2001).
240 ENODEV No such device (POSIX.1-2001).
242 ENOENT No such file or directory (POSIX.1-2001).
244 Typically, this error results when a specified pathname
246 does not exist, or one of the components in the direc‐
247 tory prefix of a pathname does not exist, or the speci‐
248 fied pathname is a dangling symbolic link.
249 ENOEXEC Exec format error (POSIX.1-2001).
251 ENOKEY Required key not available.
253 ENOLCK No locks available (POSIX.1-2001).
255 ENOLINK Link has been severed (POSIX.1-2001).
257 ENOMEDIUM No medium found.
259 ENOMEM Not enough space/cannot allocate memory (POSIX.1-2001).
261 ENOMSG No message of the desired type (POSIX.1-2001).
263 ENONET Machine is not on the network.
265 ENOPKG Package not installed.
267 ENOPROTOOPT Protocol not available (POSIX.1-2001).
269 ENOSPC No space left on device (POSIX.1-2001).
271 ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option)).
273 ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option)).
275 ENOSYS Function not implemented (POSIX.1-2001).
277 ENOTBLK Block device required.
279 ENOTCONN The socket is not connected (POSIX.1-2001).
281 ENOTDIR Not a directory (POSIX.1-2001).
283 ENOTEMPTY Directory not empty (POSIX.1-2001).
285 ENOTRECOVERABLE State not recoverable (POSIX.1-2008).
287 ENOTSOCK Not a socket (POSIX.1-2001).
289 ENOTSUP Operation not supported (POSIX.1-2001).
291 ENOTTY Inappropriate I/O control operation (POSIX.1-2001).
293 ENOTUNIQ Name not unique on network.
295 ENXIO No such device or address (POSIX.1-2001).
297 EOPNOTSUPP Operation not supported on socket (POSIX.1-2001).
299 (ENOTSUP and EOPNOTSUPP have the same value on Linux,
301 but according to POSIX.1 these error values should be
302 distinct.)
303 EOVERFLOW Value too large to be stored in data type
305 (POSIX.1-2001).
306 EOWNERDEAD Owner died (POSIX.1-2008).
308 EPERM Operation not permitted (POSIX.1-2001).
310 EPFNOSUPPORT Protocol family not supported.
312 EPIPE Broken pipe (POSIX.1-2001).
314 EPROTO Protocol error (POSIX.1-2001).
316 EPROTONOSUPPORT Protocol not supported (POSIX.1-2001).
318 EPROTOTYPE Protocol wrong type for socket (POSIX.1-2001).
320 ERANGE Result too large (POSIX.1, C99).
322 EREMCHG Remote address changed.
324 EREMOTE Object is remote.
326 EREMOTEIO Remote I/O error.
328 ERESTART Interrupted system call should be restarted.
330 ERFKILL Operation not possible due to RF-kill.
332 EROFS Read-only filesystem (POSIX.1-2001).
334 ESHUTDOWN Cannot send after transport endpoint shutdown.
336 ESPIPE Invalid seek (POSIX.1-2001).
338 ESOCKTNOSUPPORT Socket type not supported.
340 ESRCH No such process (POSIX.1-2001).
342 ESTALE Stale file handle (POSIX.1-2001).
344 This error can occur for NFS and for other filesystems.
346 ESTRPIPE Streams pipe error.
348 ETIME Timer expired (POSIX.1 (XSI STREAMS option)).
350 (POSIX.1 says "STREAM ioctl(2) timeout".)
352 ETIMEDOUT Connection timed out (POSIX.1-2001).
354 ETOOMANYREFS Too many references: cannot splice.
356 ETXTBSY Text file busy (POSIX.1-2001).
358 EUCLEAN Structure needs cleaning.
360 EUNATCH Protocol driver not attached.
362 EUSERS Too many users.
364 EWOULDBLOCK Operation would block (may be same value as EAGAIN)
366 (POSIX.1-2001).
367 EXDEV Improper link (POSIX.1-2001).
369 EXFULL Exchange full.
371
373 A common mistake is to do
374 if (somecall() == -1) {
376 printf("somecall() failed\n");
377 if (errno == ...) { ... }
378 }
379 where errno no longer needs to have the value it had upon return from
381 somecall() (i.e., it may have been changed by the printf(3)). If the
382 value of errno should be preserved across a library call, it must be
383 saved:
384 if (somecall() == -1) {
386 int errsv = errno;
387 printf("somecall() failed\n");
388 if (errsv == ...) { ... }
389 }
390 Note that the POSIX threads APIs do not set errno on error. Instead,
392 on failure they return an error number as the function result. These
393 error numbers have the same meanings as the error numbers returned in
394 errno by other APIs.
395 On some ancient systems, <errno.h> was not present or did not declare
397 errno, so that it was necessary to declare errno manually (i.e., extern
398 int errno). Do not do this. It long ago ceased to be necessary, and
399 it will cause problems with modern versions of the C library.
400
402 errno(1), err(3), error(3), perror(3), strerror(3)
403
405 This page is part of release 5.10 of the Linux man-pages project. A
406 description of the project, information about reporting bugs, and the
407 latest version of this page, can be found at
408 https://www.kernel.org/doc/man-pages/.
409 2020-11-01 ERRNO(3)