errno(3) - Linux manual page (original) (raw)

errno(3) Library Functions Manual errno(3)

NAME top

   errno - number of last error

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <errno.h>**

DESCRIPTION top

   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.

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)](../man2/getpriority.2.html)), -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)](../man3/perror.3.html) and [strerror(3)](../man3/strerror.3.html)
   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) 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 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**
          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 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
          sequence".

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

   **EINTR** Interrupted function call (POSIX.1-2001); see [signal(7)](../man7/signal.7.html).

   **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.

   **ELNRNG** 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
          [getrlimit(2)](../man2/getrlimit.2.html).  Can also be caused by exceeding the limit
          specified in _/proc/sys/fs/nropen_.

   **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)](../man5/proc.5.html)).

   **ENOANO** No anode.

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

   **ENODATA**
          The named attribute does not exist, or the process has no
          access to this attribute; see [xattr(7)](../man7/xattr.7.html).

          In POSIX.1-2001 (XSI STREAMS option), this error was
          described as "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 directory
          prefix of a pathname does not exist, or the specified
          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
          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 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)](../man2/ioctl.2.html) 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** Invalid cross-device link (POSIX.1-2001).

   **EXFULL** Exchange full.

NOTES top

   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)](../man3/printf.3.html)).
   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 function
   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_ 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.

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Linux man-pages 6.10 2024-07-23 errno(3)