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     NAME
          errno - number of last error

     SYNOPSIS
          #include <errno.h>

     DESCRIPTION
          The <errno.h> header file defines the integer variable
          errno, which is set by system calls and some library func-
          tions in the event of an error to indicate what went wrong.

        errno
          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.  On Linux, these two have the same value on
          all architectures.

          The error numbers that correspond to each symbolic name vary
          across UNIX systems, and even across different architectures
          on Linux.  Therefore, 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

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          using the errno(1) command (part of the moreutils package):

              $ 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 numbers 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 indi-
             cated.

          *  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-

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                          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       On most architectures, a synonym for
                          EDEADLK.  On some architectures (e.g., Linux
                          MIPS, PowerPC, SPARC), it is a separate
                          error code "File locking deadlock error".

          EDESTADDRREQ    Destination address required (POSIX.1-2001).

          EDOM            Mathematics argument out of domain of func-
                          tion (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).

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          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 sequence".

          EINPROGRESS     Operation in progress (POSIX.1-2001).

          EINTR           Interrupted function call (POSIX.1-2001);
                          see signal(7).

          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

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          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).  Com-
                          monly caused by exceeding the RLIMIT_NOFILE
                          resource limit described in getrlimit(2).
                          Can also be caused by exceeding the limit
                          specified in /proc/sys/fs/nr_open.

          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 option)).

          ENODATA         No message is available on the STREAM head
                          read queue (POSIX.1-2001).

          ENODEV          No such device (POSIX.1-2001).

          ENOENT          No such file or directory (POSIX.1-2001).

                          Typically, this error results when a speci-
                          fied pathname does not exist, or one of the
                          components in the directory prefix of a
                          pathname does not exist, or the specified
                          pathname is a dangling symbolic link.

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          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).

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          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 distinct.)

          EOVERFLOW       Value too large to be stored in data type
                          (POSIX.1-2001).

          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 shut-
                          down.

          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.

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          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) (POSIX.1-2001).

          EXDEV           Improper link (POSIX.1-2001).

          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 == ...) { ... }
              }

          Note that the POSIX threads APIs do not set errno on error.
          Instead, on failure they return an error number as the func-
          tion result.  These error numbers have the same meanings as
          the error numbers returned in errno by other APIs.

          On some ancient systems, <errno.h> was not present or did
          not declare errno, so that it was necessary to declare errno

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          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.

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

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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