fopen a directory

Author:Wojciech Muła
Added on:2013-12-25

It's not clear how function fopen applied to a directory should behave, manual pages don't say anything about this. So, our common sense fail — at least when use a standard library shipped with GCC, beacuse fopen returns a valid handle.

Discussion on stackoverflow pointed that fseek or ftell` would fail. But on my system it's not true, ftell(f, 0, SEEK_END) returns the size of an opened directory.

Only when we trying to read data using fread or fgetc the errno variable is set to EISDIR error code.

Here is output from simple test program:

$ ./a.out ~
testing '/home/wojtek'...
fopen: Success [errno=0]
fseek: Success [errno=0]
fseek result: 0
ftell: Success [errno=0]
ftell result: 24576
feof: Success [errno=0]
feof result: 0 (EOF=no)
fgetc: Is a directory [errno=21]
fgetc result: -1 (EOF=yes)
fread: Is a directory [errno=21]
fread result: 0