In this lab, you will learn the following topics and practice C programming skills.

1. Writing simple Makefiles
2. open(), close(), read(), and write() system calls
3. File statistics: fstat() system call
4. Altering file modes: chmod() system call
5. Printing file information: getpwuid(), getgrgid(), strmode() library functions
6. Altering file modification times: utimes(), getimeofday() system calls
7. Printing System Call Errors: perror() and errno

Run the following command

~aviv/bin/ic221-up

Change into the lab directory

cd ~/ic221/lab/06

All the material you need to complete the lab can be found in the lab directory. All material you will submit, you should be place within the lab directory. Throughout this lab, we refer to the lab directory, which you should interpret as the above path.

For this lab, all scripts for submission should be placed in the following folder:

~/ic221/lab/06

This directory contains 5 sub-directories; examples, makefile, mycp, myls, and mytouch. In the examples directory you will find any source code in this lab document. All lab work should be done in the remaining directories directory.

• Only source files found in the folder will be graded.
• Do not change the names of any source files

Finally, in the top level of the lab directory, you will find a README file. You must complete the README file, and include any additional details that might be needed to complete this lab.

You are required to provide your own Makefiles for this lab. Each of the source folders, mycp, myls, and mytouch, must have a Makefile. We should be able to compile your programs by typing make in each source directory.

In the top level of the lab directory, you will find a README file. You must fill out the README file with your name and alpha. Please include a short summary of each of the tasks and any other information you want to provide to the instructor.

You are provided a test script to test your submission. It is found in the base of the lab directory: test.sh.

One of the advantages of C is that you can stage your compilation process. You did this already in the last lab when we had to compile simplefs into an object file filesystem.o and then compile that object file with other source, like the shell or testfile.

Let's review the compilation process. When we have source broken across multiple file, we first have to compile those files to object code, an intermediate compilation stage.

gcc -c source.c -o source.o

Next we can compile multiple object files to assemble an executable.

gcc source.o main.o -o executable

If you look at the above compilation command, you can see the target and dependancies. The target is the executable, executable, the three dependancies are the object files, source.o, and main.o, which each have dependencies, the associate source (and header) files. Let's translate that into a Makefile.

all: executable

executable: source.o main.o
        gcc source.o main.o -o executable

source.o: source.c source.h
        gcc -c source.c -o source.o

main.o: main.c
        gcc -c main.c -o main.o

Tracing the dependencies and the commands starting with all, we can see that to reach the compilation command for executable, first source.c and main.c must be compiled to object files, source.o and main.o. You will also notice the header file, source.h, is listed as a dependency for source.o, which is common so that recompilation will occur whenever the header file changes.

The operating system maintains file information for each file on the system. You can retrieve this information with the stat() and fstat() system call, as follows:

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int stat(const char *path, struct stat *buf);
int fstat(int fd, struct stat *buf);

The stat() system call takes a path to a file and a pointer to a struct stat. It will then set the value of the struct stat pointed to by buf with the file statistics. The fstat() system call does the same, but takes an open file descriptor rather than a path.

The struct stat, which is defined in the man pages, has the following fields, and a longer description of each is provided in the man pages.

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 */
};

Of particular relevance to this lab is the st_mode and st_mtime fields. The former, st_mode, is the mode for the file which defines the permissions of the file, who can read/write/exec the file, as well as the disposition of the file, such as directory of file status. There are a number of macros define in the sys/stat.h header file for interpreting the mode of the file. For example, here is a small program to test the provided path is a directory.

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char * argv[]){

  struct stat st;

  if( argc < 2){
    fprintf(stderr, "ERROR: Require a path\n");
    return 2;   //return status error                                                                                                                                        
  }


  if( stat(argv[1], &st) < 0){     //error, cannot stat file                                                                                                                 
    perror(argv[0]);               //report erro with perror                                                                                                                 
    return 2;                     //return status error                                                                                                                      
  }

  if ( S_ISDIR(st.st_mode) ){
    printf("It's a directory!\n");
    return 0;  //return status true                                                                                                                                          
  }

  printf("Not a directory :(\n");
  return 1; //return status false                                                                                                                                            

}

The mode of a file, since it is maintained by the operating system, can only be changed via a system call. The system call to do that is chmod(), which is orally familiar to the command line tool chmod you've already used. To view the man page, be sure to look in section 2 of the manual:

#> man 2 chmod

Here are the two forms, one taking a file descriptor and the other taking a file path, just like stat()

#include <sys/stat.h>

int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);

The mode_t mode argument is the same type as the st_mode from the stat() output, and is defined using an ORing like file creation. Here are the relevant constants:

S_IRUSR  (00400)  read by owner

S_IWUSR  (00200)  write by owner

S_IXUSR  (00100)  execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed)

S_IRGRP  (00040)  read by group

S_IWGRP  (00020)  write by group

S_IXGRP  (00010)  execute/search by group

S_IROTH  (00004)  read by others

S_IWOTH  (00002)  write by others

S_IXOTH  (00001)  execute/search by others

So to set the mode of a file to read/write own, and read group:

chmod( "path/to/file", S_IRUSR | S_IWUSR | S_IRGRP);

But, as you can see from the constants, they are also defined as octets, like how we use chmod on the command line, and the following is equivalent to the above:

chmod( "path/to/file", 0640);

It's a octet, so the leading 0 is important and tells C that this number should be interpreted in octal.

All system calls have the same general function prototype for their return value. They always return an integer: On success, 0 is return, and on failure, a negative value is returned. This means we can always check for system call errors using the same pattern:

if( stat(argv[1], &st) < 0){     //error, cannot stat file
  perror(argv[0]);               //report error with perror
  return 2;                      //return status error
}

Simply place the system call in an if statement and check that the return value is less then 0. If so, we want to report that error. We can then exit the program, if that is the appropriate action to take; it isn't always, depending on the task.

Due to the simplicity of the return value, the actual cause of the error is not reported via the return value. Instead, there exists a global variable errno which is set to the value of the error. For example, here are the possible values of errno for a fairly of stat() (note these #define'ed constants):

EACCES Search permission is denied for one of the directories in the path prefix of path.  (See also path_resolution(7).)

EBADF  fd is bad.

EFAULT Bad address.

ELOOP  Too many symbolic links encountered while traversing the path.

ENAMETOOLONG
       path is too long.

ENOENT A component of path does not exist, or path is an empty string.

ENOMEM Out of memory (i.e., kernel memory).

ENOTDIR
       A component of the path prefix of path is not a directory.

EOVERFLOW
       (stat()) path refers to a file whose size cannot be represented in the type off_t.  This can occur when  an  application  compiled  on  a  32-bit  platform  without  -D_FILE_OFF‐
       SET_BITS=64 calls stat() on a file whose size exceeds (1<<31)-1 bits.

The most likely error to occur is EACCES, cannot access the file at the path, or ENOENT, the file or directory doesn't exist. It's good practice to report the precise error to the user so that the error can be corrected, but it's a huge pain to have to type these error messages yourself into every program. Instead, the C standard library has a built in error reporting tool: perror() or print error. It will automatically check the value of the errno and print an appropriate message. Here is a sample output of isdir with a bad path.

#> ./isdir bad/path
./isdir: No such file or directory

Notice, that it prints useful information. I passed to perror() the name of the program, argv[0], so that perror() will output the name of the program in addition to printing the error. This way it looks like a real command line tool.

We've already discussed the st_mode field, which stores the various dispositions of the file. It's just a number really, but we'd like to view that information in a human readable, which can be done with the library function strmode(), which will convert a mode_t into a -rwxrwxrwx string, just like in ls. Here is an example program:

/*examples/printmode.c*/
#include <stdio.h>
#include <stdlib.h>
#include <bsd/string.h>

int main(int argc, char * argv[]){

  char smode[12]; //mode strings are always 11 chars long
                  // +1 for the NULL, makes 12!

  strmode(0644, smode);
  printf("0644 : %s\n", smode);

  strmode(0742, smode);
  printf("0742 : %s\n", smode);

}

To compile a program that calls strmode() on a linux system, which you will be, you need to use the bsd library. Add the -lbsd option to gcc to link the library to your executable. Here is the sample compilation from the Makefile for printmode:

printmode: printmode.c
        gcc printmode.c -o printmode -lbsd

Each file has an owner, a user, and a group. As humans, we like to refer to these values as strings and not numbers. The owner is aviv and the group is scs, for example. The operating system doesn't think like a human, and instead stores these values a numbers. Each user has a uid and can be member of any number of groups, identified by a number gid. Similarly, files also have an associated user and group for ownership purposes, st_uid and st_gid in the struct stat.

To convert these numbers to human readable formats, we could look in the /etc/passwd and /etc/group files like we did when programming bash, but that's way, way too much work. Fortunately, C provides two library functions to do that conversion for us.

Let's start with retrieving the username. We first need to look up the password file entry for that user. We use getpwuid() for that.

#include <sys/types.h>
#include <pwd.h>

struct passwd *getpwuid(uid_t uid);

Provided the uid, which we have from the stat() output, getpwuid() returns a pointer to a struct passwd data type. It has the following fields:

struct passwd {
  char   *pw_name;       /* username */
  char   *pw_passwd;     /* user password */
  uid_t   pw_uid;        /* user ID */
  gid_t   pw_gid;        /* group ID */
  char   *pw_gecos;      /* user information */
  char   *pw_dir;        /* home directory */
  char   *pw_shell;      /* shell program */
};

The relevant field for this lab is pw_name, which is a string referencing the user name. Here is a sample program to print a username. My uid on the linux lab is 35001. (BTW, I know what you're thinking, "oohh, password!" But, the pw_passwd field is the encrypted password not the plaint text password. Nice try, though, and even then, the actual encrypted password is stored in a different file called /etc/shadow.)

/*examples/printusername.c*/
#include <sys/types.h>
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>


int main(int argc, char * argv[]){

  uid_t my_uid = 35001;
  struct passwd * pwd;

  pwd = getpwuid(my_uid);

  printf("My username is: %s\n", pwd->pw_name);

  return 0;
}

Retrieving the string version of the gid uses a similar process to that of retrieving the username. We use the getgrgid() library function:

#include <sys/types.h>
#include <grp.h>

struct group *getgrgid(gid_t gid);

Given a gid, the getgrid() function returns a pointer to a struct group, which has the following fields:

struct group {
    char   *gr_name;       /* group name */
    char   *gr_passwd;     /* group password */
    gid_t   gr_gid;        /* group ID */
    char  **gr_mem;        /* group members */
};

The relevant field is gr_name, which is the human readable name of the group. Here is a sample program to print group name for the SCS group, which is group id 10120:

#include <sys/types.h>
#include <grp.h>
#include <stdio.h>
#include <stdlib.h>


int main(int argc, char * argv[]){

  uid_t my_gid = 10120;
  struct group * grp;

  grp = getgrgid(my_gid);

  printf("My groupname is: %s\n", grp->gr_name);

  return 0;
}

The utimes() system call changes a files last access and modification time. Here is the prototype from the man page:

#include <sys/types.h>
#include <sys/time.h>

int utimes(const char *filename, const struct timeval times[2]);

It takes an argument filename, which is the path to the file, and a array of struct timeval. The size of the array is 2, and times[0] is the new access time and times[1] is the new modification time. Note, that struct timeval is a different time that the time_t data types we've been using for managing time stamps.

A struct timeval has the following fields:

struct timeval {
    long tv_sec;        /* seconds */
    long tv_usec;       /* microseconds */
};

The tv_sec is like a time_t, seconds since the epoch, and the timeval offers even finer precision. The tv_usec is the additional microsecond calculation. To get the current tiemval from the system clock, you use the gettimeofday() system call.

#include <sys/time.h>

int gettimeofday(struct timeval *tv, struct timezone *tz);

The gettimeofday() system call takes a pointer to a timeval and a timezone. It will set the current time at the memory referenced by those pointers. We don't really care about the timezone, so we'll call gettimeofday() like this, generally:

struct timeval tv;
gettimeofday(&tv, NULL);

Putting it all together, here is a sample program to print the current time using ctime() and getimeofday():

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>

int main(int argc, char * argv[]){

  struct timeval tv;

  gettimeofday(&tv,NULL);

  //ctime takes a pointer to the seconds since epoch
  printf("%s", ctime(&(tv.tv_sec)));

}