Tuesday, March 3, 2009

Simple Code To Implement C's stat() function On Linux And Unix

Hey There,

Today's post loosely ties back to our post last week on Perl stat() basics for Linux and Unix, although this post is not a continuation of that one. Just trying to keep it confusing ;)

I've often been bashed for my C programming skills (the "bash" in that sentence wasn't meant to be a pun), and, although I've come to expect the vicious backlash, I still like to come back to C every once in a while to look at some of the things you can do with it and to keep me awake ;) That being said, any competent C programmer out there who finds fault with this post's attached C-code is, most probably, correct. This could be written better, have more robust error-checking, implement measures to prevent buffer over-runs, under-runs, and a thousand other exploits. That being said; it works, which is miracle enough for me :) Feel free to modify it in any way, shape or form you prefer. Everything I put on this blog is considered at-your-own-risk material. There's not all that much to worry about, though, if you just use this code for yourself to test and play with. Just secure it with simple ownership and file permission settings and delete the binary when you're done.

It's been a long time since I've had to write this, but typing it will be faster than trying to find another post with C code in it ;) If you want to compile the attached code, you can do the following (should work with most any C compiler, I'm just using gcc as an example since that's what I have installed on my machine):

host # gcc -o filestat filestat.c

and the program is ready to run. That part's pretty simple, too. You just execute it and follow it with one or more file names (they can be "any" type of file: file, directory, link, socket, etc). If you want to see the usage information, just run it without any arguments, like so:

host # ./filestat
Usage: ./filestat file1 file2 fileN...

The most interesting part for me was figuring out how to decipher the "file mode" information. If you check your system's "stat" manpage, you'll see that there are actually a good deal of macros already written-in that make printing out plain English file types and modes much simpler, but I decided to keep it cryptic. I take my modes like I take my permissions; in octal ;)

Here's a sample run of the program - just using one file to make it simple (it should be noted, also, that you can only run this command against file types that you have permission to read. If you run this as your regular user on a file owned by root with 700 permissions, you'll get the "File or Directory Not Found!" error, which is, technically, incorrect. My God, am I a lazy C programmer. No wonder I get beat up so bad every time I goof around with it on here ;)

NOTE: This program interacts with the shell you're using, so if, for instance, you can access a file named "myfile" by typing "my*", the same globbing rules will be taken care of, applied to the filename and expanded by the shell before this program processes it!

host # ./filestat tct-1.18
Checking tct-1.18
tct-1.18 stat values are:
File Mode: 40700
Inode Number: 17540
Device ID: 35651599
Raw Device ID: 0
Number Of Links: 12
User ID: 0
Group ID: 0
Bytes Size: 1024
Access Time: Mon, March 02, 2009 - 14:15:31
Modification Time: Mon, April 09, 2007 - 08:36:09
Status Change Time: Mon, March 02, 2009 - 15:47:38
Block Size: 8192
Number Of Blocks: 2
File System Type: ufs

Most of the above is self-explanatory. The few things to note (if you just "really" don't want to read the manpage ;) are that the "Raw Device ID" is almost always zero unless you're dealing with a disk, or other file type, that supports raw access. From my own testing, you can definitely get back a "Raw Device ID" from any disk (/dev/rdsk/c0t0d0s0, /dev/dsk/c0t0d0s0, /dev/hda1, etc). The "Bytes Size" is simply the Size in Bytes. I left it the way it is because it reminds me that I need to eat less ;)

Lastly, you may have noted that the "File Mode" (listed first in the output, since I just went through the "stat" struct and used each value in order) seems a bit off. The last 3 digits are probably familiar to most Linux and Unix users, as they represent the file permissions. By the same virtue, the first 4 (including those original 3), probably seem familiar as well, since that 4th-left bit is used in octal permissions settings to represent setuid, setgid, and sticky-bit, etc, special permissions. So, for the above "File Mode" of 40700, you know that the file permissions are "0700" (no setuid, setgid or sticky-bit - read/write/execute for the user and no permissions for the group and world).

The only question, once you've got the right side of the "File Mode" down is, what do the variable number of numerals to the left of those stand for? The answer is pretty simple and (as I noted above) there are standard macros that you can use in your C code to test the mode and print it out so that it's more human-readable. Basically, they determine the type of file you're interrogating. Since our one number to the left of the four permission bits is "4," we know that the file we're accessing is a directory. You can see this in the table below (shamelessly lifted from the system header file - /usr/include/sys/stat.h ;) <-- This is just a subset of all available macros. There's a lot more information in that header file than I want to jam into this post... It's getting long enough now. Especially bad if you're an "F" reader ;)

As one last interesting side-note, the "File Mode" for the gzipped tarball, that was extracted to create this directory, is "100700" which gives us the same permissions, but a "10" on the left-hand "file type" side. You can also see this showing up below as a "regular" file.

#define S_IFMT  0170000  /* type of file mask */
#define S_IFIFO 0010000 /* named pipe (fifo) */
#define S_IFCHR 0020000 /* character special */
#define S_IFDIR 0040000 /* directory */
#define S_IFBLK 0060000 /* block special */
#define S_IFREG 0100000 /* regular */
#define S_IFDB 0110000 /* record access file */
#define S_IFLNK 0120000 /* symbolic link */
#define S_IFSOCK 0140000 /* socket */
#define S_IFWHT 0160000 /* whiteout */
#define S_ISVTX 0001000 /* save swapped text even after use */
#define S_IRWXU 00700 /* read, write, execute: owner */
#define S_IRUSR 00400 /* read permission: owner */
#define S_IWUSR 00200 /* write permission: owner */
#define S_IXUSR 00100 /* execute permission: owner */
#define S_IRWXG 00070 /* read, write, execute: group */
#define S_IRGRP 00040 /* read permission: group */
#define S_IWGRP 00020 /* write permission: group */
#define S_IXGRP 00010 /* execute permission: group */
#define S_IRWXO 00007 /* read, write, execute: other */
#define S_IROTH 00004 /* read permission: other */
#define S_IWOTH 00002 /* write permission: other */
#define S_IXOTH 00001 /* execute permission: other */

Hope you enjoy the attached C code and get some use out of it (or your own Frankenstein'ed version of it ;)


Creative Commons License

This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>
#include <pwd.h>
#include <grp.h>
#include <time.h>
#include <locale.h>
#include <langinfo.h>
#include <fcntl.h>

filestat.c - Would you like fries with that?
2009 - Mike Golvach - eggi@comcast.net
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
int main (int argc, char *argv[])
if (argc <= 1) {
printf("Usage: %s file1 file2 fileN...\n", argv[0]);
struct stat dirfilebuf;
int arguments, dirfilestate;
const char *dirfilename;
char *nicetime, *errormessage, *divider;


errormessage="File or Directory Not Found!";

for (arguments = 1; arguments < argc; arguments++) {
dirfilename = argv[arguments];
printf("%s\nChecking %s\n%s\n", divider,dirfilename,divider);
dirfilestate = stat(dirfilename, &dirfilebuf);
if ( dirfilestate == -1 ) {
printf("%s - Quitting!\n", errormessage);
} else{
printf("%s stat values are:\n%s\n", dirfilename,divider);
printf("File Mode: %lo\n", dirfilebuf.st_mode);
printf("Inode Number: %-10d\n", dirfilebuf.st_ino);
printf("Device ID: %d\n", dirfilebuf.st_dev);
printf("Raw Device ID: %d\n", dirfilebuf.st_rdev);
printf("Number Of Links: %d\n", dirfilebuf.st_nlink);
printf("User ID: %-8d\n", dirfilebuf.st_uid);
printf("Group ID: %-8d\n", dirfilebuf.st_gid);
printf("Bytes Size: %jd\n", (intmax_t)dirfilebuf.st_size);
strftime(nicetime,34,"%a, %B %d, %Y - %H:%M:%S", localtime(&dirfilebuf.st_atime));
printf("Access Time: %s\n", nicetime);
strftime(nicetime,34,"%a, %B %d, %Y - %H:%M:%S", localtime(&dirfilebuf.st_mtime));
printf("Modification Time: %s\n", nicetime);
strftime(nicetime,34,"%a, %B %d, %Y - %H:%M:%S", localtime(&dirfilebuf.st_ctime));
printf("Status Change Time: %s\n", nicetime);
printf("Block Size: %d\n", dirfilebuf.st_blksize);
printf("Number Of Blocks: %d\n", dirfilebuf.st_blocks);
printf("File System Type: %s\n", dirfilebuf.st_fstype);
printf("%s\n", divider);

, Mike

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