C Interview Questions and Answers

Answer - 51 : - This is paranoia based on long experience. After a pointer has been freed, you can no longer use the pointed-to data. The pointer is said to dangle; it doesn’t point at anything useful. If you NULL out or zero out a pointer immediately after freeing it, your program can no longer get in trouble by using that pointer. True, you might go indirect on the null pointer instead, but that’s something your debugger might be able to help you with immediately. Also, there still might be copies of the pointer that refer to the memory that has been deallocated; that’s the nature of C. Zeroing out pointers after freeing them won’t solve all problems;

Answer - 52 : - These are all serious errors, symptoms of a wild pointer or subscript. Null pointer assignment is a message you might get when an MS-DOS program finishes executing. Some such programs can arrange for a small amount of memory to be available “where the NULL pointer points to (so to speak). If the program tries to write to that area, it will overwrite the data put there by the compiler. When the program is done, code generated by the compiler examines that area. If that data has been changed, the compiler-generated code complains with null pointer assignment. This message carries only enough information to get you worried. There’s no way to tell, just from a null pointer assignment message, what part of your program is responsible for the error. Some debuggers, and some compilers, can give you more help in finding the problem. Bus error: core dumped and Memory fault: core dumped are messages you might see from a program running under UNIX. They’re more programmer friendly. Both mean that a pointer or an array subscript was wildly out of bounds. You can get these messages on a read or on a write. They aren’t restricted to null pointer problems. The core dumped part of the message is telling you about a file, called core, that has just been written in your current directory. This is a dump of everything on the stack and in the heap at the time the program was running. With the help of a debugger, you can use the core dump to find where the bad pointer was used. That might not tell you why the pointer was bad, but it’s a step in the right direction. If you don’t have write permission in the current directory, you won’t get a core file, or the core dumped message

Answer - 53 : - There are times when it’s necessary to have a pointer that doesn’t point to anything. The macro NULL, defined in , has a value that’s guaranteed to be different from any valid pointer. NULL is a literal zero, possibly cast to void* or char*. Some people, notably C++ programmers, prefer to use 0 rather than NULL. The null pointer is used in three ways: 1) To stop indirection in a recursive data structure 2) As an error value 3) As a sentinel value

Answer - 54 : - The strcpy() function is designed to work exclusively with strings. It copies each byte of the source string to the destination string and stops when the terminating null character () has been moved. On the other hand, the memcpy() function is designed to work with any type of data. Because not all data ends with a null character, you must provide the memcpy() function with the number of bytes you want to copy from the source to the destination.

Answer - 55 : - The standard C library provides several functions for converting strings to numbers of all formats (integers, longs, floats, and so on) and vice versa. The following functions can be used to convert strings to numbers: Function Name Purpose atof() Converts a string to a double-precision floating-point value. atoi() Converts a string to an integer. atol() Converts a string to a long integer. strtod() Converts a string to a double-precision floating-point value and reports any leftover numbers that could not be converted. strtol() Converts a string to a long integer and reports any leftover numbers that could not be converted. strtoul() Converts a string to an unsigned long integer and reports any leftover numbers that could not be converted.

Answer - 56 : - The standard C library provides several functions for converting numbers of all formats (integers, longs, floats, and so on) to strings and vice versa The following functions can be used to convert integers to strings: Function Name Purpose itoa() Converts an integer value to a string. ltoa() Converts a long integer value to a string. ultoa() Converts an unsigned long integer value to a string. The following functions can be used to convert floating-point values to strings: Function Name Purpose ecvt() Converts a double-precision floating-point value to a string without an embedded decimal point. fcvt() Same as ecvt(), but forces the precision to a specified number of digits. gcvt() Converts a double-precision floating-point value to a string with an embedded decimal point. Is it possible to execute code even after the program exits the main() function? The standard C library provides a function named atexit() that can be used to perform cleanup operations when your program terminates. You can set up a set of functions you want to perform automatically when your program exits by passing function pointers to the at exit() function.

Answer - 57 : - The stack is where all the functions’ local (auto) variables are created. The stack also contains some information used to call and return from functions. A stack trace is a list of which functions have been called, based on this information. When you start using a debugger, one of the first things you should learn is how to get a stack trace. The stack is very inflexible about allocating memory; everything must be deallocated in exactly the reverse order it was allocated in. For implementing function calls, that is all that’s needed. Allocating memory off the stack is extremely efficient. One of the reasons C compilers generate such good code is their heavy use of a simple stack. There used to be a C function that any programmer could use for allocating memory off the stack. The memory was automatically deallocated when the calling function returned. This was a dangerous function to call; it’s not available anymore.

Answer - 58 : - The safest way is to use printf() (or fprintf() or sprintf()) with the %P specification. That prints a void pointer (void*). Different compilers might print a pointer with different formats. Your compiler will pick a format that’s right for your environment. If you have some other kind of pointer (not a void*) and you want to be very safe, cast the pointer to a void*: printf( %Pn, (void*) buffer );

Answer - 59 : - The preprocessor will include whatever file you specify in your #include statement. Therefore, if you have the line #include in your program, the file macros.inc will be included in your precompiled program. It is, however, unusual programming practice to put any file that does not have a .h or .hpp extension in an #include statement. You should always put a .h extension on any of your C files you are going to include. This method makes it easier for you and others to identify which files are being used for preprocessing purposes. For instance, someone modifying or debugging your program might not know to look at the macros.inc file for macro definitions. That person might try in vain by searching all files with .h extensions and come up empty. If your file had been named macros.h, the search would have included the macros.h file, and the searcher would have been able to see what macros you defined in it.

Answer - 60 : - The preprocessor is used to modify your program according to the preprocessor directives in your source code. Preprocessor directives (such as #define) give the preprocessor specific instructions on how to modify your source code. The preprocessor reads in all of your include files and the source code you are compiling and creates a preprocessed version of your source code. This preprocessed version has all of its macros and constant symbols replaced by their corresponding code and value assignments. If your source code contains any conditional preprocessor directives (such as #if), the preprocessor evaluates the condition and modifies your source code accordingly. The preprocessor contains many features that are powerful to use, such as creating macros, performing conditional compilation, inserting predefined environment variables into your code, and turning compiler features on and off. For the professional programmer, in-depth knowledge of the features of the preprocessor can be one of the keys to creating fast, efficient programs.