errno(3)                                              Library Functions Manual                                             errno(3)

       errno - number of last error

       Standard C library (libc, -lc)

       #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  suc‐
       cessful return can be distinguished from an error return by setting errno to zero before the call, and then, if the call re‐
       turns 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 er‐
       ror 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:

              The name is defined by POSIX.1-2001, and is defined in later POSIX.1 versions, unless otherwise indicated.

              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 se‐

       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  di‐
                       rectory 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

       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.

       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 manu‐
       ally  (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)

Linux man-pages 6.03                                         2022-12-04                                                    errno(3)