Convert POSIX integer errno to compile-time constant

Question

Occasionally (e.g. using strace, gdb etc) one finds a POSIX call sets errno to an integer value, and one wants to know the compile time C constant (more accurately pre-processor define) to check for it (e.g. ECHILD) - see e.g. waitpid for child process not succeeding.

EG in the above linked question, the integer error number 10 was returned in errno. I want to get back from that to the string ECHILD. Not to what perror or strerror would give me ("No child processes" or similar).

Here's the obvious way does to do it, which does not work:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int
main (int argc, char **argv)
{
  printf ("%s\n", strerror (10));
  exit (0);
}

This prints the output:

No child processes

not ECHILD, hence does not do the required.

Is there any easier way to do this than a somewhat manual grep through /usr/include?

Things you might think are duplicates but aren't:

• How to convert errno in UNIX to corresponding string? - says strerror (obviously) converts the integer to a human readable string, and perror prints it. I do not want to do that. I want to print the symbolic equivalent, i.e. the preprocessor define that could be used to test for that errno.

• Linux, convert errno to name - closer, but is looking for an API call. Clearly there isn't one of those. The accepted answer also incorrectly states the errors are in errno.h - in my system they are spread between multiple files making the search more entertaining. How can I print the symbolic name of an errno in C? is similar.

An appropriate answer might involve some magic to preprocess appropriate parts of /usr/include and display any constants beginning with E with the appropriate value.

Answer 1

You can call the C preprocessor directly. For the GCC toolchain, the preprocessor executable is cpp.

(Edit: I realize you specifically mentioned POSIX, and this example is specific to GCC, but maybe it's a start)

Here's an example I came up with for your situation:

$ cpp -dM -include /usr/include/errno.h | grep '^#define E' | sed 's/^#define \(E[A-Z0-9]*\)\s*\(.*\)$/\2 \1/g' | sort -n
EAGAIN EWOULDBLOCK
EDEADLK EDEADLOCK
EOPNOTSUPP ENOTSUP
1 EPERM
2 ENOENT
3 ESRCH
4 EINTR
5 EIO
6 ENXIO
7 E2BIG
8 ENOEXEC
9 EBADF
10 ECHILD
11 EAGAIN
12 ENOMEM
13 EACCES
14 EFAULT
15 ENOTBLK
16 EBUSY
17 EEXIST
18 EXDEV
19 ENODEV
20 ENOTDIR
21 EISDIR
22 EINVAL
23 ENFILE
24 EMFILE
25 ENOTTY
26 ETXTBSY
27 EFBIG
28 ENOSPC
29 ESPIPE
30 EROFS
31 EMLINK
32 EPIPE
33 EDOM
34 ERANGE
35 EDEADLK
36 ENAMETOOLONG
37 ENOLCK
38 ENOSYS
39 ENOTEMPTY
40 ELOOP
42 ENOMSG
43 EIDRM
44 ECHRNG
45 EL2NSYNC
46 EL3HLT
47 EL3RST
48 ELNRNG
49 EUNATCH
50 ENOCSI
51 EL2HLT
52 EBADE
53 EBADR
54 EXFULL
55 ENOANO
56 EBADRQC
57 EBADSLT
59 EBFONT
60 ENOSTR
61 ENODATA
62 ETIME
63 ENOSR
64 ENONET
65 ENOPKG
66 EREMOTE
67 ENOLINK
68 EADV
69 ESRMNT
70 ECOMM
71 EPROTO
72 EMULTIHOP
73 EDOTDOT
74 EBADMSG
75 EOVERFLOW
76 ENOTUNIQ
77 EBADFD
78 EREMCHG
79 ELIBACC
80 ELIBBAD
81 ELIBSCN
82 ELIBMAX
83 ELIBEXEC
84 EILSEQ
85 ERESTART
86 ESTRPIPE
87 EUSERS
88 ENOTSOCK
89 EDESTADDRREQ
90 EMSGSIZE
91 EPROTOTYPE
92 ENOPROTOOPT
93 EPROTONOSUPPORT
94 ESOCKTNOSUPPORT
95 EOPNOTSUPP
96 EPFNOSUPPORT
97 EAFNOSUPPORT
98 EADDRINUSE
99 EADDRNOTAVAIL
100 ENETDOWN
101 ENETUNREACH
102 ENETRESET
103 ECONNABORTED
104 ECONNRESET
105 ENOBUFS
106 EISCONN
107 ENOTCONN
108 ESHUTDOWN
109 ETOOMANYREFS
110 ETIMEDOUT
111 ECONNREFUSED
112 EHOSTDOWN
113 EHOSTUNREACH
114 EALREADY
115 EINPROGRESS
116 ESTALE
117 EUCLEAN
118 ENOTNAM
119 ENAVAIL
120 EISNAM
121 EREMOTEIO
122 EDQUOT
123 ENOMEDIUM
124 EMEDIUMTYPE
125 ECANCELED
126 ENOKEY
127 EKEYEXPIRED
128 EKEYREVOKED
129 EKEYREJECTED
130 EOWNERDEAD
131 ENOTRECOVERABLE
132 ERFKILL
133 EHWPOISON

A couple of things to note:

• This is almost certainly not foolproof. It's highly likely this will miss some things, and get other things wrong. For one thing, if there are other macro definitions, in any file #include-ed (directly or indirectly) by errno.h, then it is possible that grep '^#define E' is not sufficient to filter out the desired errno definitions from the output of the cpp command. And this is by no means the only way in which this approach is likely to fail.
• Evidently, there are cases, such as #define EWOULDBLOCK EAGAIN, where one Exxxx value is defined as a synonym of another previously defined value.
• There appear to be some missing values in the sequence, such as 41. I'm not sure if this is normal, or if this is an example of something that is missed by this approach.

With those disclaimers out of the way, it should be possible to use this approach as the basis of a script (which you could then optionally call from your Makefile) to auto-generate a lookup table (e.g., errno-lookup.c) and reference this table in your code to obtain the associated symbol for any given errno value.

Answer 2

The set of EFOO constants defined (directly or indirectly) in <errno.h>, and their values, varies from one system to another. And often more than one EFOO constant will have; for example, on my system both EAGAIN and EWOULDBLOCK have the value 11. So there isn't necessarily a unique EFOO symbol for a given numeric errno value.

I've collected a list of 160 E* symbols from various systems. It is neither definitive nor exhaustive. You can write a script that takes the list as input and generates a C source program that prints the numeric value of each symbol. For each symbol, the program would contain something like:

#ifdef E2BIG
    printf("E2BIG %d\n", E2BIG);
#endif

From that, you can generate a C function that returns the appropriate symbol as a string, or something like "?" if there's no such symbol.

I know this is an incomplete answer, but it's a good starting point. I might implement a more complete solution later. If so, I'll probably create it as a Github project and update this answer with a link to it.

Here's the list:

E2BIG EACCES EADDRINUSE EADDRNOTAVAIL EADV EAFNOSUPPORT EAGAIN
EALREADY EBADCOOKIE EBADE EBADF EBADFD EBADHANDLE EBADMSG EBADR EBADRQC
EBADSLT EBADTYPE EBFONT EBUSY ECANCELED ECANCELLED ECHILD ECHRNG ECOMM
ECONNABORTED ECONNREFUSED ECONNRESET EDEADLK EDEADLOCK EDESTADDRREQ
EDOM EDOTDOT EDQUOT EEXIST EFAULT EFBIG EHOSTDOWN EHOSTUNREACH EHWPOISON
EIDRM EILSEQ EINIT EINPROGRESS EINTR EINVAL EIO EIOCBQUEUED EIOCBRETRY
EISCONN EISDIR EISNAM EJUKEBOX EKEYEXPIRED EKEYREJECTED EKEYREVOKED
EL2HLT EL2NSYNC EL3HLT EL3RST ELIBACC ELIBBAD ELIBEXEC ELIBMAX ELIBSCN
ELNRNG ELOCKUNMAPPED ELOOP EMAXERRNO EMEDIUMTYPE EMFILE EMLINK EMSGSIZE
EMULTIHOP ENAMETOOLONG ENAVAIL ENETDOWN ENETRESET ENETUNREACH ENFILE
ENOANO ENOBUFS ENOCSI ENODATA ENODEV ENOENT ENOEXEC ENOIOCTLCMD ENOKEY
ENOLCK ENOLINK ENOMEDIUM ENOMEM ENOMSG ENONET ENOPKG ENOPROTOOPT ENOSPC
ENOSR ENOSTR ENOSYM ENOSYS ENOTACTIVE ENOTBLK ENOTCONN ENOTDIR ENOTEMPTY
ENOTNAM ENOTRECOVERABLE ENOTSOCK ENOTSUP ENOTSUPP ENOTSYNC ENOTTY
ENOTUNIQ ENXIO EOPNOTSUPP EOVERFLOW EOWNERDEAD EPERM EPFNOSUPPORT EPIPE
EPROCLIM EPROTO EPROTONOSUPPORT EPROTOTYPE ERANGE EREFUSED EREMCHG EREMDEV
EREMOTE EREMOTEIO EREMOTERELEASE ERESTART ERESTARTNOHAND ERESTARTNOINTR
ERESTARTSYS ERESTART_RESTARTBLOCK ERFKILL EROFS ERREMOTE ESERVERFAULT
ESHUTDOWN ESOCKTNOSUPPORT ESPIPE ESRCH ESRMNT ESTALE ESTRPIPE ETIME
ETIMEDOUT ETOOMANYREFS ETOOSMALL ETXTBSY EUCLEAN EUNATCH EUSERS
EWOULDBLOCK EXDEV EXFULL

Answer 3

I recently wrote the errnoname library that has an errnoname function which does exactly this.

So you can take your example program from the question, #include "errnoname.h", just replace strerror with errnoname, and compile/link it with errnoname.c.

As the other answers make clear, there is nothing hard or complex about the errnoname function itself, it is just really tedious to implement by hand, to make sure all errno names are included, and all the little corner-cases are handled.

Examples:

• As another answer points out, there are pairs, like EWOULDBLOCK is equal to EAGAIN, that have the same value on some systems, different values on other systems, and
• errno names are not at all guaranteed to be contiguous (hence why I recommend implementing this as a switch statement instead of as an array, because on some rare or future systems it might be a very, very inefficiently large array with many unused entries, if it even compiles).

Anyway, since it is released under the "Zero-Clause BSD license" (0BSD), which is a permissive license, or more accurately a public-domain-equivalent license, you can do whatever you want with it.

Below is a direct copy-paste of the function from my library, so that this answer can stand alone. Couple of notes:

1. This covers all errno names I could find for Linux, Darwin (Mac OS X and iOS X), FreeBSD, NetBSD, OpenBSD, DragonflyBSD, and several closed source Unixes, as of the start of August 2019.

2. It returns a NULL pointer if you give it an errno value it does not know the name of.

#include <errno.h>

char const * errnoname(int errno_)
{
    switch(errno_)
    {
#ifdef E2BIG
        case E2BIG: return "E2BIG";
#endif
#ifdef EACCES
        case EACCES: return "EACCES";
#endif
#ifdef EADDRINUSE
        case EADDRINUSE: return "EADDRINUSE";
#endif
#ifdef EADDRNOTAVAIL
        case EADDRNOTAVAIL: return "EADDRNOTAVAIL";
#endif
#ifdef EADI
        case EADI: return "EADI";
#endif
#ifdef EADV
        case EADV: return "EADV";
#endif
#ifdef EAFNOSUPPORT
        case EAFNOSUPPORT: return "EAFNOSUPPORT";
#endif
#ifdef EAGAIN
        case EAGAIN: return "EAGAIN";
#endif
#ifdef EAIO
        case EAIO: return "EAIO";
#endif
#ifdef EALIGN
        case EALIGN: return "EALIGN";
#endif
#ifdef EALREADY
        case EALREADY: return "EALREADY";
#endif
#ifdef EASYNC
        case EASYNC: return "EASYNC";
#endif
#ifdef EAUTH
        case EAUTH: return "EAUTH";
#endif
#ifdef EBADARCH
        case EBADARCH: return "EBADARCH";
#endif
#ifdef EBADE
        case EBADE: return "EBADE";
#endif
#ifdef EBADEXEC
        case EBADEXEC: return "EBADEXEC";
#endif
#ifdef EBADF
        case EBADF: return "EBADF";
#endif
#ifdef EBADFD
        case EBADFD: return "EBADFD";
#endif
#ifdef EBADMACHO
        case EBADMACHO: return "EBADMACHO";
#endif
#ifdef EBADMSG
        case EBADMSG: return "EBADMSG";
#endif
#ifdef EBADR
        case EBADR: return "EBADR";
#endif
#ifdef EBADRPC
        case EBADRPC: return "EBADRPC";
#endif
#ifdef EBADRQC
        case EBADRQC: return "EBADRQC";
#endif
#ifdef EBADSLT
        case EBADSLT: return "EBADSLT";
#endif
#ifdef EBADVER
        case EBADVER: return "EBADVER";
#endif
#ifdef EBFONT
        case EBFONT: return "EBFONT";
#endif
#ifdef EBUSY
        case EBUSY: return "EBUSY";
#endif
#ifdef ECANCELED
        case ECANCELED: return "ECANCELED";
#endif
#if defined(ECANCELLED) && (!defined(ECANCELED) || ECANCELLED != ECANCELED)
        case ECANCELLED: return "ECANCELLED";
#endif
#ifdef ECAPMODE
        case ECAPMODE: return "ECAPMODE";
#endif
#ifdef ECHILD
        case ECHILD: return "ECHILD";
#endif
#ifdef ECHRNG
        case ECHRNG: return "ECHRNG";
#endif
#ifdef ECKPT
        case ECKPT: return "ECKPT";
#endif
#ifdef ECLONEME
        case ECLONEME: return "ECLONEME";
#endif
#ifdef ECOMM
        case ECOMM: return "ECOMM";
#endif
#ifdef ECONFIG
        case ECONFIG: return "ECONFIG";
#endif
#ifdef ECONNABORTED
        case ECONNABORTED: return "ECONNABORTED";
#endif
#ifdef ECONNREFUSED
        case ECONNREFUSED: return "ECONNREFUSED";
#endif
#ifdef ECONNRESET
        case ECONNRESET: return "ECONNRESET";
#endif
#ifdef ECORRUPT
        case ECORRUPT: return "ECORRUPT";
#endif
#ifdef ECVCERORR
        case ECVCERORR: return "ECVCERORR";
#endif
#ifdef ECVPERORR
        case ECVPERORR: return "ECVPERORR";
#endif
#ifdef EDEADLK
        case EDEADLK: return "EDEADLK";
#endif
#if defined(EDEADLOCK) && (!defined(EDEADLK) || EDEADLOCK != EDEADLK)
        case EDEADLOCK: return "EDEADLOCK";
#endif
#ifdef EDESTADDREQ
        case EDESTADDREQ: return "EDESTADDREQ";
#endif
#ifdef EDESTADDRREQ
        case EDESTADDRREQ: return "EDESTADDRREQ";
#endif
#ifdef EDEVERR
        case EDEVERR: return "EDEVERR";
#endif
#ifdef EDIRIOCTL
        case EDIRIOCTL: return "EDIRIOCTL";
#endif
#ifdef EDIRTY
        case EDIRTY: return "EDIRTY";
#endif
#ifdef EDIST
        case EDIST: return "EDIST";
#endif
#ifdef EDOM
        case EDOM: return "EDOM";
#endif
#ifdef EDOOFUS
        case EDOOFUS: return "EDOOFUS";
#endif
#ifdef EDOTDOT
        case EDOTDOT: return "EDOTDOT";
#endif
#ifdef EDQUOT
        case EDQUOT: return "EDQUOT";
#endif
#ifdef EDUPFD
        case EDUPFD: return "EDUPFD";
#endif
#ifdef EDUPPKG
        case EDUPPKG: return "EDUPPKG";
#endif
#ifdef EEXIST
        case EEXIST: return "EEXIST";
#endif
#ifdef EFAIL
        case EFAIL: return "EFAIL";
#endif
#ifdef EFAULT
        case EFAULT: return "EFAULT";
#endif
#ifdef EFBIG
        case EFBIG: return "EFBIG";
#endif
#ifdef EFORMAT
        case EFORMAT: return "EFORMAT";
#endif
#ifdef EFSCORRUPTED
        case EFSCORRUPTED: return "EFSCORRUPTED";
#endif
#ifdef EFTYPE
        case EFTYPE: return "EFTYPE";
#endif
#ifdef EHOSTDOWN
        case EHOSTDOWN: return "EHOSTDOWN";
#endif
#ifdef EHOSTUNREACH
        case EHOSTUNREACH: return "EHOSTUNREACH";
#endif
#ifdef EHWPOISON
        case EHWPOISON: return "EHWPOISON";
#endif
#ifdef EIDRM
        case EIDRM: return "EIDRM";
#endif
#ifdef EILSEQ
        case EILSEQ: return "EILSEQ";
#endif
#ifdef EINIT
        case EINIT: return "EINIT";
#endif
#ifdef EINPROG
        case EINPROG: return "EINPROG";
#endif
#ifdef EINPROGRESS
        case EINPROGRESS: return "EINPROGRESS";
#endif
#ifdef EINTEGRITY
        case EINTEGRITY: return "EINTEGRITY";
#endif
#ifdef EINTR
        case EINTR: return "EINTR";
#endif
#ifdef EINVAL
        case EINVAL: return "EINVAL";
#endif
#ifdef EIO
        case EIO: return "EIO";
#endif
#ifdef EIPSEC
        case EIPSEC: return "EIPSEC";
#endif
#ifdef EISCONN
        case EISCONN: return "EISCONN";
#endif
#ifdef EISDIR
        case EISDIR: return "EISDIR";
#endif
#ifdef EISNAM
        case EISNAM: return "EISNAM";
#endif
#ifdef EJUSTRETURN
        case EJUSTRETURN: return "EJUSTRETURN";
#endif
#ifdef EKEEPLOOKING
        case EKEEPLOOKING: return "EKEEPLOOKING";
#endif
#ifdef EKEYEXPIRED
        case EKEYEXPIRED: return "EKEYEXPIRED";
#endif
#ifdef EKEYREJECTED
        case EKEYREJECTED: return "EKEYREJECTED";
#endif
#ifdef EKEYREVOKED
        case EKEYREVOKED: return "EKEYREVOKED";
#endif
#ifdef EL2HLT
        case EL2HLT: return "EL2HLT";
#endif
#ifdef EL2NSYNC
        case EL2NSYNC: return "EL2NSYNC";
#endif
#ifdef EL3HLT
        case EL3HLT: return "EL3HLT";
#endif
#ifdef EL3RST
        case EL3RST: return "EL3RST";
#endif
#ifdef ELIBACC
        case ELIBACC: return "ELIBACC";
#endif
#ifdef ELIBBAD
        case ELIBBAD: return "ELIBBAD";
#endif
#ifdef ELIBEXEC
        case ELIBEXEC: return "ELIBEXEC";
#endif
#ifdef ELIBMAX
        case ELIBMAX: return "ELIBMAX";
#endif
#ifdef ELIBSCN
        case ELIBSCN: return "ELIBSCN";
#endif
#ifdef ELNRNG
        case ELNRNG: return "ELNRNG";
#endif
#ifdef ELOCKUNMAPPED
        case ELOCKUNMAPPED: return "ELOCKUNMAPPED";
#endif
#ifdef ELOOP
        case ELOOP: return "ELOOP";
#endif
#ifdef EMEDIA
        case EMEDIA: return "EMEDIA";
#endif
#ifdef EMEDIUMTYPE
        case EMEDIUMTYPE: return "EMEDIUMTYPE";
#endif
#ifdef EMFILE
        case EMFILE: return "EMFILE";
#endif
#ifdef EMLINK
        case EMLINK: return "EMLINK";
#endif
#ifdef EMOUNTEXIT
        case EMOUNTEXIT: return "EMOUNTEXIT";
#endif
#ifdef EMOVEFD
        case EMOVEFD: return "EMOVEFD";
#endif
#ifdef EMSGSIZE
        case EMSGSIZE: return "EMSGSIZE";
#endif
#ifdef EMTIMERS
        case EMTIMERS: return "EMTIMERS";
#endif
#ifdef EMULTIHOP
        case EMULTIHOP: return "EMULTIHOP";
#endif
#ifdef ENAMETOOLONG
        case ENAMETOOLONG: return "ENAMETOOLONG";
#endif
#ifdef ENAVAIL
        case ENAVAIL: return "ENAVAIL";
#endif
#ifdef ENEEDAUTH
        case ENEEDAUTH: return "ENEEDAUTH";
#endif
#ifdef ENETDOWN
        case ENETDOWN: return "ENETDOWN";
#endif
#ifdef ENETRESET
        case ENETRESET: return "ENETRESET";
#endif
#ifdef ENETUNREACH
        case ENETUNREACH: return "ENETUNREACH";
#endif
#ifdef ENFILE
        case ENFILE: return "ENFILE";
#endif
#ifdef ENFSREMOTE
        case ENFSREMOTE: return "ENFSREMOTE";
#endif
#ifdef ENOANO
        case ENOANO: return "ENOANO";
#endif
#ifdef ENOATTR
        case ENOATTR: return "ENOATTR";
#endif
#ifdef ENOBUFS
        case ENOBUFS: return "ENOBUFS";
#endif
#ifdef ENOCONNECT
        case ENOCONNECT: return "ENOCONNECT";
#endif
#ifdef ENOCSI
        case ENOCSI: return "ENOCSI";
#endif
#ifdef ENODATA
        case ENODATA: return "ENODATA";
#endif
#ifdef ENODEV
        case ENODEV: return "ENODEV";
#endif
#ifdef ENOENT
        case ENOENT: return "ENOENT";
#endif
#ifdef ENOEXEC
        case ENOEXEC: return "ENOEXEC";
#endif
#ifdef ENOIOCTL
        case ENOIOCTL: return "ENOIOCTL";
#endif
#ifdef ENOKEY
        case ENOKEY: return "ENOKEY";
#endif
#ifdef ENOLCK
        case ENOLCK: return "ENOLCK";
#endif
#ifdef ENOLINK
        case ENOLINK: return "ENOLINK";
#endif
#ifdef ENOLOAD
        case ENOLOAD: return "ENOLOAD";
#endif
#ifdef ENOMATCH
        case ENOMATCH: return "ENOMATCH";
#endif
#ifdef ENOMEDIUM
        case ENOMEDIUM: return "ENOMEDIUM";
#endif
#ifdef ENOMEM
        case ENOMEM: return "ENOMEM";
#endif
#ifdef ENOMSG
        case ENOMSG: return "ENOMSG";
#endif
#ifdef ENONET
        case ENONET: return "ENONET";
#endif
#ifdef ENOPKG
        case ENOPKG: return "ENOPKG";
#endif
#ifdef ENOPOLICY
        case ENOPOLICY: return "ENOPOLICY";
#endif
#ifdef ENOPROTOOPT
        case ENOPROTOOPT: return "ENOPROTOOPT";
#endif
#ifdef ENOREG
        case ENOREG: return "ENOREG";
#endif
#ifdef ENOSPC
        case ENOSPC: return "ENOSPC";
#endif
#ifdef ENOSR
        case ENOSR: return "ENOSR";
#endif
#ifdef ENOSTR
        case ENOSTR: return "ENOSTR";
#endif
#ifdef ENOSYM
        case ENOSYM: return "ENOSYM";
#endif
#ifdef ENOSYS
        case ENOSYS: return "ENOSYS";
#endif
#ifdef ENOTACTIVE
        case ENOTACTIVE: return "ENOTACTIVE";
#endif
#ifdef ENOTBLK
        case ENOTBLK: return "ENOTBLK";
#endif
#ifdef ENOTCAPABLE
        case ENOTCAPABLE: return "ENOTCAPABLE";
#endif
#ifdef ENOTCONN
        case ENOTCONN: return "ENOTCONN";
#endif
#ifdef ENOTDIR
        case ENOTDIR: return "ENOTDIR";
#endif
#ifdef ENOTEMPTY
        case ENOTEMPTY: return "ENOTEMPTY";
#endif
#ifdef ENOTNAM
        case ENOTNAM: return "ENOTNAM";
#endif
#ifdef ENOTREADY
        case ENOTREADY: return "ENOTREADY";
#endif
#ifdef ENOTRECOVERABLE
        case ENOTRECOVERABLE: return "ENOTRECOVERABLE";
#endif
#ifdef ENOTRUST
        case ENOTRUST: return "ENOTRUST";
#endif
#ifdef ENOTSOCK
        case ENOTSOCK: return "ENOTSOCK";
#endif
#ifdef ENOTSUP
        case ENOTSUP: return "ENOTSUP";
#endif
#ifdef ENOTTY
        case ENOTTY: return "ENOTTY";
#endif
#ifdef ENOTUNIQ
        case ENOTUNIQ: return "ENOTUNIQ";
#endif
#ifdef ENOUNLD
        case ENOUNLD: return "ENOUNLD";
#endif
#ifdef ENOUNREG
        case ENOUNREG: return "ENOUNREG";
#endif
#ifdef ENXIO
        case ENXIO: return "ENXIO";
#endif
#ifdef EOPCOMPLETE
        case EOPCOMPLETE: return "EOPCOMPLETE";
#endif
#if defined(EOPNOTSUPP) && (!defined(ENOTSUP) || EOPNOTSUPP != ENOTSUP)
        case EOPNOTSUPP: return "EOPNOTSUPP";
#endif
#ifdef EOVERFLOW
        case EOVERFLOW: return "EOVERFLOW";
#endif
#ifdef EOWNERDEAD
        case EOWNERDEAD: return "EOWNERDEAD";
#endif
#ifdef EPASSTHROUGH
        case EPASSTHROUGH: return "EPASSTHROUGH";
#endif
#ifdef EPATHREMOTE
        case EPATHREMOTE: return "EPATHREMOTE";
#endif
#ifdef EPERM
        case EPERM: return "EPERM";
#endif
#ifdef EPFNOSUPPORT
        case EPFNOSUPPORT: return "EPFNOSUPPORT";
#endif
#ifdef EPIPE
        case EPIPE: return "EPIPE";
#endif
#ifdef EPOWERF
        case EPOWERF: return "EPOWERF";
#endif
#ifdef EPROCLIM
        case EPROCLIM: return "EPROCLIM";
#endif
#ifdef EPROCUNAVAIL
        case EPROCUNAVAIL: return "EPROCUNAVAIL";
#endif
#ifdef EPROGMISMATCH
        case EPROGMISMATCH: return "EPROGMISMATCH";
#endif
#ifdef EPROGUNAVAIL
        case EPROGUNAVAIL: return "EPROGUNAVAIL";
#endif
#ifdef EPROTO
        case EPROTO: return "EPROTO";
#endif
#ifdef EPROTONOSUPPORT
        case EPROTONOSUPPORT: return "EPROTONOSUPPORT";
#endif
#ifdef EPROTOTYPE
        case EPROTOTYPE: return "EPROTOTYPE";
#endif
#ifdef EPWROFF
        case EPWROFF: return "EPWROFF";
#endif
#ifdef EQFULL
        case EQFULL: return "EQFULL";
#endif
#ifdef EQSUSPENDED
        case EQSUSPENDED: return "EQSUSPENDED";
#endif
#ifdef ERANGE
        case ERANGE: return "ERANGE";
#endif
#ifdef ERECYCLE
        case ERECYCLE: return "ERECYCLE";
#endif
#ifdef EREDRIVEOPEN
        case EREDRIVEOPEN: return "EREDRIVEOPEN";
#endif
#ifdef EREFUSED
        case EREFUSED: return "EREFUSED";
#endif
#ifdef ERELOC
        case ERELOC: return "ERELOC";
#endif
#ifdef ERELOCATED
        case ERELOCATED: return "ERELOCATED";
#endif
#ifdef ERELOOKUP
        case ERELOOKUP: return "ERELOOKUP";
#endif
#ifdef EREMCHG
        case EREMCHG: return "EREMCHG";
#endif
#ifdef EREMDEV
        case EREMDEV: return "EREMDEV";
#endif
#ifdef EREMOTE
        case EREMOTE: return "EREMOTE";
#endif
#ifdef EREMOTEIO
        case EREMOTEIO: return "EREMOTEIO";
#endif
#ifdef EREMOTERELEASE
        case EREMOTERELEASE: return "EREMOTERELEASE";
#endif
#ifdef ERESTART
        case ERESTART: return "ERESTART";
#endif
#ifdef ERFKILL
        case ERFKILL: return "ERFKILL";
#endif
#ifdef EROFS
        case EROFS: return "EROFS";
#endif
#ifdef ERPCMISMATCH
        case ERPCMISMATCH: return "ERPCMISMATCH";
#endif
#ifdef ESAD
        case ESAD: return "ESAD";
#endif
#ifdef ESHLIBVERS
        case ESHLIBVERS: return "ESHLIBVERS";
#endif
#ifdef ESHUTDOWN
        case ESHUTDOWN: return "ESHUTDOWN";
#endif
#ifdef ESOCKTNOSUPPORT
        case ESOCKTNOSUPPORT: return "ESOCKTNOSUPPORT";
#endif
#ifdef ESOFT
        case ESOFT: return "ESOFT";
#endif
#ifdef ESPIPE
        case ESPIPE: return "ESPIPE";
#endif
#ifdef ESRCH
        case ESRCH: return "ESRCH";
#endif
#ifdef ESRMNT
        case ESRMNT: return "ESRMNT";
#endif
#ifdef ESTALE
        case ESTALE: return "ESTALE";
#endif
#ifdef ESTART
        case ESTART: return "ESTART";
#endif
#ifdef ESTRPIPE
        case ESTRPIPE: return "ESTRPIPE";
#endif
#ifdef ESYSERROR
        case ESYSERROR: return "ESYSERROR";
#endif
#ifdef ETIME
        case ETIME: return "ETIME";
#endif
#ifdef ETIMEDOUT
        case ETIMEDOUT: return "ETIMEDOUT";
#endif
#ifdef ETOOMANYREFS
        case ETOOMANYREFS: return "ETOOMANYREFS";
#endif
#ifdef ETXTBSY
        case ETXTBSY: return "ETXTBSY";
#endif
#ifdef EUCLEAN
        case EUCLEAN: return "EUCLEAN";
#endif
#ifdef EUNATCH
        case EUNATCH: return "EUNATCH";
#endif
#ifdef EUSERS
        case EUSERS: return "EUSERS";
#endif
#ifdef EVERSION
        case EVERSION: return "EVERSION";
#endif
#if defined(EWOULDBLOCK) && (!defined(EAGAIN) || EWOULDBLOCK != EAGAIN)
        case EWOULDBLOCK: return "EWOULDBLOCK";
#endif
#ifdef EWRONGFS
        case EWRONGFS: return "EWRONGFS";
#endif
#ifdef EWRPROTECT
        case EWRPROTECT: return "EWRPROTECT";
#endif
#ifdef EXDEV
        case EXDEV: return "EXDEV";
#endif
#ifdef EXFULL
        case EXFULL: return "EXFULL";
#endif
    }
    return 0;
}

Answer 4

Here's my own answer, based on a combination of the others plus some perl:

#!/usr/bin/perl
use strict;
use warnings;
die "Syntax: error ERRORNUM" unless ($#ARGV==0);
open (my $p, "cpp -dM /usr/include/errno.h |") || die ("Cannot preprocess headers: $!");
while (<$p>)
{
    chomp;
    print "$1\n" if /^#define (E\w+) (\d+)/ && $2==$ARGV[0];
}
close ($p);

Usage example:

$ ./error 10
ECHILD

I'm sure it would be improved by some error handling.