Open function : how to protect against directory opening? - c

Question

I am using the open function like this in my file to get some coordinates from a file :
t_coo *get_buffer(char **av, t_coo **head)
{
int ret;
int fd;
char *line;
int y;
t_coo *cur;
cur = NULL;
*head = NULL;
y = 0;
ret = 0;
fd = open(av[1], O_RDONLY);
while ((ret = get_next_line(fd, &line) > 0))
{
head = get_map(line, head, y);
y++;
}
close(fd);
cur = *head;
return (cur);
}
It is working perfectly but the problem is when I try to make it open a directory, my program segfault. I want to protect it against directory opening so that I dont segault anymore. I tried to look at the flags on the internet and tried many of them but I could not find this one. Can anybody tell me which one it is? Thank you.

Answer 1

You need to use the lstat function to tell you whether the given file name represents a regular file or a directory.
struct stat statbuf;
int rval;
rval = lstat(argv[1], &statbuf);
if (rval == -1) {
perror("error getting file status");
exit(1);
}
if (S_ISREG(statbuf.st_mode)) {
printf("%s is a regular file\n", argv[1]);
} else if (S_ISDIR(statbuf.st_mode)) {
printf("%s is a directory\n", argv[1]);
} else {
printf("%s is something else\n", argv[1]);
}

Answer 2

I would suggest to open the file (which could be a directory) so get a file descriptor, then use fstat(2) on that file descriptor (and check the result of fstat by using statresult.st_mode & S_IFMT == S_IFDIR ...)
This would avoid an (improbable) race condition with the lstat (or stat) then open approach (suggested in Dbush's answer): some other process might (with very bad luck) remove or rename the file between these two system calls. You might also opendir or else open (but that also suffers from a similar race condition).
PS. The race conditions I am suggesting here are so improbable that we can normally ignore them... But they might be a security flaw (than an attacker could use)