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Linux 2.6 - man page for errno (linux section 3)

ERRNO(3)			    Linux Programmer's Manual				 ERRNO(3)

       errno - number of last error

       #include <errno.h>

       The <errno.h> header file defines the integer variable 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 return value of the call indicated an error (i.e., -1
       from most system calls; -1 or NULL from most library functions); a function that  succeeds
       is allowed to change errno.

       Valid  error  numbers  are  all	nonzero; errno is never set to zero by any system call or
       library function.

       For some system calls and library functions (e.g., getpriority(2)), -1 is a  valid  return
       on  success.  In such cases, a successful return can be distinguished from an error return
       by setting errno to zero before the call, and then, if the  call  returns  a  status  that
       indicates that an error may have occurred, checking to see if errno has a nonzero value.

       errno is defined by the ISO C standard to be a modifiable lvalue of type int, and must not
       be explicitly declared; errno may be a macro.  errno is thread-local; setting  it  in  one
       thread does not affect its value in any other thread.

       All  the error names specified by POSIX.1 must have distinct values, with the exception of
       EAGAIN and EWOULDBLOCK, which may be the same.

       Below is a list of the symbolic error names that are defined on Linux.  Some of these  are
       marked  POSIX.1,  indicating  that the name is defined by POSIX.1-2001, or C99, indicating
       that the name is defined by C99.

       E2BIG	       Argument list too long (POSIX.1)

       EACCES	       Permission denied (POSIX.1)

       EADDRINUSE      Address already in use (POSIX.1)

       EADDRNOTAVAIL   Address not available (POSIX.1)

       EAFNOSUPPORT    Address family not supported (POSIX.1)

       EAGAIN	       Resource temporarily unavailable (may be the same  value  as  EWOULDBLOCK)

       EALREADY        Connection already in progress (POSIX.1)

       EBADE	       Invalid exchange

       EBADF	       Bad file descriptor (POSIX.1)

       EBADFD	       File descriptor in bad state

       EBADMSG	       Bad message (POSIX.1)

       EBADR	       Invalid request descriptor

       EBADRQC	       Invalid request code

       EBADSLT	       Invalid slot

       EBUSY	       Device or resource busy (POSIX.1)

       ECANCELED       Operation canceled (POSIX.1)

       ECHILD	       No child processes (POSIX.1)

       ECHRNG	       Channel number out of range

       ECOMM	       Communication error on send

       ECONNABORTED    Connection aborted (POSIX.1)

       ECONNREFUSED    Connection refused (POSIX.1)

       ECONNRESET      Connection reset (POSIX.1)

       EDEADLK	       Resource deadlock avoided (POSIX.1)

       EDEADLOCK       Synonym for EDEADLK

       EDESTADDRREQ    Destination address required (POSIX.1)

       EDOM	       Mathematics argument out of domain of function (POSIX.1, C99)

       EDQUOT	       Disk quota exceeded (POSIX.1)

       EEXIST	       File exists (POSIX.1)

       EFAULT	       Bad address (POSIX.1)

       EFBIG	       File too large (POSIX.1)

       EHOSTDOWN       Host is down

       EHOSTUNREACH    Host is unreachable (POSIX.1)

       EIDRM	       Identifier removed (POSIX.1)

       EILSEQ	       Illegal byte sequence (POSIX.1, C99)

       EINPROGRESS     Operation in progress (POSIX.1)

       EINTR	       Interrupted function call (POSIX.1); see signal(7).

       EINVAL	       Invalid argument (POSIX.1)

       EIO	       Input/output error (POSIX.1)

       EISCONN	       Socket is connected (POSIX.1)

       EISDIR	       Is a directory (POSIX.1)

       EISNAM	       Is a named type file

       EKEYEXPIRED     Key has expired

       EKEYREJECTED    Key was rejected by service

       EKEYREVOKED     Key has been revoked

       EL2HLT	       Level 2 halted

       EL2NSYNC        Level 2 not synchronized

       EL3HLT	       Level 3 halted

       EL3RST	       Level 3 halted

       ELIBACC	       Cannot access a needed shared library

       ELIBBAD	       Accessing a corrupted shared library

       ELIBMAX	       Attempting to link in too many shared libraries

       ELIBSCN	       lib section in a.out corrupted

       ELIBEXEC        Cannot exec a shared library directly

       ELOOP	       Too many levels of symbolic links (POSIX.1)

       EMEDIUMTYPE     Wrong medium type

       EMFILE	       Too many open files (POSIX.1)

       EMLINK	       Too many links (POSIX.1)

       EMSGSIZE        Message too long (POSIX.1)

       EMULTIHOP       Multihop attempted (POSIX.1)

       ENAMETOOLONG    Filename too long (POSIX.1)

       ENETDOWN        Network is down (POSIX.1)

       ENETRESET       Connection aborted by network (POSIX.1)

       ENETUNREACH     Network unreachable (POSIX.1)

       ENFILE	       Too many open files in system (POSIX.1)

       ENOBUFS	       No buffer space available (POSIX.1 (XSI STREAMS option))

       ENODATA	       No message is available on the STREAM head read queue (POSIX.1)

       ENODEV	       No such device (POSIX.1)

       ENOENT	       No such file or directory (POSIX.1)

       ENOEXEC	       Exec format error (POSIX.1)

       ENOKEY	       Required key not available

       ENOLCK	       No locks available (POSIX.1)

       ENOLINK	       Link has been severed (POSIX.1)

       ENOMEDIUM       No medium found

       ENOMEM	       Not enough space (POSIX.1)

       ENOMSG	       No message of the desired type (POSIX.1)

       ENONET	       Machine is not on the network

       ENOPKG	       Package not installed

       ENOPROTOOPT     Protocol not available (POSIX.1)

       ENOSPC	       No space left on device (POSIX.1)

       ENOSR	       No STREAM resources (POSIX.1 (XSI STREAMS option))

       ENOSTR	       Not a STREAM (POSIX.1 (XSI STREAMS option))

       ENOSYS	       Function not implemented (POSIX.1)

       ENOTBLK	       Block device required

       ENOTCONN        The socket is not connected (POSIX.1)

       ENOTDIR	       Not a directory (POSIX.1)

       ENOTEMPTY       Directory not empty (POSIX.1)

       ENOTSOCK        Not a socket (POSIX.1)

       ENOTSUP	       Operation not supported (POSIX.1)

       ENOTTY	       Inappropriate I/O control operation (POSIX.1)

       ENOTUNIQ        Name not unique on network

       ENXIO	       No such device or address (POSIX.1)

       EOPNOTSUPP      Operation not supported on socket (POSIX.1)

		       (ENOTSUP  and  EOPNOTSUPP  have	the same value on Linux, but according to
		       POSIX.1 these error values should be distinct.)

       EOVERFLOW       Value too large to be stored in data type (POSIX.1)

       EPERM	       Operation not permitted (POSIX.1)

       EPFNOSUPPORT    Protocol family not supported

       EPIPE	       Broken pipe (POSIX.1)

       EPROTO	       Protocol error (POSIX.1)

       EPROTONOSUPPORT Protocol not supported (POSIX.1)

       EPROTOTYPE      Protocol wrong type for socket (POSIX.1)

       ERANGE	       Result too large (POSIX.1, C99)

       EREMCHG	       Remote address changed

       EREMOTE	       Object is remote

       EREMOTEIO       Remote I/O error

       ERESTART        Interrupted system call should be restarted

       EROFS	       Read-only filesystem (POSIX.1)

       ESHUTDOWN       Cannot send after transport endpoint shutdown

       ESPIPE	       Invalid seek (POSIX.1)

       ESOCKTNOSUPPORT Socket type not supported

       ESRCH	       No such process (POSIX.1)

       ESTALE	       Stale file handle (POSIX.1)

		       This error can occur for NFS and for other filesystems

       ESTRPIPE        Streams pipe error

       ETIME	       Timer expired (POSIX.1 (XSI STREAMS option))

		       (POSIX.1 says "STREAM ioctl(2) timeout")

       ETIMEDOUT       Connection timed out (POSIX.1)

       ETXTBSY	       Text file busy (POSIX.1)

       EUCLEAN	       Structure needs cleaning

       EUNATCH	       Protocol driver not attached

       EUSERS	       Too many users

       EWOULDBLOCK     Operation would block (may be same value as EAGAIN) (POSIX.1)

       EXDEV	       Improper link (POSIX.1)

       EXFULL	       Exchange full

       A common mistake is to do

	   if (somecall() == -1) {
	       printf("somecall() failed\n");
	       if (errno == ...) { ... }

       where errno no longer needs to have the value it had upon return from somecall() (i.e., it
       may have been changed by the printf(3)).  If the value of errno should be preserved across
       a library call, it must be saved:

	   if (somecall() == -1) {
	       int errsv = errno;
	       printf("somecall() failed\n");
	       if (errsv == ...) { ... }

       It was common in traditional C to declare errno manually (i.e., extern int errno)  instead
       of  including  <errno.h>.  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 <errno.h> and the  declara-
       tion is needed.

       err(3), error(3), perror(3), strerror(3)

       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

					    2008-07-09					 ERRNO(3)

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