Data Files (Part - 3), C Programming Interview Questions | Placement Papers - Technical & HR Questions - Interview Preparation PDF Download

12. How do you view the PATH?

Your C compiler library contains a function called getenv() that can retrieve any specified environment variable. It has one argument, which is a pointer to a string containing the environment variable you want to retrieve. It returns a pointer to the desired environment string on successful completion. If the function cannot find your environment variable, it returns NULL.

The following example program shows how to obtain the PATH environment variable and print it on-screen:

#include <stdio.h>
#include <stdlib.h>
void main(void);
void main(void)
{
     char* env_string;
     env_string = getenv("PATH");
     if (env_string == (char*) NULL)
          printf("\nYou have no PATH!\n");
     else
          printf("\nYour PATH is: %s\n", env_string);
}

13. How can I open a file so that other programs can update it at the same time?

Your C compiler library contains a low-level file function called sopen() that can be used to open a file in shared mode. Beginning with DOS 3.0, files could be opened in shared mode by loading a special program named SHARE.EXE. Shared mode, as the name implies, allows a file to be shared with other programs as well as your own. Using this function, you can allow other programs that are running to update the same file you are updating.

The sopen() function takes four parameters: a pointer to the filename you want to open, the operational mode you want to open the file in, the file sharing mode to use, and, if you are creating a file, the mode to create the file in. The second parameter of the sopen() function, usually referred to as the "operation flag" parameter, can have the following values assigned to it:

The third parameter of the sopen() function, usually referred to as the "sharing flag," can have the following values assigned to it:

If the sopen() function is successful, it returns a non-negative number that is the file's handle. If an error occurs, -1 is returned, and the global variable errno is set to one of the following values:

The following example shows how to open a file in shared mode:

#include <stdio.h>
#include <fcntl.h>
#include <sys\stat.h>
#include <io.h>
#include <share.h>
void main(void);
void main(void)
{
     int file_handle;
     /* Note that sopen() is not ANSI compliant */
     file_handle = sopen("C:\\DATA\\TEST.DAT", O_RDWR, SH_DENYNO);
     close(file_handle);
}

Whenever you are sharing a file's contents with other programs, you should be sure to use the standard C library function named locking() to lock a portion of your file when you are updating it.

14. How can I make sure that my program is the only one accessing a file?

By using the sopen() function, you can open a file in shared mode and explicitly deny reading and writing permissions to any other program but yours. This task is accomplished by using the SH_DENYWR shared flag to denote that your program is going to deny any writing or reading attempts by other programs. For example, the following snippet of code shows a file being opened in shared mode, denying access to all other files:

/* Note that the sopen() function is not ANSI compliant... */
fileHandle = sopen("C:\\DATA\\SETUP.DAT", O_RDWR, SH_DENYWR);

By issuing this statement, all other programs are denied access to the SETUP.DAT file. If another program were to try to open SETUP.DAT for reading or writing, it would receive an EACCES error code, denoting that access is denied to the file.

 

15. How can I prevent another program from modifying part of a file that I am modifying?

If your C compiler library comes with a function named locking() that can be used to lock and unlock portions of shared files.

The locking function takes three arguments: a handle to the shared file you are going to lock or unlock, the operation you want to perform on the file, and the number of bytes you want to lock. The file lock is placed relative to the current position of the file pointer, so if you are going to lock bytes located anywhere but at the beginning of the file, you need to reposition the file pointer by using the lseek() function.

The following example shows how a binary index file named SONGS.DAT can be locked and unlocked:

#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <process.h>
#include <string.h>
#include <share.h>
#include <sys\locking.h>
void main(void);
void main(void)
{
     int file_handle, ret_code;
     char* song_name = "Six Months In A Leaky Boat";
     char rec_buffer[50];
     file_handle = sopen("C:\\DATA\\SONGS.DAT", O_RDWR, SH_DENYNO);
     /* Assuming a record size of 50 bytes, position the file
        pointer to the 10th record. */
     lseek(file_handle, 450, SEEK_SET);
     /* Lock the 50-byte record. */
     ret_code = locking(file_handle, LK_LOCK, 50);
     /* Write the data and close the file. */
     memset(rec_buffer, '\0', sizeof(rec_buffer));
     sprintf(rec_buffer, "%s", song_name);
     write(file_handle, rec_buffer, sizeof(rec_buffer));
     lseek(file_handle, 450, SEEK_SET);
     locking(file_handle, LK_UNLCK, 50);
     close(file_handle);
}

Notice that before the record is locked, the record pointer is positioned to the 10th record (450th byte) by using the lseek() function. Also notice that to write the record to the file, the record pointer has to be repositioned to the beginning of the record before unlocking the record.