Data Files - C Interview Questions IX

Q. 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.

Q. 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 thelseek() 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.