The errno.h header file defines the symbolic constants that are returned in the external variable errno.
| EACCES | Permission denied |
| EADDRINUSE | Address in use |
| EADDRNOTAVAIL | Address not available |
| EADV | Advertise error |
| EAFNOSUPPORT | Address family not supported |
| EAGAIN | Resource temporarily unavailable |
| EALREADY | Connection already in progress |
| EBADF | Bad file descriptor |
| EBADMSG | Bad message |
| EBUSY | Resource busy |
| ECANCELED | Operation canceled |
| ECHILD | No child processes |
| ECICS | Function not supported under CICS® |
| ECOMM | Communication error on send |
| ECONNABORTED | Connection aborted |
| ECONNREFUSED | Connection refused |
| ECONNRESET | Connection reset |
| EDEADLK | Resource deadlock avoided |
| EDESTADDRREQ | Destination address required |
| EDOM | Domain error |
| EDOTDOT | Cross mount point (not an error) |
| EDQUOT | Reserved |
| EEXIST | File exists |
| EFAULT | Bad address |
| EFBIG | File too large |
| EHOSTDOWN | Host is down |
| EHOSTUNREACH | Destination host can not be reached |
| EIBMBADCALL | A bad socket-call constant in IUCV header |
| EIBMBADPARM | Other IUCV header error |
| EIBMCANCELLED | Request canceled |
| EIBMCONFLICT | Conflicting call outstanding on socket |
| EIBMIUCVERR | Request failed due to IUCV error |
| EIBMSOCKINUSE | Assigned socket number already in use |
| EIBMSOCKOUTOFRANGE | Assigned socket number out of range |
| EIDRM | Identifier removed |
| EILSEQ | Illegal byte sequence |
| EINPROGRESS | Connection in progress |
| EINTR | Interrupted function call |
| EINTRNODATA | Function call interrupted before any data received |
| EINVAL | Invalid argument |
| EIO | Input/output error |
| EISCONN | Socket is already connected |
| EISDIR | Is a directory |
| ELEMSGERR | Message file was not found in the hierarchical file system |
| ELEMULTITHREAD | Function not allowed in a multithreaded environment |
| ELEMULTITHREADFORK | Function not allowed in child of fork() in multithreaded environment |
| ELENOFORK | Language Environment member language cannot tolerate a fork() |
| ELOOP | A loop exists in symbolic links encountered during resolution of the path argument |
| EMFILE | Too many open files |
| EMLINK | Too many links |
| EMSGSIZE | Message too long |
| EMULTIHOP | Multihop is not allowed |
| EMVSBADCHAR | Bad character in environment variable name |
| EMVSCATLG | Catalog obtain error |
| EMVSCPLERROR | A CPL service failed |
| EMVSCVAF | Catalog Volume Access Facility error |
| EMVSDYNALC | Dynamic allocation error |
| EMVSERR | An MVS™ internal error |
| EMVSEXPIRE | Password has expired |
| EMVSINITIAL | Process initialization err |
| EMVSNORTL | Access to the z/OS® UNIX version of the C RTL is denied |
| EMVSNOTXP | z/OS UNIX are not active |
| EMVSPARM | Bad parameters were passed to the service |
| EMVSPASSWORD | Password is invalid |
| EMVSPATHOPTS | Access mode argument conflicts with PATHOPTS parameter |
| EMVSPFSFILE | PDSE/X encountered a permanent file error |
| EMVSPFSPERM | PDSE/X encountered a system error |
| EMVSSAF2ERR | SAF/RACF error |
| EMVSSAFEXTRERR | SAF/RACF extract error |
| EMVSTODNOTSET | System TOD clock not set |
| ENAMETOOLONG | File name too long |
| ENETDOWN | The local interface to use or reach the destination |
| ENETRESET | Network dropped connection on reset |
| ENETUNREACH | Network unreachable |
| ENFILE | Too many open files in system |
| ENOBUFS | No buffer space available |
| ENODATA | No message available |
| ENODEV | No such device |
| ENOENT | No such file or directory |
| ENOEXEC | Exec format error |
| ENOLINK | The link has been severed |
| ENOLCK | No locks available |
| ENOMEM | Not enough space |
| ENOMSG | No message of desired type |
| ENONET | Machine is not on the network |
| ENOPROTOOPT | Protocol not available |
| ENOREUSE | The socket cannot be reused |
| ENOSPC | No space left on device |
| ENOSR | No stream resource |
| ENOSYS | Function not implemented |
| ENOSTR | Not a stream |
| ENOTBLK | Block device required |
| ENOTCONN | The socket is not connected. |
| ENOTDIR | Not a directory |
| ENOTEMPTY | Directory not empty |
| ENOTSOCK | Descriptor does not refer to a socket |
| ENOTSUP | Not supported. |
| ENOTTY | Inappropriate I/O control operation |
| ENXIO | No such device or address |
| EOFFLOADboxDOWN | Offload box down |
| EOFFLOADboxERROR | Offload box error |
| EOFFLOADboxRESTART | Offload box restarted |
| EOPNOTSUPP | Operation not supported on socket |
| EOVERFLOW | Value too large to be stored in data type |
| EPERM | Operation not permitted |
| EPFNOSUPPORT | Protocol family not supported |
| EPIPE | Broken pipe |
| EPROCLIM | Too many processes |
| EPROTO | Protocol error |
| EPROTONOSUPPORT | Protocol not supported |
| EPROTOTYPE | The socket type is not supported by the protocol |
| ERANGE | Range error or pole error |
| EREMCHG | Remote address changed |
| EREMOTE | Too many levels of remote in path |
| EROFS | Read-only file system |
| ERREMOTE | Object is remote |
| ESHUTDOWN | Cannot send after socket shutdown |
| ESOCKTNOSUPPORT | Socket type not supported |
| ESPIPE | Invalid seek |
| ESRCH | No such process |
| ESRMNT | srmount error |
| ESTALE | The file handle is stale |
| ETIME | Stream ioctl() timeout |
| ETIMEDOUT | Socket not connected |
| ETOOMANYREFS | Too many references: cannot splice |
| ETXTBSY | Text file busy |
| EUSERS | Too many users |
| EWOULDBLOCK | Problem on nonblocking socket |
| EXDEV | A link to a file on another file system was attempted |
| E2BIG | Argument list too long |
The errno.h header file also defines errno, which is a modifiable lvalue having type int. If you intend to test the value of errno after library function calls, first set it to 0, because the library functions do not reset the value to 0.
strerror() or perror() functions can be used to print the description of the message associated with a particular errno.
To test for the explicit error, use the macro names defined in errno.h, rather than specific values of these macros. Doing so will ensure future compatibility and portability.
errno.h also declares the __errno2() prototype.