Errno

• fopen and fgetsboth return NULL if there is an error (fgetc and fscanf return EOF on error, and fprintf returns a negative integer). To get more detailed information, check errno. This is a global int variable defined in errno.h that many library routines use to report error conditions.

• errno.h also defines a large number of symbolic constants that specify what the various error numbers mean. Here is a sampling:

  #define EPERM   1       /* Operation not permitted              */
  #define ENOENT  2       /* No such file or directory            */
  #define EIO     5       /* I/O error                            */
  #define EBADF   9       /* Bad file descriptor                  */
  #define ENOMEM  12      /* Not enough space                     */
  #define EACCES  13      /* Permission denied                    */
  #define EFAULT  14      /* Bad address                          */
  #define EEXIST  17      /* File exists                          */
  #define EISDIR  21      /* Is a directory                       */
  #define ENFILE  23      /* Too many open files in system        */
  #define EMFILE  24      /* Too many open files                  */
  #define EFBIG   27      /* File too large                       */
  #define ENOSPC  28      /* No space left on device              */
  #define ENOTREADY       46      /* Device not ready             */
  #define EWRPROTECT      47      /* Write-protected media        */
  #define EFORMAT         48      /* Unformatted media            */
  #define EDQUOT          88      /* Disc quota exceeded */
  

• While these constants are useful for checking for specific types of errors, most of the time you just want to report the error to the user in the semi-user friendly way. For this purpose, there is char *strerror(int errno) which converts errno numbers into descriptive error strings.

• If you aren't sure if you've hit the end of the file or if you've got an honest to goodness error, call int ferror() or int feof().