What Is The Heap?
The heap is where malloc(), calloc(), and realloc() get memory.
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.
Difference Between Null And Nul?
NULL is a macro defined in for the null pointer. NUL is the name of the first character in the ASCII character set. It corresponds to a zero value. There’s no standard macro NUL in C, but some people like to define it.
The digit 0 corresponds to a value of 80, decimal. Don’t confuse the digit 0 with the value of ‘’ (NUL)!
NULL can be defined as ((void*)0), NUL as ‘ ’.
What Is The Stack?
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.
When Should A Far Pointer Be Used?
Sometimes you can get away with using a small memory model in most of a given program. There might be just a few things that don’t fit in your small data and code segments. When that happens, you can use explicit far pointers and function declarations to get at the rest of memory. A far function can be outside the 64KB segment most functions are shoehorned into for a small-code model. (Often, libraries are declared explicitly far, so they’ll work no matter what code model the program uses.)
A far pointer can refer to information outside the 64KB data segment. Typically, such pointers are used with farmalloc () and such, to manage a heap separate from where all the rest of the data lives. If you use a small-data, large-code model, you should explicitly make your function pointers far.
Differentiate Between Far And Near?
Some compilers for PC compatibles use two types of pointers. Near pointers are 16 bits long and can address a 64KB range. far pointers are 32 bits long and can address a 1MB range.
Near pointers operate within a 64KB segment. There’s one segment for function addresses and one segment for data. far pointers have a 16-bit base (the segment address) and a 16-bit offset. The base is multiplied by 16, so a far pointer is effectively 20 bits long. Before you compile your code, you must tell the compiler which memory model to use. If you use a small code memory model, near pointers are used by default for function addresses.
That means that all the functions need to fit in one 64KB segment. With a large-code model, the default is to use far function addresses. You’ll get near pointers with a small data model, and far pointers with a large data model. These are just the defaults; you can declare variables and functions as explicitly near or far.
Far pointers are a little slower. Whenever one is used, the code or data segment register needs to be swapped out. Far pointers also have odd semantics for arithmetic and comparison. For example, the two far pointers in the preceding example point to the same address, but they would compare as different! If your program fits in a small-data, small-code memory model, your life will be easier.
Is It Better To Use Malloc () Or Calloc ()?
Both the malloc() and the calloc() functions are used to allocate dynamic memory. Each operates slightly different from the other. malloc() takes a size and returns a pointer to a chunk of memory at least that big
void *malloc( size_t size );
calloc() takes a number of elements, and the size of each, and returns a pointer to a chunk of memory at least big enough to hold them all
void *calloc( size_t numElements,size_t sizeOfElement );
There’s one major difference and one minor difference between the two functions. The major difference is that malloc () doesn’t initialize the allocated memory. The first time malloc () gives you a particular chunk of memory, the memory might be full of zeros. If memory has been allocated, freed, and reallocated, it probably has whatever junk was left in it. That means, unfortunately, that a program might run in simple cases (when memory is never reallocated) but break when used harder (and when memory is reused). calloc() fills the allocated memory with all zero bits. That means that anything there you’re going to use as a char or an int of any length, signed or unsigned, is guaranteed to be zero. Anything you’re going to use as a pointer is set to all zero bits. That’s usually a null pointer, but it’s not guaranteed. Anything you’re going to use as a float or double is set to all zero bits; that’s a floating-point zero on some types of machines, but not on all.
The minor difference between the two is that calloc () returns an array of objects; malloc () returns one object. Some people use calloc () to make clear that they want an array.
Explain The Purpose Of Main( ) Function?
The function main( ) invokes other functions within it.It is the first function to be called when the program starts execution.
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 argument count and.
argument vector (represents strings passed).
Any user-defined name can also be used as parameters for main( ) instead of argc and argv.
Write The Equivalent Expression For X%8?
x&7.
Why N++ Executes Faster Than N+1?
The expression n++ requires a single machine instruction such as INR to carry out the increment operation whereas n+1 requires more instructions to carry out this operation.
Can The Sizeof Operator Be Used To Tell The Size Of An Array Passed To A Function?
No. There’s no way to tell, at runtime, how many elements are in an array parameter just by looking at the array parameter itself. Remember, passing an array to a function is exactly the same as passing a pointer to the first element.
Is Using Exit () The Same As Using Return?
No. The exit () function is used to exit your program and return control to the operating system. The return statement is used to return from a function and return control to the calling function. If you issue a return from the main () function, you are essentially returning control to the calling function, which is the operating system. In this case, the return statement and exit () function are similar.
What Is A Function And Built-in Function?
A large program is subdivided into a number of smaller programs or subprograms. Each subprogram specifies one or more actions to be performed for a large program. Such subprograms are functions. The function supports only static and extern storage classes. By default, function assumes extern storage class. Functions have global scope. Only register or auto storage class is allowed in the function parameters. Built-in functions that predefined and supplied along with the compiler are known as built-in functions. They are also known as library functions.
Write About Modular Programming?
If a program is large, it is subdivided into a number of smaller programs that are called modules or subprograms. If a complex problem is solved using more modules, this approach is known as modular programming.
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.
The heap is where malloc(), calloc(), and realloc() get memory.
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.
Difference Between Null And Nul?
NULL is a macro defined in for the null pointer. NUL is the name of the first character in the ASCII character set. It corresponds to a zero value. There’s no standard macro NUL in C, but some people like to define it.
The digit 0 corresponds to a value of 80, decimal. Don’t confuse the digit 0 with the value of ‘’ (NUL)!
NULL can be defined as ((void*)0), NUL as ‘ ’.
What Is The Stack?
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.
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| Apigee Most Frequently Asked C Language Interview Questions Answers |
When Should A Far Pointer Be Used?
Sometimes you can get away with using a small memory model in most of a given program. There might be just a few things that don’t fit in your small data and code segments. When that happens, you can use explicit far pointers and function declarations to get at the rest of memory. A far function can be outside the 64KB segment most functions are shoehorned into for a small-code model. (Often, libraries are declared explicitly far, so they’ll work no matter what code model the program uses.)
A far pointer can refer to information outside the 64KB data segment. Typically, such pointers are used with farmalloc () and such, to manage a heap separate from where all the rest of the data lives. If you use a small-data, large-code model, you should explicitly make your function pointers far.
Differentiate Between Far And Near?
Some compilers for PC compatibles use two types of pointers. Near pointers are 16 bits long and can address a 64KB range. far pointers are 32 bits long and can address a 1MB range.
Near pointers operate within a 64KB segment. There’s one segment for function addresses and one segment for data. far pointers have a 16-bit base (the segment address) and a 16-bit offset. The base is multiplied by 16, so a far pointer is effectively 20 bits long. Before you compile your code, you must tell the compiler which memory model to use. If you use a small code memory model, near pointers are used by default for function addresses.
That means that all the functions need to fit in one 64KB segment. With a large-code model, the default is to use far function addresses. You’ll get near pointers with a small data model, and far pointers with a large data model. These are just the defaults; you can declare variables and functions as explicitly near or far.
Far pointers are a little slower. Whenever one is used, the code or data segment register needs to be swapped out. Far pointers also have odd semantics for arithmetic and comparison. For example, the two far pointers in the preceding example point to the same address, but they would compare as different! If your program fits in a small-data, small-code memory model, your life will be easier.
Is It Better To Use Malloc () Or Calloc ()?
Both the malloc() and the calloc() functions are used to allocate dynamic memory. Each operates slightly different from the other. malloc() takes a size and returns a pointer to a chunk of memory at least that big
void *malloc( size_t size );
calloc() takes a number of elements, and the size of each, and returns a pointer to a chunk of memory at least big enough to hold them all
void *calloc( size_t numElements,size_t sizeOfElement );
There’s one major difference and one minor difference between the two functions. The major difference is that malloc () doesn’t initialize the allocated memory. The first time malloc () gives you a particular chunk of memory, the memory might be full of zeros. If memory has been allocated, freed, and reallocated, it probably has whatever junk was left in it. That means, unfortunately, that a program might run in simple cases (when memory is never reallocated) but break when used harder (and when memory is reused). calloc() fills the allocated memory with all zero bits. That means that anything there you’re going to use as a char or an int of any length, signed or unsigned, is guaranteed to be zero. Anything you’re going to use as a pointer is set to all zero bits. That’s usually a null pointer, but it’s not guaranteed. Anything you’re going to use as a float or double is set to all zero bits; that’s a floating-point zero on some types of machines, but not on all.
The minor difference between the two is that calloc () returns an array of objects; malloc () returns one object. Some people use calloc () to make clear that they want an array.
Explain The Purpose Of Main( ) Function?
The function main( ) invokes other functions within it.It is the first function to be called when the program starts execution.
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 argument count and.
argument vector (represents strings passed).
Any user-defined name can also be used as parameters for main( ) instead of argc and argv.
Write The Equivalent Expression For X%8?
x&7.
Why N++ Executes Faster Than N+1?
The expression n++ requires a single machine instruction such as INR to carry out the increment operation whereas n+1 requires more instructions to carry out this operation.
Can The Sizeof Operator Be Used To Tell The Size Of An Array Passed To A Function?
No. There’s no way to tell, at runtime, how many elements are in an array parameter just by looking at the array parameter itself. Remember, passing an array to a function is exactly the same as passing a pointer to the first element.
Is Using Exit () The Same As Using Return?
No. The exit () function is used to exit your program and return control to the operating system. The return statement is used to return from a function and return control to the calling function. If you issue a return from the main () function, you are essentially returning control to the calling function, which is the operating system. In this case, the return statement and exit () function are similar.
What Is A Function And Built-in Function?
A large program is subdivided into a number of smaller programs or subprograms. Each subprogram specifies one or more actions to be performed for a large program. Such subprograms are functions. The function supports only static and extern storage classes. By default, function assumes extern storage class. Functions have global scope. Only register or auto storage class is allowed in the function parameters. Built-in functions that predefined and supplied along with the compiler are known as built-in functions. They are also known as library functions.
Write About Modular Programming?
If a program is large, it is subdivided into a number of smaller programs that are called modules or subprograms. If a complex problem is solved using more modules, this approach is known as modular programming.
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.
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