This function returns non-zero if an error has occurred on the file descriptor since the last read, because the read and readb functions clear the file descriptors error code.
When the read or readb function return the empty string, that is caused either by an error, an eof, or a blank line. This function will tell you if it was an error. The eof function will tell you if it was an end of file.
The file descriptor must have been a value previously returned by the open function.