# Errno.h

errno.h is a header file in the standard library of the C programming language. It defines macros for reporting and retrieving error conditions using the symbol errno (short for "error number").
errno acts like an integer variable. A value (the error number) is stored in errno by certain library functions when they detect errors. At program startup, the value stored is zero. Library functions store only values greater than zero. Any library function can alter the value stored before return, whether or not they detect errors. Most functions indicate that they detected an error by returning a special value, typically NULL for functions that return pointers, and -1 for functions that return integers. A few functions require the caller to preset errno to zero and test it afterwards to see if an error was detected.
The errno macro expands to an lvalue with type int, sometimes with the extern and/or volatile type specifiers depending upon the platform. Originally this was a static memory location, but macros are almost always used today to allow for multi-threading, so that each thread will see its own thread-local error number.
The header file also defines macros that expand to integer constants that represent the error codes. The C standard library only requires three to be defined:
| EDOM | A parameter was outside a function's domain, e.g. sqrt(-1) |
| ERANGE | A result outside a function's range, e.g. strtol("0xfffffffff", NULL, 0) on systems with a 32-bit wide long |
| EILSEQ | (Required since 1994 Amendment 1 to C89 standard) Illegal byte sequence, e.g. mbstowcs(buf, "\xff", 1) on systems that use UTF-8. |
POSIX compliant operating systems like AIX, Linux or Solaris include many other error values, many of which are used much more often than the above ones, such as EACCES for when a file cannot be opened for reading. C++11 additionally defines many of the same values found within the POSIX specification.
Traditionally, the first page of Unix system manuals, named intro(2), lists all errno.h macros, but this is not the case with Linux, where these macros are instead listed in the errno(3).
An errno can be translated to a descriptive string using strerror (defined in string.h) or a BSD extension called sys_errlist. The translation can be printed directly to the standard error stream using perror (defined in stdio.h). As strerror in many Unix-like systems is not thread-safe, a thread-safe version strerror_r is used, but conflicting definitions from POSIX and GNU makes it even less portable than the sys_errlist table.

## POSIX errors

The GNU C library (GLIBC) provides the additional POSIX error values macros in the header file errno.h. These are the descriptions of the macros provided by strerror, excluding 41 and 58 as they are not in the POSIX standard:
| Symbol | Value | Description |
| 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 |
| ENXIO | 6 | No such device or address |
| E2BIG | 7 | Argument list too long |
| ENOEXEC | 8 | Exec format error |
| EBADF | 9 | Bad file descriptor |
| ECHILD | 10 | No child processes |
| EAGAIN | 11 | Resource temporarily unavailable |
| ENOMEM | 12 | Cannot allocate memory |
| EACCES | 13 | Permission denied |
| EFAULT | 14 | Bad address |
| ENOTBLK | 15 | Block device required |
| EBUSY | 16 | Device or resource busy |
| EEXIST | 17 | File exists |
| EXDEV | 18 | Invalid cross-device link |
| ENODEV | 19 | No such device |
| ENOTDIR | 20 | Not a directory |
| EISDIR | 21 | Is a directory |
| EINVAL | 22 | Invalid argument |
| ENFILE | 23 | Too many open files in system |
| EMFILE | 24 | Too many open files |
| ENOTTY | 25 | Inappropriate ioctl for device |
| ETXTBSY | 26 | Text file busy |
| EFBIG | 27 | File too large |
| ENOSPC | 28 | No space left on device |
| ESPIPE | 29 | Illegal seek |
| EROFS | 30 | Read-only file system |
| EMLINK | 31 | Too many links |
| EPIPE | 32 | Broken pipe |
| EDOM | 33 | Numerical argument out of domain |
| ERANGE | 34 | Numerical result out of range |
| EDEADLK | 35 | Resource deadlock avoided |
| ENAMETOOLONG | 36 | File name too long |
| ENOLCK | 37 | No locks available |
| ENOSYS | 38 | Function not implemented |
| ENOTEMPTY | 39 | Directory not empty |
| ELOOP | 40 | Too many levels of symbolic links |
| ENOMSG | 42 | No message of desired type |
| EIDRM | 43 | Identifier removed |
| ECHRNG | 44 | Channel number out of range |
| EL2NSYNC | 45 | Level 2 not synchronized |
| EL3HLT | 46 | Level 3 halted |
| EL3RST | 47 | Level 3 reset |
| ELNRNG | 48 | Link number out of range |
| EUNATCH | 49 | Protocol driver not attached |
| ENOCSI | 50 | No CSI structure available |
| EL2HLT | 51 | Level 2 halted |
| EBADE | 52 | Invalid exchange |
| EBADR | 53 | Invalid request descriptor |
| EXFULL | 54 | Exchange full |
| ENOANO | 55 | No anode |
| EBADRQC | 56 | Invalid request code |
| EBADSLT | 57 | Invalid slot |
| EBFONT | 59 | Bad font file format |
| ENOSTR | 60 | Device not a stream |
| ENODATA | 61 | No data available |
| ETIME | 62 | Timer expired |
| ENOSR | 63 | Out of streams resources |
| ENONET | 64 | Machine is not on the network |
| ENOPKG | 65 | Package not installed |
| EREMOTE | 66 | Object is remote |
| ENOLINK | 67 | Link has been severed |
| EADV | 68 | Advertise error |
| ESRMNT | 69 | Srmount error |
| ECOMM | 70 | Communication error on send |
| EPROTO | 71 | Protocol error |
| EMULTIHOP | 72 | Multihop attempted |
| EDOTDOT | 73 | RFS specific error |
| EBADMSG | 74 | Bad message |
| EOVERFLOW | 75 | Value too large for defined data type |
| ENOTUNIQ | 76 | Name not unique on network |
| EBADFD | 77 | File descriptor in bad state |
| EREMCHG | 78 | Remote address changed |
| ELIBACC | 79 | Can not access a needed shared library |
| ELIBBAD | 80 | Accessing a corrupted shared library |
| ELIBSCN | 81 | .lib section in a.out corrupted |
| ELIBMAX | 82 | Attempting to link in too many shared libraries |
| ELIBEXEC | 83 | Cannot exec a shared library directly |
| EILSEQ | 84 | Invalid or incomplete multibyte or wide character |
| ERESTART | 85 | Interrupted system call should be restarted |
| ESTRPIPE | 86 | Streams pipe error |
| EUSERS | 87 | Too many users |
| ENOTSOCK | 88 | Socket operation on non-socket |
| EDESTADDRREQ | 89 | Destination address required |
| EMSGSIZE | 90 | Message too long |
| EPROTOTYPE | 91 | Protocol wrong type for socket |
| ENOPROTOOPT | 92 | Protocol not available |
| EPROTONOSUPPORT | 93 | Protocol not supported |
| ESOCKTNOSUPPORT | 94 | Socket type not supported |
| EOPNOTSUPP | 95 | Operation not supported |
| EPFNOSUPPORT | 96 | Protocol family not supported |
| EAFNOSUPPORT | 97 | Address family not supported by protocol |
| EADDRINUSE | 98 | Address already in use |
| EADDRNOTAVAIL | 99 | Cannot assign requested address |
| ENETDOWN | 100 | Network is down |
| ENETUNREACH | 101 | Network is unreachable |
| ENETRESET | 102 | Network dropped connection on reset |
| ECONNABORTED | 103 | Software caused connection abort |
| ECONNRESET | 104 | Connection reset by peer |
| ENOBUFS | 105 | No buffer space available |
| EISCONN | 106 | Transport endpoint is already connected |
| ENOTCONN | 107 | Transport endpoint is not connected |
| ESHUTDOWN | 108 | Cannot send after transport endpoint shutdown |
| ETOOMANYREFS | 109 | Too many references: cannot splice |
| ETIMEDOUT | 110 | Connection timed out |
| ECONNREFUSED | 111 | Connection refused |
| EHOSTDOWN | 112 | Host is down |
| EHOSTUNREACH | 113 | No route to host |
| EALREADY | 114 | Operation already in progress |
| EINPROGRESS | 115 | Operation now in progress |
| ESTALE | 116 | Stale file handle |
| EUCLEAN | 117 | Structure needs cleaning |
| ENOTNAM | 118 | Not a XENIX named type file |
| ENAVAIL | 119 | No XENIX semaphores available |
| EISNAM | 120 | Is a named type file |
| EREMOTEIO | 121 | Remote I/O error |
| EDQUOT | 122 | Disk quota exceeded |
| ENOMEDIUM | 123 | No medium found |
| EMEDIUMTYPE | 124 | Wrong medium type |
| ECANCELED | 125 | Operation canceled |
| ENOKEY | 126 | Required key not available |
| EKEYEXPIRED | 127 | Key has expired |
| EKEYREVOKED | 128 | Key has been revoked |
| EKEYREJECTED | 129 | Key was rejected by service |
| EOWNERDEAD | 130 | Owner died |
| ENOTRECOVERABLE | 131 | State not recoverable |
| ERFKILL | 132 | Operation not possible due to RF-kill |
| EHWPOISON | 133 | Memory page has hardware error |