errno


SYNOPSIS
       #include <errno.h>

DESCRIPTION
       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 suc-
       ceeds is allowed to change errno.

       Valid error numbers are all non-zero; 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 non-zero 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) (POSIX.1)

       EALREADY        Connection already in progress (POSIX.1)

       EBADE           Invalid exchange

       EBADF           Bad file descriptor (POSIX.1)

       EBADFD          File descriptor in bad state

       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

       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)

       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 file system (POSIX.1)

       ESHUTDOWN       Cannot send after transport endpoint shutdown

       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

NOTES
       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 declaration is needed.

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

COLOPHON
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