errno(3) — Linux manual page

NAME \| LIBRARY \| SYNOPSIS \| DESCRIPTION \| NOTES \| SEE ALSO \| COLOPHON

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)), -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 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).

       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).  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
              The named attribute does not exist, or the process has no
              access to this attribute; see xattr(7).

              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) 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)).
       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.

COLOPHON top

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Linux man-pages 6.15            2025-05-17                       errno(3)

errno(3) — Linux manual page

NAME top

LIBRARY top

SYNOPSIS top

DESCRIPTION top

NOTES top

SEE ALSO top

COLOPHON top