How to detect if a C filestream points to a file or a serial device?

Question

I'm working on a C application that evaluates data from a USB laser scanner, which acts as a serial device. For testing purpose, I'm also allowing test data to be read from a file, because it is not convenient to always have the scanner connected.

I open the file/device like this:

FILE *fp = fopen(argv[1], "a+b");

And depending on whether I want to read from a file or the device, I pass a file path or something like /dev/cu.usbmodemfd121 (I'm on a Mac).

This works fine as long as I've previously initialized the laser scanner, but I'd rather have my application do that. In order to do that, though, I must first figure out if I'm reading from a file or the device. How can I do that, given the FILE * returned by fopen?

I've tried to use fseek(fp, 1, SEEK_END) which I expected to fail for the scanner, since it's stream doesn't have an "end", but for some reasons fseek does not fail..

Answer 1

You could get the file descriptor using fileno and then do a fstat on it. The struct stat it populates contains thinks like st_mode which shows the type of fd. I am guessing for your non-file device S_ISCHR will be true or at least S_ISREG will be false.

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If you have control over it, don't do fopen at all. Use open directly to get the file descriptor and then use fdopen if you really want C streams.

Answer 2

@cnicutar's solution worked just fine. Here's what I ended up with, in case it helps somebody (error checking removed for clarity):

#include <fcntl.h> /* open syscall */
#include <sys/stat.h>

int fd = -1;
int status;
FILE *fp = NULL;
struct stat fd_stat;
bool serial_device = false;

fd = open("/foo/bar/baz", O_RDONLY);
fp = fdopen(fd, "rb");
status = fstat(fd, &fd_stat);

printf("S_ISCHR: %d\n", S_ISCHR(fd_stat.st_mode));
printf("S_ISREG %d\n", S_ISREG(fd_stat.st_mode));

if(!S_ISREG(fd_stat.st_mode)) {
    serial_device = true;
}