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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:

 Constant Description
 O_APPEND-Appends all writes to the end of the file
 O_BINARY-Opens the file in binary (untranslated) mode
 O_CREAT-If the file does not exist, it is created
 O_EXCL-If the O_CREAT flag is used and the file exists, returns an error
 O_RDONLY-Opens the file in read-only mode
 O_RDWR-Opens the file for reading and writing
 O_TEXT-Opens the file in text (translated) mode
 O_TRUNC-Opens an existing file and writes over its contents
 O_WRONLY-Opens the file in write-only mode

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

 Constant Description
 SH_COMPAT-No other program can access the file
 SH_DENYRW-No other program can read from or write to the file
 SH_DENYWR-No other program can write to the file
 SH_DENYRD-No other program can read from the file
 SH_DENYNO-Any program can read from or write to the file

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:

 Constant Description
 ENOENT-File or path not found
 EMFILE-No more file handles are available
 EACCES-Permission denied to access file
 EINVACC-Invalid access code

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.