Advanced stat(2) in Linux

Prior to reading this, a common working knowledge of the C programming language is very helpful. Furthermore, reading up on stat(2) and fstatat(2) will be helpful.

As a means of preparing our environment, let us create a file on our machine.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
    
int main() {
    int my_fd, bytes_written;
    char *initial_contents = "this is the contents of myfile\n";
    
    if ((my_fd = open("myfile.txt", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR)) < 0) {
        fprintf(stderr, "Failed to create file: %s\n", strerror(errno));
        return EXIT_FAILURE;
    }
    bytes_written = write(my_fd, initial_contents, strlen(initial_contents));
    close(my_fd);
    
    if (bytes_written < 0) {
        fprintf(stderr, "Failed to write to file: %s\n", strerror(errno));
        return EXIT_FAILURE;
    }
    // remaining code snippets will be put in here
}

Now, suppose we created a file some time ago, and don't already know everything about it. Suppose we want to know more information about it. Well, we can get that information with the stat(2) function! Let us set that up, and fill a struct stat with information about the file.

struct stat statbuf;

// note that I had to reference statbuf to fill it. Why is this?
if (stat("myfile.txt", &statbuf) < 0) {
    fprintf(stderr, "Failed to get information on the file: %s\n", strerror(errno));
    return EXIT_FAILURE;
}

At this point, statbuf is populated with information about the file. For a full list of what information can be obtained therein, see the description in man 2 stat. Well, there are a few things I'm interested in related to this file.

What is the file's size?

printf("Size of file: %lu characters\n", statbuf.st_size);

What is the user ID of the owner of the file?

printf("File owner's UID: %u\n", statbuf.st_uid);

When was the file last accessed? Modified? Look at man 3 ctime for how it is printed.

#include <time.h>

printf("Last access time: %s", ctime(&statbuf.st_atime));
printf("Last modification time: %s", ctime(&statbuf.st_mtime));

Lastly, how can I find the permissions of a file?

printf("Permissions of file: %d\n", statbuf.st_mode);

What?? That's weird, it's just some number that doesn't make a lot of sense. How can I really read into this?

Well, if you will recall, file permissions are broken up as follows:

For the above table:

So we see that the 9 possible combinations for those 6 characteristics provide 9 possible permission values for a file.

These constants (S_IRUSR, S_IXGRP, etc) are just declarations whose values are 1 shifted some number of bits to the left. As such, we can use them to figure out the values of different permission bits in the statbuf's mode field, or its permission field, by performing a bitwise AND between the two. For example, after doing the following:

int user_read_permissions = statbuf.st_mode & S_IRUSR;

user_read_permissions will be greater than 0 if the owner can read from the file, and zero if the owner cannot read from the file. Like this, we can systematically check each bit in the permission string. Let us set up a quick example of how this might work:

int perms[] = {S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP, S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH};
int permission_valid;
printf("-"); // this is to print the file type bit, as displayed in `$ ls -l`.
for (int i = 0; i < 9; i += 3) {
    permission_valid = statbuf.st_mode & perms[i];
    if (permission_valid) {
        printf("r");
    } else {
        printf("-");
    }
    permission_valid = statbuf.st_mode & perms[i+1];
    if (permission_valid) {
        printf("w");
    } else {
        printf("-");
    }
    permission_valid = statbuf.st_mode & perms[i+2];
    if (permission_valid) {
        printf("x");
    } else {
        printf("-");
    }
}
printf("\n");

You will notice that this printout looks more familiar and human readable!

Elliot Wasem, <elliotbielwasem@gmail.com>

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