How to notice stdin redirected to directory

Question

How can you notice in a program that stdin is redirected to a directory?

If for example I execute:

$ cat < .

I get a message from cat telling me: cat: -: Is a directory. How can I make my C program notice that stdin has been redirected to a directory?

Answer 1

You can get the file descriptor of stdin via fileno() and then do fstat on the file descriptor.

Here's a simple demonstration:

$ cat test.c
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int main(void)
{
    struct stat sb;
    if ( fstat(fileno(stdin), &sb) == -1) {
        perror("fstat");
        exit(1);
    }

    switch (sb.st_mode & S_IFMT) {
        case S_IFBLK:  printf("block device\n");            break;
        case S_IFCHR:  printf("character device\n");        break;
        case S_IFDIR:  printf("directory\n");               break;
        case S_IFIFO:  printf("FIFO/pipe\n");               break;
        case S_IFLNK:  printf("symlink\n");                 break;
        case S_IFREG:  printf("regular file\n");            break;
        case S_IFSOCK: printf("socket\n");                  break;
        default:       printf("unknown?\n");                break;
    }
}
$ gcc -Wall -Wextra test.c
$ ./a.out < .
directory