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 path is in char* path1 and the second path is in char* path2. can I fopen() both paths, call fileno() on each and get the same number?

To explain more clearly:

char* path1 = "/asdf"
char* path2 = "/asdf"

FILE* f1 = fopen(path1, "r");
FILE* f2 = fopen(path2, "w");

int fd1 = fileno(f1);
int fd2 = fileno(f2);

if(fd1 == fd2) {
  printf("These are the same file, you really shouldn't do this\n");
}

EDIT:

I do not want to compare filenames because one could easily defeat that with paths like /asdf/./asdf or by using symlinks. Ultimately, I do not want to write my output into the file that I’m reading from (could cause serious issues).

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);
}