plain C: opening a directory with fopen()

问题:

I have a program which opens a file and checks its length.

FILE* fd = fopen(argv[1], "rb");
fseek(fd, 0, SEEK_END);
size_t flen = ftell(fd);
if (flen == ((size_t)-1)) {
    printf("%s is a directory.\n", argv[1]);
    fclose(fd);
    exit(1);
}

Now, at least under Linux, fopen() returns a valid file descriptor when opening a directory. This results in the seek operation returning -1 (or, as size_t is unsigned, 0xFFFFFFFFFFFFFFFF=2⁶⁴-1 on a 64-bit system).

Unfortunately, the condition in the above code (flen == ((size_t)-1)) does not catch that case, neither does flen == 0xFFFFFFFF (EDIT: should be 0xFFFFFFFFFFFFFFFF). printf()-Commands with %x ord %d as format string show that both sides of the comparison should have the same value.

Why does the comparison operator behave in such a strange way, even when both sides are of the same type (size_t)? I am using gcc 4.8.1 as compiler.

回答1:

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).

回答2:

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.)