plain C: opening a directory with fopen()

Solution 1:

From http://pubs.opengroup.org/onlinepubs/7908799/xsh/fopen.html:

The fopen() function will fail if: 
[EISDIR] The named file is a directory and mode requires write access.

At least on Linux, if you try to fopen("dirname", "wb") you get an EISDIR error. I also tried with a directory with d-------- access rights, and I still get EISDIR (and not EACCES.)

Solution 2:

Directories do not exist in the C99 standard (or the C2011 one). So by definition, fopen-ing a directory is either implementation specific or undefined behavior.

fopen(3) can fail (giving a NULL result). fseek(3) can also fail (by returning -1). And then you should preferably check errno(3) or use perror(3)

ftell is documented to return a long, and -1L on failure. On 64 bits Linux this is 0xffffffffffffffff.

You code should be instead

FILE* fd = fopen(argv[1], "rb");
if (!fd) 
  { perror(argv[1]); exit(EXIT_FAILURE); };
if (fseek(fd, 0, SEEK_END)<0) 
  { perror("fseek"); exit(EXIT_FAILURE); };
long flen = ftell(fd);
if (flen == -1L)
  { perror("ftell"); exit(EXIT_FAILURE); };

^{BTW, On Linux/Debian/Sid/AMD64 with libc-2.17 and 3.10.6 kernel, that codes runs ok when argv[1] is /tmp; surprizingly, flen is LONG_MAX i.e. 0x7fffffffffffffff}

BTW, on Linux, directories are a special case of files. Use stat(2) on a file path (and fstat on a file descriptor, perhaps obtained with fileno(3) from some FILE*) to know more meta data about some file, including its "type" (thru its mode). You want opendir(3), readdir(3) & closedir(3) to operate on directory contents. See also inode(7).