Definiciones de errno ANSI C

La definición de estas constantes puede depender del compilador, y se incluyen aquí, sólo como ejemplo.

En el caso de Dev-C++, estas constantes son:

#define EPERM		1    /* Operation not permitted */
#define    ENOFILE		2    /* No such file or directory */
#define    ENOENT		2
#define    ESRCH		3    /* No such process */
#define    EINTR		4    /* Interrupted function call */
#define    EIO		5    /* Input/output error */
#define    ENXIO		6    /* No such device or address */
#define    E2BIG		7    /* Arg list too long */
#define    ENOEXEC		8    /* Exec format error */
#define    EBADF		9    /* Bad file descriptor */
#define    ECHILD		10    /* No child processes */
#define    EAGAIN		11    /* Resource temporarily unavailable */
#define    ENOMEM		12    /* Not enough space */
#define    EACCES		13    /* Permission denied */
#define    EFAULT		14    /* Bad address */
/* 15 - Unknown Error */
#define    EBUSY		16    /* strerror reports "Resource device" */
#define    EEXIST		17    /* File exists */
#define    EXDEV		18    /* Improper link (cross-device link?) */
#define    ENODEV		19    /* No such device */
#define    ENOTDIR		20    /* Not a directory */
#define    EISDIR		21    /* Is a directory */
#define    EINVAL		22    /* Invalid argument */
#define    ENFILE		23    /* Too many open files in system */
#define    EMFILE		24    /* Too many open files */
#define    ENOTTY		25    /* Inappropriate I/O control operation */
/* 26 - Unknown Error */
#define    EFBIG		27    /* File too large */
#define    ENOSPC		28    /* No space left on device */
#define    ESPIPE		29    /* Invalid seek (seek on a pipe?) */
#define    EROFS		30    /* Read-only file system */
#define    EMLINK		31    /* Too many links */
#define    EPIPE		32    /* Broken pipe */
#define    EDOM		33    /* Domain error (math functions) */
#define    ERANGE		34    /* Result too large (possibly too small) */
/* 35 - Unknown Error */
#define    EDEADLOCK    36    /* Resource deadlock avoided (non-Cyg) */
#define    EDEADLK		36
/* 37 - Unknown Error */
#define    ENAMETOOLONG    38    /* Filename too long (91 in Cyg?) */
#define    ENOLCK		39    /* No locks available (46 in Cyg?) */
#define    ENOSYS		40    /* Function not implemented (88 in Cyg?) */
#define    ENOTEMPTY    41    /* Directory not empty (90 in Cyg?) */
#define    EILSEQ		42    /* Illegal byte sequence */

En el caso de Borland C++, son:

#define EZERO    0      /* Error 0                  */
#define EINVFNC  1      /* Invalid function number  */
#define ENOFILE  2      /* File not found           */
#define ENOPATH  3      /* Path not found           */
#define ECONTR   7      /* Memory blocks destroyed  */
#define EINVMEM  9      /* Invalid memory block address */
#define EINVENV 10      /* Invalid environment      */
#define EINVFMT 11      /* Invalid format           */
#define EINVACC 12      /* Invalid access code      */
#define EINVDAT 13      /* Invalid data             */
#define EINVDRV 15      /* Invalid drive specified  */
#define ECURDIR 16      /* Attempt to remove CurDir */
#define ENOTSAM 17      /* Not same device          */
#define ENMFILE 18      /* No more files            */

#define ENOENT   2      /* No such file or directory*/
#define EMFILE   4      /* Too many open files      */
#define EACCES   5      /* Permission denied        */
#define EBADF    6      /* Bad file number          */
#define ENOMEM   8      /* Not enough core          */
#define EFAULT  14      /* Unknown error            */
#define ENODEV  15      /* No such device           */
#define EINVAL  19      /* Invalid argument         */
#define E2BIG   20      /* Arg list too long        */
#define ENOEXEC 21      /* Exec format error        */
#define EXDEV   22      /* Cross-device link        */
#define ENFILE  23      /* Too many open files      */
#define ECHILD  24      /* No child process         */
#define ENOTTY  25      /* UNIX - not MSDOS         */
#define ETXTBSY 26      /* UNIX - not MSDOS         */
#define EFBIG   27      /* UNIX - not MSDOS         */
#define ENOSPC  28      /* No space left on device  */
#define ESPIPE  29      /* Illegal seek             */
#define EROFS   30      /* Read-only file system    */
#define EMLINK  31      /* UNIX - not MSDOS         */
#define EPIPE   32      /* Broken pipe              */
#define EDOM    33      /* Math argument            */
#define ERANGE  34      /* Result too large         */
#define EEXIST  35      /* File already exists      */
#define EDEADLOCK 36    /* Locking violation        */
#define EPERM   37      /* Operation not permitted  */
#define ESRCH   38      /* UNIX - not MSDOS         */
#define EINTR   39      /* Interrupted function call */
#define EIO     40      /* Input/output error       */
#define ENXIO   41      /* No such device or address */
#define EAGAIN  42      /* Resource temporarily unavailable */
#define ENOTBLK 43      /* UNIX - not MSDOS         */
#define EBUSY   44      /* Resource busy            */
#define ENOTDIR 45      /* UNIX - not MSDOS         */
#define EISDIR  46      /* UNIX - not MSDOS         */
#define EUCLEAN 47      /* UNIX - not MSDOS         */

También se declara la variable:

extern int errno; 

Esta variable se usa por la función perror de stdio.h, para obtener los mensajes de error.