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C Programming

C language from basics to advanced placement prep

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Difficulty: All Easy Medium Hard 71–80 of 499

Topics in C Programming

All Basics & Syntax 100 Data Types & Variables 100 Control Flow 100 Functions 100 Arrays & Strings 100 Pointers 100 Structures & Unions 100 File Handling 100 Dynamic Memory 100 Preprocessor 100

Q.71 Medium Dynamic Memory

In a competitive programming scenario, you need to store strings dynamically. What is safe practice?

A char *str = malloc(100); without checking for NULL

B char *str = malloc(strlen(input)+1); with NULL check and strncpy

C char str[100] always on stack

D No allocation needed, use global arrays

Correct Answer: B. char *str = malloc(strlen(input)+1); with NULL check and strncpy

EXPLANATION

Safe practice includes allocating exact size (input length + 1 for null terminator), checking for NULL, and using bounded string functions.

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Q.72 Medium Dynamic Memory

What happens if you call free() twice on the same pointer?

A Program immediately crashes

B Double free error - undefined behavior

C Memory is freed twice successfully

D No effect, second free is ignored

Correct Answer: B. Double free error - undefined behavior

EXPLANATION

Double free is undefined behavior and can cause program crash or corruption. After first free(), the pointer should not be used.

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Q.73 Medium Dynamic Memory

Consider a program that repeatedly allocates memory in a loop but never frees it. What problem occurs?

A Stack overflow

B Memory leak - heap memory is exhausted

C Segmentation fault immediately

D Integer overflow

Correct Answer: B. Memory leak - heap memory is exhausted

EXPLANATION

Allocating without freeing causes memory leak, where allocated memory is not returned to the system, eventually exhausting heap.

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Q.74 Medium Dynamic Memory

A competitive programmer allocates a 2D array dynamically. What is the correct approach?

A int **arr = (int**)malloc(rows * sizeof(int*)); for(i=0; i

B int arr[rows][cols] = (int**)malloc(...);

C int **arr = calloc(rows*cols, sizeof(int));

D int arr[][] = malloc(rows * cols);

Correct Answer: A. int **arr = (int**)malloc(rows * sizeof(int*)); for(i=0; i

EXPLANATION

2D arrays are allocated as array of pointers, where each row pointer points to a dynamically allocated array.

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Q.75 Medium Dynamic Memory

What is the correct way to allocate a dynamic array of struct for 'n' elements?
struct Node { int data; char name[20]; };

A struct Node *arr = malloc(n);

B struct Node *arr = malloc(n * sizeof(struct Node));

C struct Node arr = malloc(n);

D struct Node *arr = calloc(n, 1);

Correct Answer: B. struct Node *arr = malloc(n * sizeof(struct Node));

EXPLANATION

Must allocate n * sizeof(struct Node) bytes to hold all n structures.

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Q.76 Medium Dynamic Memory

In competitive exams, why is dynamic allocation preferred over static arrays for unknown input sizes?

A It's faster than static arrays

B Allows allocation only as much memory as needed, preventing stack overflow

C Static arrays don't work in C

D It's automatically freed by compiler

Correct Answer: B. Allows allocation only as much memory as needed, preventing stack overflow

EXPLANATION

Dynamic allocation allocates memory on heap based on runtime input, preventing fixed-size limitations and stack overflow.

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Q.77 Medium Dynamic Memory

If realloc() cannot expand memory at the current location, what does it do?

A Fails and returns NULL

B Allocates new memory, copies old data, frees old memory, returns new pointer

C Extends memory anyway

D Keeps old pointer valid

Correct Answer: B. Allocates new memory, copies old data, frees old memory, returns new pointer

EXPLANATION

realloc() finds new space if needed, copies data, deallocates old block, and returns new pointer.

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Q.78 Medium Dynamic Memory

What is the output of this code?
int *p = (int*)calloc(3, sizeof(int));
printf("%d %d %d", p[0], p[1], p[2]);

A Garbage values

B 0 0 0

C 3 bytes each

D Compiler error

Correct Answer: B. 0 0 0

EXPLANATION

calloc() initializes all allocated memory to zero, so all elements will be 0.

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Q.79 Medium Dynamic Memory

Which scenario best demonstrates a memory leak?

A char *s = malloc(10); free(s);

B char *s = malloc(10); s = malloc(20);

C char *s = malloc(10); free(s); free(s);

D char *s = NULL; free(s);

Correct Answer: B. char *s = malloc(10); s = malloc(20);

EXPLANATION

Original malloc(10) pointer is lost before freeing. The 10 bytes are now inaccessible - a memory leak.

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Q.80 Medium Dynamic Memory

What happens if you use a pointer after calling free() on it (without reallocating)?

A Program runs normally

B Pointer becomes NULL

C Use-after-free: undefined behavior, possible crash or data corruption

D Compiler automatically prevents it

Correct Answer: C. Use-after-free: undefined behavior, possible crash or data corruption

EXPLANATION

After free(), accessing the pointer causes undefined behavior as memory is returned to the heap and may be reused.

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