C Interview Questions Set-5

21. 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
Constant Description

22. How can method defined in multiple base classes with same name can be invoked from derived class simultaneously

ex:

class x
{
public:
m1();

};

class y
{
public:
m1();

};

class z :public x, public y
{
public:
m1()
{
x::m1();
y::m1();
}

};

23. Write a program to interchange 2 variables without using the third one

a=7;
b=2;
a = a + b;
b = a - b;
a = a - b;

24. What is the result of using Option Explicit?

When writing your C program, you can include files in two ways.
The first way is to surround the file you want to include with the angled brackets < and >.
This method of inclusion tells the preprocessor to look for the file in the predefined default location.
This predefined default location is often an INCLUDE environment variable that denotes the path to your include files.
For instance, given the INCLUDE variable
INCLUDE=C:\COMPILER\INCLUDE;S:\SOURCE\HEADERS;
using the #include version of file inclusion, the compiler first checks the
C:\COMPILER\INCLUDE
directory for the specified file. If the file is not found there, the compiler then checks the
S:\SOURCE\HEADERS directory. If the file is still not found, the preprocessor checks the current directory.
The second way to include files is to surround the file you want to include with double quotation marks. This method of inclusion tells the preprocessor to look for the file in the current directory first, then look for it in the predefined locations you have set up. Using the #include file version of file inclusion and applying it to the preceding example, the preprocessor first checks the current directory for the specified file. If the file is not found in the current directory, the C:COMPILERINCLUDE directory is searched. If the file is still not found, the preprocessor checks the S:SOURCEHEADERS directory.
The #include method of file inclusion is often used to include standard headers such as stdio.h or
stdlib.h.
This is because these headers are rarely (if ever) modified, and they should always be read from your compiler’s standard include file directory.
The #include file method of file inclusion is often used to include nonstandard header files that you have created for use in your program. This is because these headers are often modified in the current directory, and you will want the preprocessor to use your newly modified version of the header rather than the older, unmodified version.

25. What is the benefit of using an enum rather than a #define constant?

The use of an enumeration constant (enum) has many advantages over using the traditional symbolic constant style of #define. These advantages include a lower maintenance requirement, improved program readability, and better debugging capability.
1) The first advantage is that enumerated constants are generated automatically by the compiler. Conversely, symbolic constants must be manually assigned values by the programmer.
For instance, if you had an enumerated constant type for error codes that could occur in your program, your enum definition could look something like this:
enum Error_Code
{
OUT_OF_MEMORY,
INSUFFICIENT_DISK_SPACE,
LOGIC_ERROR,
FILE_NOT_FOUND
};
In the preceding example, OUT_OF_MEMORY is automatically assigned the value of 0 (zero) by the compiler because it appears first in the definition. The compiler then continues to automatically assign numbers to the enumerated constants, making INSUFFICIENT_DISK_SPACE equal to 1, LOGIC_ERROR equal to 2, and FILE_NOT_FOUND equal to 3, so on.
If you were to approach the same example by using symbolic constants, your code would look something like this:
#define OUT_OF_MEMORY 0
#define INSUFFICIENT_DISK_SPACE 1
#define LOGIC_ERROR 2
#define FILE_NOT_FOUND 3
values by the programmer. Each of the two methods arrives at the same result: four constants assigned numeric values to represent error codes. Consider the maintenance required, however, if you were to add two constants to represent the error codes DRIVE_NOT_READY and CORRUPT_FILE. Using the enumeration constant method, you simply would put these two constants anywhere in the enum definition. The compiler would generate two unique values for these constants. Using the symbolic constant method, you would have to manually assign two new numbers to these constants. Additionally, you would want to ensure that the numbers you assign to these constants are unique.
2) Another advantage of using the enumeration constant method is that your programs are more readable and thus can be understood better by others who might have to update your program later.

3) A third advantage to using enumeration constants is that some symbolic debuggers can print the value of an enumeration constant. Conversely, most symbolic debuggers cannot print the value of a symbolic constant. This can be an enormous help in debugging your program, because if your program is stopped at a line that uses an enum, you can simply inspect that constant and instantly know its value. On the other hand, because most debuggers cannot print #define values, you would most likely have to search for that value by manually looking it up in a header file.