| 1.Difference between arrays and pointers? - Pointers are used to manipulate data using the address. Pointers use * operator to access the data pointed to by them - Arrays use subscripted variables to access and manipulate data.Array variables can be equivalently written using pointer expression. 2. What are the advantages of the functions?Answer – Debugging is easier - It is easier to understand the logic involved in the program 3 How can I open a file so that other programs can update it at the same time? Answer 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 Constant Description O_APPEND Appends all writes to the end of the file O_BINARY Opens the file in binary (untranslated) 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 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 4. Can static variables be declared in a header file? You can’t declare a static variable without defining it as well (this is because the storage class modifiers You can use the #ifdef and #ifndef preprocessor directives to check whether a symbol has been defined 7.How do you override a defined macro? Answer 10. Can a variable be both const and volatile? Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. For instance, in the example in Many programmers like to create a custom header file that has #include statements for every header needed for each module. This is perfectly acceptable and can help avoid potential problems relating to #include files, such as accidentally omitting an #include file in a module. 12. Can static variables be declared in a header file? 13. When does the compiler not implicitly generate the address of the first element of an array? Whenever an array name appears in an expression such as - array as an operand of the sizeof operator - array as an operand of & operator - array as a string literal initializer for a character array Then the compiler does not implicitly generate the address of the address of the first element of an array. 14.What is the difference between #include <file> and #include “file”? When writing your C program, you can include files in two ways. The first way is to surround the file you INCLUDE=C:\COMPILER\INCLUDE;S:\SOURCE\HEADERS; using the #include <file> version of file inclusion, the compiler first checks the C:\COMPILER\INCLUDE 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 “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. 15.When function say abc() calls another function say xyz(), what happens in stack? 16.How do you print an address? there is a another specifier %Fp which prints both the code segment and offset address 18.How to find entered number is EVEN or ODD without using conditional statement(not using if.. else,if.. , else if..,while, do… while…., for….) 19.How to break cycle in circular single link list? 20.How can I convert a number to a string? 21.How to swap the content oftwo variables without a temporary variable { a =a+b; b=a-b; a=a-b; } 22. How can send unlimited no of arguments to a function, eg printf function can take any no of arguments 23.What is the benefit of using #define to declare a constant? For instance, if your program used the value of pi (approximately 3.14159) several times, you might want to declare a constant for pi as follows: #define PI 3.14159 Using the #define method of declaring a constant is probably the most familiar way of declaring constants to traditional C programmers. Besides being the most common method of declaring constants, it also takes up the least memory. Constants defined in this manner are simply placed directly into your source code, with no variable space allocated in memory. Unfortunately, this is one reason why most debuggers cannot inspect constants created using the #define method. 24.How do you write a C program which can calculate lines of code but not counting comments? Then using while,check for the character ‘/’,'*’ (as the comments start with these characters) and end with (‘*’ and ‘/’).if condition of this is true then break and come out of the block else increment the line. 25.How can I search for data in a linked list? Unfortunately, the only way to search a linked list is with a linear search, because the only way a linked list’s members can be accessed is sequentially. Sometimes it is quicker to take the data from a linked list and store it in a different data structure so that searches can be more efficient. 26.How to write a C program to find the power of 2 in a normal way and in single step? 27.What is hashing? The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can’t perform comparison searches. If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you’ll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array. To search for an item, you simply hash it and look at all the data whose hash values match that of the data you’re looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one. One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value. There are two ways to resolve this problem. In “open addressing,” the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its The second method of resolving a hash collision is called “chaining.” In this method, a “bucket” or linked list holds all the elements whose keys hash to the same value. When the hash table is searched, the list must be searched linearly. 28.Diffence arrays and pointers? to access data stored in array we use indexes such as a[0]. 30.Can a variable be both const and volatile? 31.Why should we assign NULL to the elements (pointer) after freeing them? Answer 32.What is a “null pointer assignment” error? What are bus errors, memory faults, and core dumps? Null pointer assignment is a message you might get when an MS-DOS program finishes executing. Some 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 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. 33.What are storage class in c Answer static auto register extern 34.Following declarations are different from one another Answer 35.When should a type cast be used?
The second case is to cast pointer types to and from void * in order to interface with functions that expect or return void pointers. For example, the following line type casts the return value of the call to malloc() to be a pointer to a foo structure. struct foo *p = (struct foo *) malloc(sizeof(struct foo)); 36.What is a null pointer? There are times when it’s necessary to have a pointer that doesn’t point to anything. The macro NULL, defined in <stddef.h>, 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 36.What is a const pointer?
37.when should the volatile modifier be used?
Most computers have a set of registers that can be accessed faster than the computer’s main memory. A good compiler will perform a kind of optimization called “redundant load and store removal.” The compiler looks for places in the code where it can either remove an instruction to load data from memory because the value is already in a register, or remove an instruction to store data to memory because the value can stay in a register until it is changed again anyway. If a variable is a pointer to something other than normal memory, such as memory-mapped ports on a time_t time_addition(volatile const struct timer *t, int a) In this code, the variable t->value is actually a hardware counter that is being incremented as time passes. The function adds the value of a to x 1000 times, and it returns the amount the timer was incremented by while the 1000 additions were being performed. Without the volatile modifier, a clever optimizer might assume that the value of t does not change during the execution of the function, because there is no statement that explicitly changes it. In that case, there’s no need to read it from memory a second time and subtract it, because the answer will always be 0. The compiler might therefore “optimize” the function by making it always return 0. If a variable points to data in shared memory, you also don’t want the compiler to perform redundant load and store optimizations. Shared memory is normally used to enable two programs to communicate with each other by having one program store data in the shared portion of memory and the other program read the same portion of memory. If the compiler optimizes away a load or store of shared memory, communication between the two programs will be affected. 38.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 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 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. 39.When is a switch statement better than multiple if statements? 40.What is the difference between a string copy (strcpy) and a memory copy (memcpy)? When should each be used? 41.How can I convert a string to a number?
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. 41.How can I convert a number to a string?
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. 42.Is it possible to execute code even after the program exits the main() function? What is the stack? 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 How do you print an address? 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 ); 45.When should the register modifier be used? Does it really help? The register modifier hints to the compiler that the variable will be heavily used and should be kept in the CPU’s registers, if possible, so that it can be accessed faster. There are several restrictions on the use of the register modifier. First, the variable must be of a type that can be held in the CPU’s register. This usually means a single value of a size less than or equal to the size of an integer. Some machines have registers that can hold floating-point numbers as well. Second, because the variable might not be stored in memory, its address cannot be taken with the unary & operator. An attempt to do so is flagged as an error by the compiler. Some additional rules affect how useful the register modifier is. Because the number of registers is limited, and because some registers can hold only certain types of data (such as pointers or floating-point numbers), the number and types of register modifiers that will actually have any effect are dependent on what machine the Also, in some cases, it might actually be slower to keep a variable in a register because that register So when should the register modifier be used? The answer is never, with most modern compilers. Early C compilers did not keep any variables in registers unless directed to do so, and the register modifier was a valuable addition to the language. C compiler design has advanced to the point, however, where the compiler will usually make better decisions than the programmer about which variables should be stored in registers. In fact, many compilers actually ignore the register modifier, which is perfectly legal, because it is only a hint and not a directive. 46.Can a file other than a .h file be included with #include? The preprocessor will include whatever file you specify in your #include statement. Therefore, if you have the line #include <macros.inc> 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. 47.What is Preprocessor? 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. 48.How can you restore a redirected standard stream? The dup() function duplicates a file handle. You can use the dup() function to save the file handle 53.What is the heap? Getting memory from the heap is much slower than getting it from the stack. On the other hand, the heap is much more flexible than the stack. Memory can be allocated at any time and deallocated in any order. Such memory isn’t deallocated automatically; you have to call free(). Recursive data structures are almost always implemented with memory from the heap. Strings often come from there too, especially strings that could be very long at runtime. If you can keep data in a local variable (and allocate it from the stack), your code will run faster than if you put the data on the heap. Sometimes you can use a better algorithm if you use the heap—faster, or more robust, or more flexible. It’s a tradeoff. If memory is allocated from the heap, it’s available until the program ends. That’s great if you remember to deallocate it when you’re done. If you forget, it’s a problem. A “memory leak” is some allocated memory that’s no longer needed but isn’t deallocated. If you have a memory leak inside a loop, you can use up all the memory on the heap and not be able to get any more. (When that happens, the allocation functions return a null pointer.) In some environments, if a program doesn’t deallocate everything it allocated, memory stays unavailable even after the program ends. 54.How do you use a pointer to a function? The hardest part about using a pointer-to-function is declaring it. Consider an example. You want to create a pointer, pf, that points to the strcmp() function. The strcmp() function is declared in this way: int strcmp(const char *, const char * ) To set up pf to point to the strcmp() function, you want a declaration that looks just like the strcmp() function’s declaration, but that has *pf rather than strcmp: int (*pf)( const char *, const char * ); After you’ve gotten the declaration of pf, you can #include <string.h> and assign the address of strcmp() to pf: pf = strcmp; 55.What is the purpose of realloc( )? 56.What is the purpose of main( ) function?
- It is the starting function - It returns an int value to the environment that called the program - Recursive call is allowed for main( ) also. - It is a user-defined function - Program execution ends when the closing brace of the function main( ) is reached. - It has two arguments 1)argument count and 2) argument vector (represents strings passed). - Any user-defined name can also be used as parameters for main( ) instead of argc and argv * more here |
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