C File Operations: Check opened file pointer access mode

Question

A simple question:

How do I check the access mode of an already opened file pointer?

So say a function is passed an already opened FILE pointer:

Answer 1

Warning: this answer is specific to Visual Studio 2010.

---

The stdio.h file that comes with Visual Studio 2010 defines the FILE type like this:

struct _iobuf {
    char *_ptr;
    int   _cnt;
    char *_base;
    int   _flag;
    int   _file;
    int   _charbuf;
    int   _bufsiz;
    char *_tmpfname;
};
typedef struct _iobuf FILE;

When fopening a file with the "rb" mode, it gets the value of 0x00000001.

In the fopen function, some of the flags you're interesed in can be mapped as this:

r    _IOREAD
w    _IOWRT
a    _IOWRT
+    _IORW

These constants are defined in stdio.h:

#define _IOREAD         0x0001
#define _IOWRT          0x0002
#define _IORW           0x0080

The underlying file descriptor contains more info, I haven't dug further yet.

Answer 2

There is no standard way to achieve that.

Answer 3

On Linux (and possibly all UNIX systems) you could use fcntl to get the access mode of the file:

int get_file_status(FILE* f) {
    int fd = fileno(f);
    return fcntl(fd, F_GETFL);
}

Note that the returned value is an integer as combination of flags like O_RDONLY or O_RDWR, not the "r" or "w+" strings. See http://pubs.opengroup.org/onlinepubs/007908799/xsh/open.html for some of these flags.

Not sure about Windows, see On Windows/mingw, what is the equivalent of `fcntl(fd, F_GETFL) | O_ACCMODE`?.