1ERRNO(3) Library functions 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. Its value is significant only when the
15 call returned an error (usually -1), and a function that does succeed
16 is allowed to change errno.
17 Sometimes, when -1 is also a valid successful return value one has to
19 zero errno before the call in order to detect possible errors.
20 errno is defined by the ISO C standard to be a modifiable lvalue of
22 type int, and must not be explicitly declared; errno may be a macro.
23 errno is thread-local; setting it in one thread does not affect its
24 value in any other thread.
25 Valid error numbers are all non-zero; errno is never set to zero by any
27 library function. All the error names specified by POSIX.1 must have
28 distinct values, with the exception of EAGAIN and EWOULDBLOCK, which
29 may be the same.
30 Below is a list of the symbolic error names that are defined on Linux.
32 Some of these are marked POSIX.1, indicating that the name is defined
33 by POSIX.1-2001, or C99, indicating that the name is defined by C99.
34 E2BIG Argument list too long (POSIX.1)
36 EACCES Permission denied (POSIX.1)
38 EADDRINUSE
40 Address already in use (POSIX.1)
41 EADDRNOTAVAIL
43 Address not available (POSIX.1)
44 EAFNOSUPPORT
46 Address family not supported (POSIX.1)
47 EAGAIN Resource temporarily unavailable (may be the same value as
49 EWOULDBLOCK) (POSIX.1)
50 EALREADY
52 Connection already in progress (POSIX.1)
53 EBADE Invalid exchange
55 EBADF Bad file descriptor (POSIX.1)
57 EBADFD File descriptor in bad state
59 EBADMSG
61 Bad message (POSIX.1)
62 EBADR Invalid request descriptor
64 EBADRQC
66 Invalid request code
67 EBADSLT
69 Invalid slot
70 EBUSY Device or resource busy (POSIX.1)
72 ECANCELED
74 Operation canceled (POSIX.1)
75 ECHILD No child processes (POSIX.1)
77 ECHRNG Channel number out of range
79 ECOMM Communication error on send
81 ECONNABORTED
83 Connection aborted (POSIX.1)
84 ECONNREFUSED
86 Connection refused (POSIX.1)
87 ECONNRESET
89 Connection reset (POSIX.1)
90 EDEADLK
92 Resource deadlock avoided (POSIX.1)
93 EDEADLOCK
95 Synonym for EDEADLK
96 EDESTADDRREQ
98 Destination address required (POSIX.1)
99 EDOM Mathematics argument out of domain of function (POSIX.1, C99)
101 EDQUOT Disk quota exceeded (POSIX.1)
103 EEXIST File exists (POSIX.1)
105 EFAULT Bad address (POSIX.1)
107 EFBIG File too large (POSIX.1)
109 EHOSTDOWN
111 Host is down
112 EHOSTUNREACH
114 Host is unreachable (POSIX.1)
115 EIDRM Identifier removed (POSIX.1)
117 EILSEQ Illegal byte sequence (POSIX.1, C99)
119 EINPROGRESS
121 Operation in progress (POSIX.1)
122 EINTR Interrupted function call (POSIX.1)
124 EINVAL Invalid argument (POSIX.1)
126 EIO Input/output error (POSIX.1)
128 EISCONN
130 Socket is connected (POSIX.1)
131 EISDIR Is a directory (POSIX.1)
133 EISNAM Is a named type file
135 EKEYEXPIRED
137 Key has expired
138 EKEYREJECTED
140 Key was rejected by service
141 EKEYREVOKED
143 Key has been revoked
144 EL2HLT Level 2 halted
146 EL2NSYNC
148 Level 2 not synchronized
149 EL3HLT Level 3 halted
151 EL3RST Level 3 halted
153 ELIBACC
155 Cannot access a needed shared library
156 ELIBBAD
158 Accessing a corrupted shared library
159 ELIBMAX
161 Attempting to link in too many shared libraries
162 ELIBSCN
164 lib section in a.out corrupted
165 ELIBEXEC
167 Cannot exec a shared library directly
168 ELOOP Too many levels of symbolic links (POSIX.1)
170 EMEDIUMTYPE
172 Wrong medium type
173 EMFILE Too many open files (POSIX.1)
175 EMLINK Too many links (POSIX.1)
177 EMSGSIZE
179 Message too long (POSIX.1)
180 EMULTIHOP
182 Multihop attempted (POSIX.1)
183 ENAMETOOLONG
185 Filename too long (POSIX.1)
186 ENETDOWN
188 Network is down (POSIX.1)
189 ENETRESET
191 Connection aborted by network (POSIX.1)
192 ENETUNREACH
194 Network unreachable (POSIX.1)
195 ENFILE Too many open files in system (POSIX.1)
197 ENOBUFS
199 No buffer space available (POSIX.1 (XSI STREAMS option))
200 ENODATA
202 No message is available on the STREAM head read queue (POSIX.1)
203 ENODEV No such device (POSIX.1)
205 ENOENT No such file or directory (POSIX.1)
207 ENOEXEC
209 Exec format error (POSIX.1)
210 ENOKEY Required key not available
212 ENOLCK No locks available (POSIX.1)
214 ENOLINK
216 Link has been severed (POSIX.1)
217 ENOMEDIUM
219 No medium found
220 ENOMEM Not enough space (POSIX.1)
222 ENOMSG No message of the desired type (POSIX.1)
224 ENONET Machine is not on the network
226 ENOPKG Package not installed
228 ENOPROTOOPT
230 Protocol not available (POSIX.1)
231 ENOSPC No space left on device (POSIX.1)
233 ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option))
235 ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option))
237 ENOSYS Function not implemented (POSIX.1)
239 ENOTBLK
241 Block device required
242 ENOTCONN
244 The socket is not connected (POSIX.1)
245 ENOTDIR
247 Not a directory (POSIX.1)
248 ENOTEMPTY
250 Directory not empty (POSIX.1)
251 ENOTSOCK
253 Not a socket (POSIX.1)
254 ENOTSUP
256 Operation not supported (POSIX.1)
257 ENOTTY Inappropriate I/O control operation (POSIX.1)
259 ENOTUNIQ
261 Name not unique on network
262 ENXIO No such device or address (POSIX.1)
264 EOPNOTSUPP
266 Operation not supported on socket (POSIX.1)
267 (ENOTSUP and EOPNOTSUPP have the same value on Linux, but
269 according to POSIX.1 these error values should be distinct.)
270 EOVERFLOW
272 Value too large to be stored in data type (POSIX.1)
273 EPERM Operation not permitted (POSIX.1)
275 EPFNOSUPPORT
277 Protocol family not supported
278 EPIPE Broken pipe (POSIX.1)
280 EPROTO Protocol error (POSIX.1)
282 EPROTONOSUPPORT
284 Protocol not supported (POSIX.1)
285 EPROTOTYPE
287 Protocol wrong type for socket (POSIX.1)
288 ERANGE Result too large (POSIX.1, C99)
290 EREMCHG
292 Remote address changed
293 EREMOTE
295 Object is remote
296 EREMOTEIO
298 Remote I/O error
299 ERESTART
301 Interrupted system call should be restarted
302 EROFS Read-only file system (POSIX.1)
304 ESHUTDOWN
306 Cannot send after transport endpoint shutdown
307 ESPIPE Invalid seek (POSIX.1)
309 ESOCKTNOSUPPORT
311 Socket type not supported
312 ESRCH No such process (POSIX.1)
314 ESTALE Stale file handle (POSIX.1)
316 This error can occur for NFS and for other file systems
318 ESTRPIPE
320 Streams pipe error
321 ETIME Timer expired (POSIX.1 (XSI STREAMS option))
323 (POSIX.1 says "STREAM ioctl() timeout")
325 ETIMEDOUT
327 Connection timed out (POSIX.1)
328 ETXTBSY
330 Text file busy (POSIX.1)
331 EUCLEAN
333 Structure needs cleaning
334 EUNATCH
336 Protocol driver not attached
337 EUSERS Too many users
339 EWOULDBLOCK
341 Operation would block (may be same value as EAGAIN) (POSIX.1)
342 EXDEV Improper link (POSIX.1)
344 EXFULL Exchange full
346
348 A common mistake is to do
349 if (somecall() == -1) {
351 printf("somecall() failed\n");
352 if (errno == ...) { ... }
353 }
354 where errno no longer needs to have the value it had upon return from
356 somecall() (i.e., it may have been changed by the printf()). If the
357 value of errno should be preserved across a library call, it must be
358 saved:
359 if (somecall() == -1) {
361 int errsv = errno;
362 printf("somecall() failed\n");
363 if (errsv == ...) { ... }
364 }
365 It was common in traditional C to declare errno manually (i.e., extern
367 int errno) instead of including <errno.h>. Do not do this. It will
368 not work with modern versions of the C library. However, on (very) old
369 Unix systems, there may be no <errno.h> and the declaration is needed.
370
372 err(3), error(3), perror(3), strerror(3)
373 2006-02-09 ERRNO(3)