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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.
     The  value  in  errno  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.  The value of 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.
 Error numbers and names
     Valid  error  numbers  are  all positive numbers.  The <errno.h> header
     file defines symbolic names for each of the possible error numbers that
     may appear in errno.
     All  the  error  names  specified by POSIX.1 must have distinct values,
     with the exception of EAGAIN and EWOULDBLOCK, which may be the same.
     The error numbers that correspond to each  symbolic  name  vary  across
     UNIX systems, and even across different architectures on Linux.  There-
     fore, numeric values are not included as part  of  the  list  of  error
     names  below.   The  perror(3) and strerror(3) functions can be used to
     convert these names to corresponding textual error messages.
     On any particular Linux system, one can obtain a list of  all  symbolic
     error names and the corresponding error numbers using the errno(1) com-
         $ errno -l EPERM 1 Operation not permitted ENOENT 2 No such file or
         directory  ESRCH  3 No such process EINTR 4 Interrupted system call
         EIO 5 Input/output error ...
     The errno(1) command can also be used to look up individual error  num-
     bers  and  names, and to search for errors using strings from the error
     description, as in the following examples:
         $ errno 2 ENOENT 2 No such file or directory $ errno ESRCH ESRCH  3
         No such process $ errno -s permission EACCES 13 Permission denied
 List of error names
     In the list of the symbolic error names below, various names are marked
     as follows:
  • POSIX.1-2001: The name is defined by POSIX.1-2001, and is defined in

later POSIX.1 versions, unless otherwise indicated.

  • POSIX.1-2008: The name is defined in POSIX.1-2008, but was not

present in earlier POSIX.1 standards.

  • C99: The name is defined by C99. Below is a list of the symbolic

error names that are defined on Linux:

     E2BIG           Argument list too long (POSIX.1-2001).
     EACCES          Permission denied (POSIX.1-2001).
     EADDRINUSE      Address already in use (POSIX.1-2001).
     EADDRNOTAVAIL   Address not available (POSIX.1-2001).
     EAFNOSUPPORT    Address family not supported (POSIX.1-2001).
     EAGAIN          Resource temporarily unavailable (may be the same value
                     as EWOULDBLOCK) (POSIX.1-2001).
     EALREADY        Connection already in progress (POSIX.1-2001).
     EBADE           Invalid exchange.
     EBADF           Bad file descriptor (POSIX.1-2001).
     EBADFD          File descriptor in bad state.
     EBADMSG         Bad message (POSIX.1-2001).
     EBADR           Invalid request descriptor.
     EBADRQC         Invalid request code.
     EBADSLT         Invalid slot.
     EBUSY           Device or resource busy (POSIX.1-2001).
     ECANCELED       Operation canceled (POSIX.1-2001).
     ECHILD          No child processes (POSIX.1-2001).
     ECHRNG          Channel number out of range.
     ECOMM           Communication error on send.
     ECONNABORTED    Connection aborted (POSIX.1-2001).
     ECONNREFUSED    Connection refused (POSIX.1-2001).
     ECONNRESET      Connection reset (POSIX.1-2001).
     EDEADLK         Resource deadlock avoided (POSIX.1-2001).
     EDEADLOCK       Synonym for EDEADLK.
     EDESTADDRREQ    Destination address required (POSIX.1-2001).
     EDOM            Mathematics  argument  out  of   domain   of   function
                     (POSIX.1, C99).
     EDQUOT          Disk quota exceeded (POSIX.1-2001).
     EEXIST          File exists (POSIX.1-2001).
     EFAULT          Bad address (POSIX.1-2001).
     EFBIG           File too large (POSIX.1-2001).
     EHOSTDOWN       Host is down.
     EHOSTUNREACH    Host is unreachable (POSIX.1-2001).
     EHWPOISON       Memory page has hardware error.
     EIDRM           Identifier removed (POSIX.1-2001).
     EILSEQ          Invalid  or  incomplete  multibyte  or  wide  character
                     (POSIX.1, C99).
                     The text shown here is the glibc error description;  in
                     POSIX.1,  this  error  is  described  as  "Illegal byte
     EINPROGRESS     Operation in progress (POSIX.1-2001).
     EINTR           Interrupted  function  call  (POSIX.1-2001);  see  sig-
     EINVAL          Invalid argument (POSIX.1-2001).
     EIO             Input/output error (POSIX.1-2001).
     EISCONN         Socket is connected (POSIX.1-2001).
     EISDIR          Is a directory (POSIX.1-2001).
     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 reset.
     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.
     ELNRANGE        Link number out of range.
     ELOOP           Too many levels of symbolic links (POSIX.1-2001).
     EMEDIUMTYPE     Wrong medium type.
     EMFILE          Too many open files (POSIX.1-2001).  Commonly caused by
                     exceeding the RLIMIT_NOFILE resource limit described in
     EMLINK          Too many links (POSIX.1-2001).
     EMSGSIZE        Message too long (POSIX.1-2001).
     EMULTIHOP       Multihop attempted (POSIX.1-2001).
     ENAMETOOLONG    Filename too long (POSIX.1-2001).
     ENETDOWN        Network is down (POSIX.1-2001).
     ENETRESET       Connection aborted by network (POSIX.1-2001).
     ENETUNREACH     Network unreachable (POSIX.1-2001).
     ENFILE          Too  many  open  files  in  system  (POSIX.1-2001).  On
                     Linux, this is probably a result  of  encountering  the
                     /proc/sys/fs/file-max limit (see proc(5)).
     ENOANO          No anode.
     ENOBUFS         No   buffer   space  available  (POSIX.1  (XSI  STREAMS
     ENODATA         No message is available on the STREAM head  read  queue
     ENODEV          No such device (POSIX.1-2001).
     ENOENT          No such file or directory (POSIX.1-2001).
                     Typically, this error results when a specified pathname
                     does not exist, or one of the components in the  direc-
                     tory prefix of a pathname does not exist, or the speci-
                     fied pathname is a dangling symbolic link.
     ENOEXEC         Exec format error (POSIX.1-2001).
     ENOKEY          Required key not available.
     ENOLCK          No locks available (POSIX.1-2001).
     ENOLINK         Link has been severed (POSIX.1-2001).
     ENOMEDIUM       No medium found.
     ENOMEM          Not enough space/cannot allocate memory (POSIX.1-2001).
     ENOMSG          No message of the desired type (POSIX.1-2001).
     ENONET          Machine is not on the network.
     ENOPKG          Package not installed.
     ENOPROTOOPT     Protocol not available (POSIX.1-2001).
     ENOSPC          No space left on device (POSIX.1-2001).
     ENOSR           No STREAM resources (POSIX.1 (XSI STREAMS option)).
     ENOSTR          Not a STREAM (POSIX.1 (XSI STREAMS option)).
     ENOSYS          Function not implemented (POSIX.1-2001).
     ENOTBLK         Block device required.
     ENOTCONN        The socket is not connected (POSIX.1-2001).
     ENOTDIR         Not a directory (POSIX.1-2001).
     ENOTEMPTY       Directory not empty (POSIX.1-2001).
     ENOTRECOVERABLE State not recoverable (POSIX.1-2008).
     ENOTSOCK        Not a socket (POSIX.1-2001).
     ENOTSUP         Operation not supported (POSIX.1-2001).
     ENOTTY          Inappropriate I/O control operation (POSIX.1-2001).
     ENOTUNIQ        Name not unique on network.
     ENXIO           No such device or address (POSIX.1-2001).
     EOPNOTSUPP      Operation not supported on socket (POSIX.1-2001).
                     (ENOTSUP  and  EOPNOTSUPP have the same value on Linux,
                     but according to POSIX.1 these error values  should  be
     EOVERFLOW       Value   too   large   to   be   stored   in  data  type
     EOWNERDEAD      Owner died (POSIX.1-2008).
     EPERM           Operation not permitted (POSIX.1-2001).
     EPFNOSUPPORT    Protocol family not supported.
     EPIPE           Broken pipe (POSIX.1-2001).
     EPROTO          Protocol error (POSIX.1-2001).
     EPROTONOSUPPORT Protocol not supported (POSIX.1-2001).
     EPROTOTYPE      Protocol wrong type for socket (POSIX.1-2001).
     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.
     ERFKILL         Operation not possible due to RF-kill.
     EROFS           Read-only filesystem (POSIX.1-2001).
     ESHUTDOWN       Cannot send after transport endpoint shutdown.
     ESPIPE          Invalid seek (POSIX.1-2001).
     ESOCKTNOSUPPORT Socket type not supported.
     ESRCH           No such process (POSIX.1-2001).
     ESTALE          Stale file handle (POSIX.1-2001).
                     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.1-2001).
     ETOOMANYREFS    Too many references: cannot splice.
     ETXTBSY         Text file busy (POSIX.1-2001).
     EUCLEAN         Structure needs cleaning.
     EUNATCH         Protocol driver not attached.
     EUSERS          Too many users.
     EWOULDBLOCK     Operation  would  block  (may  be same value as EAGAIN)
     EXDEV           Improper link (POSIX.1-2001).
     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
         if (somecall() == -1) {
             int errsv = errno;
             printf("somecall() failed\n");
             if (errsv == ...) { ... } }
     On  some  ancient systems, <errno.h> was not present or did not declare
     errno, so that it was necessary to declare errno manually (i.e., extern
     int  errno).   Do not do this.  It long ago ceased to be necessary, and
     it will cause problems with modern versions of the C library.


     errno(1), err(3), error(3), perror(3), strerror(3)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
                                2018-02-02                          ERRNO(3)
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