.\" Copyright (c) 1996 Andries Brouwer (aeb@cwi.nl) .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, .\" USA. .\" .\" 5 Oct 2002, Modified by Michael Kerrisk .\" Updated for POSIX 1003.1 2001 .\" 2004-12-17 Martin Schulze , mtk .\" Removed errno declaration prototype, added notes .\" .TH ERRNO 3 2004-12-17 "" "Library functions" .SH NAME errno \- number of last error .SH SYNOPSIS .B #include .\".sp .\".BI "extern int " errno ; .SH DESCRIPTION The .I header file defines the integer variable .BR errno , which is set by system calls and some library functions in the event of an error to indicate what went wrong. Its value is significant only when the call returned an error (usually \-1), and a function that does succeed is allowed to change .BR errno . Sometimes, when \-1 is also a valid successful return value one has to zero .B errno before the call in order to detect possible errors. \fBerrno\fR is defined by the ISO C standard to be a modifiable lvalue of type \fBint\fR, and must not be explicitly declared; \fBerrno\fR may be a macro. \fBerrno\fR is thread-local; setting it in one thread does not affect its value in any other thread. Valid error numbers are all non-zero; \fBerrno\fR is never set to zero by any library function. All the error names specified by POSIX.1 must have distinct values, with the exception of .B EAGAIN and .BR EWOULDBLOCK , which may be the same. .\" FIXME EILSEQ is in C99. POSIX.1 (2001 edition) lists the following symbolic error names. Of these, \fBEDOM\fR and \fBERANGE\fR are in the ISO C standard. ISO C Amendment 1 defines the additional error number \fBEILSEQ\fR for coding errors in multibyte or wide characters. .TP .B E2BIG Arg list too long .TP .B EACCES Permission denied .TP .B EADDRINUSE Address in use .TP .B EADDRNOTAVAIL Address not available .TP .B EAFNOSUPPORT Address family not supported .TP .B EAGAIN Resource temporarily unavailable .TP .B EALREADY Connection already in progress .TP .B EBADF Bad file descriptor .TP .B EBADMSG Bad message .TP .B EBUSY Resource busy .TP .B ECANCELED Operation canceled .TP .B ECHILD No child processes .TP .B ECONNABORTED Connection aborted .TP .B ECONNREFUSED Connection refused .TP .B ECONNRESET Connection reset .TP .B EDEADLK Resource deadlock avoided .TP .B EDESTADDRREQ Destination address required .TP .B EDOM Domain error .TP .B EDQUOT Reserved .TP .B EEXIST File exists .TP .B EFAULT Bad address .TP .B EFBIG File too large .TP .B EHOSTUNREACH Host is unreachable .TP .B EIDRM Identifier removed .TP .B EILSEQ Illegal byte sequence .TP .B EINPROGRESS Operation in progress .TP .B EINTR Interrupted function call .TP .B EINVAL Invalid argument .TP .B EIO Input/output error .TP .B EISCONN Socket is connected .TP .B EISDIR Is a directory .TP .B ELOOP Too many levels of symbolic links .TP .B EMFILE Too many open files .TP .B EMLINK Too many links .TP .B EMSGSIZE Inappropriate message buffer length .TP .B EMULTIHOP Reserved .TP .B ENAMETOOLONG Filename too long .TP .B ENETDOWN Network is down .TP .B ENETRESET Connection aborted by network .TP .B ENETUNREACH Network unreachable .TP .B ENFILE Too many open files in system .TP .B ENOBUFS No buffer space available .\" ENODATA is part of XSR option .TP .B ENODATA No message is available on the STREAM head read queue .TP .B ENODEV No such device .TP .B ENOENT No such file or directory .TP .B ENOEXEC Exec format error .TP .B ENOLCK No locks available .TP .B ENOLINK Reserved .TP .B ENOMEM Not enough space .TP .B ENOMSG No message of the desired type .TP .B ENOPROTOOPT Protocol not available .TP .B ENOSPC No space left on device .\" ENOSR is part of XSR option .TP .B ENOSR No STREAM resources .\" ENOSTR is part of XSR option .TP .B ENOSTR Not a STREAM .TP .B ENOSYS Function not implemented .TP .B ENOTCONN The socket is not connected .TP .B ENOTDIR Not a directory .TP .B ENOTEMPTY Directory not empty .TP .B ENOTSOCK Not a socket .TP .B ENOTSUP Not supported .TP .B ENOTTY Inappropriate I/O control operation .TP .B ENXIO No such device or address .TP .B EOPNOTSUPP Operation not supported on socket .TP .B EOVERFLOW Value too large to be stored in data type .TP .B EPERM Operation not permitted .TP .B EPIPE Broken pipe .TP .B EPROTO Protocol error .TP .B EPROTONOSUPPORT Protocol not supported .TP .B EPROTOTYPE Protocol wrong type for socket .TP .B ERANGE Result too large .TP .B EROFS Read-only file system .TP .B ESPIPE Invalid seek .TP .B ESRCH No such process .TP .B ESTALE Stale file handle .\" Can occur for NFS and for other file systems .\" ETIME is part of XSR option .TP .B ETIME STREAM ioctl() timeout .TP .B ETIMEDOUT Operation timed out .TP .B ETXTBSY Text file busy .TP .B EWOULDBLOCK Operation would block (may be same value as .BR EAGAIN ) .TP .B EXDEV Improper link .SH NOTES A common mistake is to do .RS .nf if (somecall() == \-1) { printf("somecall() failed\en"); if (errno == ...) { ... } } .fi .RE where .I errno no longer needs to have the value it had upon return from .IR somecall () (i.e., it may have been changed by the .IR printf ()). If the value of .I errno should be preserved across a library call, it must be saved: .RS .nf if (somecall() == \-1) { int errsv = errno; printf("somecall() failed\en"); if (errsv == ...) { ... } } .fi .RE .PP It was common in traditional C to declare .I errno manually (i.e., .IR "extern int errno" ) instead of including .IR . .BR "Do not do this" . It will not work with modern versions of the C library. However, on (very) old Unix systems, there may be no .I and the declaration is needed. .SH "SEE ALSO" .BR perror (3), .BR strerror (3)