File System, Part 2: Files are inodes (everything else is just data...) - lyuanschool/SystemProgramming GitHub Wiki
Big idea: Forget names of files: The 'inode' is the file.
It is common to think of the file name as the 'actual' file. It's not! Instead consider the inode as the file. The inode holds the meta-information (last accessed, ownership, size) and points to the disk blocks used to hold the file contents.
A directory is just a mapping of names to inode numbers. POSIX provides a small set of functions to read the filename and inode number for each entry (see below)
From a shell, use ls with the -i option
$ ls -i
12983989 dirlist.c 12984068 sandwhich.c
From C, call one of the stat functions (introduced below).
Use the stat calls. For example, to find out when my 'notes.txt' file was last accessed -
struct stat s;
stat("notes.txt", & s);
printf("Last accessed %s", ctime(s.st_atime));There are actually three versions of stat;
int stat(const char *path, struct stat *buf);
int fstat(int fd, struct stat *buf);
int lstat(const char *path, struct stat *buf);For example you can use fstat to find out the meta-information about a file if you already have an file descriptor associated with that file
FILE *file = fopen("notes.txt", "r");
int fd = fileno(file); /* Just for fun - extract the file descriptor from a C FILE struct */
struct stat s;
fstat(fd, & s);
printf("Last accessed %s", ctime(s.st_atime));The third call 'lstat' we will discuss when we introduce symbolic links.
In addition to access,creation, and modified times, the stat structure includes the inode number, length of the file and owner information.
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};Let's write our own version of 'ls' to list the contents of a directory.
#include <stdio.h>
#include <dirent.h>
#include <stdlib.h>
int main(int argc, char **argv) {
if(argc == 1) {
printf("Usage: %s [directory]\n", *argv);
exit(0);
}
struct dirent *dp;
DIR *dirp = opendir(argv[1]);
while ((dp = readdir(dirp)) != NULL) {
puts(dp->d_name);
}
closedir(dirp);
return 0;
}Ans: Use opendir readdir closedir For example, here's a very simple implementation of 'ls' to list the contents of a directory.
#include <stdio.h>
#include <dirent.h>
#include <stdlib.h>
int main(int argc, char **argv) {
if(argc ==1) {
printf("Usage: %s [directory]\n", *argv);
exit(0);
}
struct dirent *dp;
DIR *dirp = opendir(argv[1]);
while ((dp = readdir(dirp)) != NULL) {
printf("%s %lu\n", dp-> d_name, (unsigned long)dp-> d_ino );
}
closedir(dirp);
return 0;
}Note: after a call to fork(), either (XOR) the parent or the child can use readdir(), rewinddir() or seekdir(). If both the parent and the child use the above, behavior is undefined.
For example, to see if a particular directory includes a file (or filename) 'name', we might write the following code. (Hint: Can you spot the bug?)
int exists(char *directory, char *name) {
struct dirent *dp;
DIR *dirp = opendir(directory);
while ((dp = readdir(dirp)) != NULL) {
puts(dp->d_name);
if (!strcmp(dp->d_name, name)) {
return 1; /* Found */
}
}
closedir(dirp);
return 0; /* Not Found */
}The above code has a subtle bug: It leaks resources! If a matching filename is found then 'closedir' is never called as part of the early return. Any file descriptors opened, and any memory allocated, by opendir are never released. This means eventually the process will run out of resources and an open or opendir call will fail.
The fix is to ensure we free up resources in every possible code-path. In the above code this means calling closedir before return 1. Forgetting to release resources is a common C programming bug because there is no support in the C lanaguage to ensure resources are always released with all codepaths.
Note If C supported exception handling this discussion would be unnecessary.
Imagine your function required several temporary resources that need to be freed before returning.
How can we write readable code that correctly frees resources under all code paths? Some system programs use goto to jump forward into the clean up code, using the following pattern:
int f() {
Acquire resource r1
if(...) goto clean_up_r1
Acquire resource r2
if(...) goto clean_up_r2
perform work
clean_up_r2:
clean up r2
clean_up_r1:
clean up r1
return result
}Whether this is a good thing or not has led to long rigorous debates that have generally helped system programmers stay warm during the cold winter months. Are there alternatives? Yes! For example using conditional logic, breaking out of do-while loops and writing secondary functions that perform the innermost work. However all choices are problematic and cumbersome as we are attempting to shoe-horn in exception handling in a language that has no inbuilt support for it.
There are two main gotchas and one consideration:
The readdir function returns "." (current directory) and ".." (parent directory). If you are looking for sub-directories, you need to explicitly exclude these directories.
For many applications it's reasonable to check the current directory first before recursively searching sub-directories. This can be achieved by storing the results in a linked list, or resetting the directory struct to restart from the beginning.
One final note of caution: readdir is not thread-safe! For multi-threaded searches use readdir_r which requires the caller to pass in the address of an existing dirent struct.
See the man page of readdir for more details.
Ans: Use S_ISDIR to check the mode bits stored in the stat structure
And to check if a file is regular file use S_ISREG,
struct stat s;
if (0 == stat(name, &s)) {
printf("%s ", name);
if (S_ISDIR( s.st_mode)) puts("is a directory");
if (S_ISREG( s.st_mode)) puts("is a regular file");
} else {
perror("stat failed - are you sure I can read this file's meta data?");
}