Memory Allocation Questions and Answers

Q: What is the output of this C code? void main() { char *p = calloc(100, 1); p = "welcome"; printf("%s\n", p); }

Correct answer: welcomeThe calloc block is allocated and then immediately abandoned: assigning the string literal to p overwrites the only pointer to it. That is a memory leak, but it is not what the question asks about.p now points at the literal "welcome", so that is what printf outputs. Note that the literal is not modifiable - writing through p afterwards would be undefined behaviour, which is why such a pointer is better declared const char *.

Q: malloc() allocates memory from the heap and not from the stack.

Correct answer: TRUEmalloc obtains storage from the heap, a region the program grows at run time through the operating system. That is what gives the block a lifetime independent of any function - it persists until free is called, whatever calls return in the meantime.Stack storage behaves quite differently: it is allocated by the function prologue and reclaimed on return, which is why returning the address of a local is a fault and returning the result of malloc is not.

Q: calloc initialises memory with all bits set to zero.

Correct answer: truecalloc sets every bit of the allocated block to zero, and the standard requires it. That is the guarantee, not a compiler-dependent convenience.One subtlety is worth knowing: all-zero bits reliably give integer zero, but the standard does not promise that they give a null pointer or a floating-point zero - those hold on essentially every current implementation but are not guaranteed. Code depending on a zeroed block containing valid null pointers is portable in practice rather than in principle.

Q: Memory returned by malloc() is automatically initialised to zero.

Correct answer: Falsemalloc() leaves the contents of the block indeterminate, so reading it before writing is undefined behaviour. calloc() is the function that clears the memory to zero, and it is the reason calloc() is preferred when a zeroed buffer is required.

Q: If p is a pointer to a block allocated with malloc(), what does sizeof(p) give ?

Correct answer: The size of a pointer on that platformsizeof is resolved at compile time and knows nothing of what malloc() did at run time, so it reports the size of the pointer itself, typically 8 bytes on a 64-bit system. C provides no portable way to recover the size of an allocated block, so the program must remember it.

Q: What is the return type of malloc() or calloc()?

Correct answer: void *Both allocators return void *, a pointer to untyped storage. The caller assigns it to a pointer of whatever type it needs, and in C that conversion is implicit.The design is what lets a single pair of functions serve every type in the language. It also means the allocator has no idea what will be stored in the block - alignment is guaranteed to suit any type, but the size must be computed by the caller, which is why sizeof appears in almost every call.

Q: Which languages necessarily need heap allocation in the run time environment?

Correct answer: Those that allow dynamic data structuresA heap is needed when the amount of storage required is not known until run time, and when objects must outlive the function that created them. Dynamic data structures - lists, trees, growable arrays - require exactly that.Recursion needs only a stack, since each activation is destroyed in reverse order of creation. Dynamic scoping is a name-resolution rule, and global variables have static storage decided at compile time. None of those three forces a heap.

Q: Which of the following is/are true

Correct answer: All of the aboveAll three statements are correct. calloc zeroes the memory it returns while malloc leaves whatever the allocator hands over, which is why an uninitialised malloc block holds arbitrary data.malloc followed by memset to zero achieves the same result, which is essentially what calloc does internally. And the signatures differ: calloc(n, size) takes a count and an element size, while malloc(size) takes a single total.

41.

What is the output of this C code?

void main()
{
char *p = calloc(100, 1);
p = "welcome";
printf("%s\n", p);
}

A.Segmentation fault
B.garbage
C.Error
D.welcome

Answer: D

Correct answer: welcome

The calloc block is allocated and then immediately abandoned: assigning the string literal to p overwrites the only pointer to it. That is a memory leak, but it is not what the question asks about.

p now points at the literal "welcome", so that is what printf outputs. Note that the literal is not modifiable - writing through p afterwards would be undefined behaviour, which is why such a pointer is better declared const char *.

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42.

malloc() allocates memory from the heap and not from the stack.

A.TRUE
B.FALSE
C.May Be
D.Can't Say

Answer: A

Correct answer: TRUE

malloc obtains storage from the heap, a region the program grows at run time through the operating system. That is what gives the block a lifetime independent of any function - it persists until free is called, whatever calls return in the meantime.

Stack storage behaves quite differently: it is allocated by the function prologue and reclaimed on return, which is why returning the address of a local is a fault and returning the result of malloc is not.

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43.

calloc initialises memory with all bits set to zero.

A.true
B.false
C.Depends on the compiler
D.Depends on the standard

Answer: A

Correct answer: true

calloc sets every bit of the allocated block to zero, and the standard requires it. That is the guarantee, not a compiler-dependent convenience.

One subtlety is worth knowing: all-zero bits reliably give integer zero, but the standard does not promise that they give a null pointer or a floating-point zero - those hold on essentially every current implementation but are not guaranteed. Code depending on a zeroed block containing valid null pointers is portable in practice rather than in principle.

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44.

Memory returned by malloc() is automatically initialised to zero.

A.True
B.False

Answer: B

Correct answer: False

malloc() leaves the contents of the block indeterminate, so reading it before writing is undefined behaviour. calloc() is the function that clears the memory to zero, and it is the reason calloc() is preferred when a zeroed buffer is required.

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45.

If p is a pointer to a block allocated with malloc(), what does sizeof(p) give ?

A.The size of the allocated block
B.The size of a pointer on that platform
C.The number of elements allocated
D.Zero

Answer: D

Correct answer: The size of a pointer on that platform

sizeof is resolved at compile time and knows nothing of what malloc() did at run time, so it reports the size of the pointer itself, typically 8 bytes on a 64-bit system. C provides no portable way to recover the size of an allocated block, so the program must remember it.

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46.

What is the return type of malloc() or calloc()?

A.int *
B.int **
C.void *
D.void **

Answer: C

Correct answer: void *

Both allocators return void *, a pointer to untyped storage. The caller assigns it to a pointer of whatever type it needs, and in C that conversion is implicit.

The design is what lets a single pair of functions serve every type in the language. It also means the allocator has no idea what will be stored in the block - alignment is guaranteed to suit any type, but the size must be computed by the caller, which is why sizeof appears in almost every call.

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47.

Which languages necessarily need heap allocation in the run time environment?

A.Those that support recursion
B.Those that use dynamic scoping
C.Those that use global variables
D.Those that allow dynamic data structures

Answer: D

Correct answer: Those that allow dynamic data structures

A heap is needed when the amount of storage required is not known until run time, and when objects must outlive the function that created them. Dynamic data structures - lists, trees, growable arrays - require exactly that.

Recursion needs only a stack, since each activation is destroyed in reverse order of creation. Dynamic scoping is a name-resolution rule, and global variables have static storage decided at compile time. None of those three forces a heap.

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48.

How long does memory allocated with malloc() remain valid ?

A.Until the enclosing function returns
B.Until it is explicitly freed or the program ends
C.Until the next call to malloc()
D.Until the end of the enclosing block

Answer: B

Correct answer: Until it is explicitly freed or the program ends

Heap memory has no automatic lifetime: it persists until free() is called or the process exits. This is precisely why a function can allocate a block and return its address safely, whereas returning the address of a local variable cannot work.

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49.

Which is the correct sequence of compilation process?

A.Assembler -> Compiler -> Preprocessor -> Linking
B.Compiler -> Assenbler -> Preprocessor -> Linking
C.Preprocessor -> Compiler -> Assembler -> Linking
D.Assembler -> Compiler -> Linking -> Preprocessor

Answer: C

Correct answer: Preprocessor -> Compiler -> Assembler -> Linking

Each stage consumes what the previous one produced. The preprocessor handles #include, #define and conditional compilation, emitting a single translation unit of pure C.

The compiler turns that into assembly, the assembler into an object file holding machine code with unresolved external references, and the linker joins the object files and libraries into an executable. A call to a function that was declared but never defined is a link error precisely because the linker is the first stage to look across files.

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50.

Which of the following is/are true

A.calloc() allocates the memory and also initializes the allocates memory to zero, while memory allocated using malloc() has random data.
B.malloc() and memset() can be used to get the same effect as calloc().
C.calloc() takes two arguments, but malloc takes only 1 argument
D.All of the above

Answer: D

Correct answer: All of the above

All three statements are correct. calloc zeroes the memory it returns while malloc leaves whatever the allocator hands over, which is why an uninitialised malloc block holds arbitrary data.

malloc followed by memset to zero achieves the same result, which is essentially what calloc does internally. And the signatures differ: calloc(n, size) takes a count and an element size, while malloc(size) takes a single total.

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