Can I check if two FILE* or file descriptor numbers refer to the same file?

Question

I have a program that reads from a file and writes to a file. I\'d like to prevent the user from specifying the same file for both (for obvious reasons). Lets say the first

Answer 1

Yes - compare the file device ID and inode. Per the <sys/stat.h> specification:

The st_ino and st_dev fields taken together uniquely identify the file within the system.

Use

int same_file(int fd1, int fd2) {
    struct stat stat1, stat2;
    if(fstat(fd1, &stat1) < 0) return -1;
    if(fstat(fd2, &stat2) < 0) return -1;
    return (stat1.st_dev == stat2.st_dev) && (stat1.st_ino == stat2.st_ino);
}