Data Files - C Interview Questions and Answers

  1. 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);

    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.

  2. 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.