Linux, convert errno to name

Question

I am looking for an API to convert an errno integer to its name.

For example:

int fd;
if((fd = open(path, O_RDONLY)) == -1)
    printf("error: %d %s %s\n", errno, strerror(errno), ERRNONAME(errno));

So, ERRNONAME would yield a name, such as "EINVAL", "EPERM", etc. Is this possible?

Answer 1

Those names exist as macros in the Errno.h file. There's no standard call to convert the error number to the short code. You could build your own table ...

Answer 2

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

There is unfortunately still no standard API to do this in C, and near as I can tell no other open source C library doing this.

There is nothing hard or complex about it, it is just really tedious to implement by hand, to make sure all errno names are included, and there are little corner-cases that need to be handled.

So exactly the sort of thing that should be concentrated in an open source library instead of everyone having to rewrite it and get all the details right over and over again.

For example EWOULDBLOCK is equal to EAGAIN on some systems, and the same system call might have different possible errors on different 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).

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;
}