Question

I have to duplicate a FILE* in C on Mac OS X (using POSIX int file descriptors all the way is unfortunately out of question), so I came up with the following function:

static FILE* fdup(FILE* fp, const char* mode)
{
    int fd = fileno(fp);
    int duplicated = dup(fd);
    return fdopen(duplicated, mode);
}

It works very well, except it has that small ugly part where I ask for the file mode again, because fdopen apparently can’t determine it itself.

This issue isn’t critical, since basically, I’m just using it for stdin, stdout and stderr (and obviously I know the access modes of those three). However, it would be more elegant if I didn’t have to know it myself; and this is probably possible since the dup call doesn’t need it.

How can I determine the access mode of a FILE* stream?

Answer 1

You can’t, but you can determine the mode for the underlying file descriptor:

int fd = fileno(f);
int accmode = fcntl(fd, F_GETFL) & O_ACCMODE;

You can then choose an appropriate mode to pass to fdopen based on whether accmode is O_RDONLY, O_WRONLY, or O_RDWR.