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+======Synopsis:======
+$[[ferror]](<<file descriptor>>)
+======Technical:======
+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 descriptor//s 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.
+======Returns:======
+|-1   |Error: //file descriptor// was not previously returned by [[open]]|
+| 0   |no error has occurred since the last [[read]]|
+| 1   |an error has occurred since the last [[read]]|