[devil_himself]

this program writes the user supplied text ( command line arg ) to the specified text file along with the real user id of the user.

problem -> the user id is not visible inside text editor .. but its present when seen using a hexeditor

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

#define FILENAME "/tmp/text"

void usage(char *prog_name, char *filename) {
   printf("Usage: %s <date to write to %s>\n", prog_name, filename);
   exit(0);
}


int main(int argc, char *argv[]) {
   int userid, fd; // file descriptor
   char *buffer;

   buffer = malloc(100);
   if (buffer == NULL) 
   {
	fprintf(stderr, "Error: could not allocate memory.\n");
	exit(1);
   }

   if(argc < 2)                // If there aren't commandline arguments
      usage(argv[0], FILENAME); // display usage message and exit

   strncpy(buffer, argv[1], strlen(argv[1]));  // copy into buffer

 // Opening the file
   fd = open(FILENAME, O_WRONLY|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR);
   if(fd == -1)
      perror("error while opening file");

   userid = getuid(); // get the real user ID

   // Writing data
   if(write(fd, &userid, 4) == -1) // write user ID before note data
      perror("error while writing userid to file");
   write(fd, "\n", 1); // terminate line

   if(write(fd, buffer, strlen(buffer)) == -1) // write note
      perror("error while writing buffer to file");
   write(fd, "\n", 1); // terminate line

// Closing file
   if(close(fd) == -1)
      perror("error while closing file");

   free(buffer);

   return 0;
}

here is the terminal session

[rohit@localhost tmp]$ gcc -Wall -o notes notes.c 
[rohit@localhost tmp]$ ./notes "this is some line of text"
[rohit@localhost tmp]$ cat /tmp/text

this is some line of text
[rohit@localhost tmp]$ od -c /tmp/text
0000000 364 001  \0  \0  \n   t   h   i   s       i   s       s   o   m
0000020   e       l   i   n   e       o   f       t   e   x   t  \n
0000037
[rohit@localhost tmp]$ od -x /tmp/text
0000000 01f4 0000 740a 6968 2073 7369 7320 6d6f
0000020 2065 696c 656e 6f20 2066 6574 7478 000a
0000037
[rohit@localhost tmp]$ id
uid=500(rohit) gid=500(rohit) groups=500(rohit) context=user_u:system_r:unconfined_t
[rohit@localhost tmp]$ exit

01f4 is 500 in decimal

thanks

rohit

[JimmySeal]

You're writing the userid to the file as binary data:

// Writing data
   if(write(fd, &userid, 4) == -1) // write user ID before note data
      perror("error while writing userid to file");

But you need to convert the userid to a string value and then write the string to the file.
Please look into the itoa function. That should help get you where you need.

http://www.daniweb.com/forums/thread11049.html#

[devil_himself]

thanks for the reply .. i used the 'sprintf' function as 'itoa' is non standard and it worked.

but can you explain why the data is being written as binary ?

all i know is that write expects a pointer for its source and that requirement is being satisfied.

ssize_t write(int fildes, const void *buf, size_t nbyte);

if(write(fd, &userid, 4) == -1) // write user ID before note data

[JimmySeal]

Originally Posted by devil_himself

but can you explain why the data is being written as binary ?

That's what write() is designed to do. It's writing buffer as binary character data too, and the only difference is that that's what text editors are designed to read.

[devil_himself]

thank you