QUESTION 05 · Pointers and Functions

Does passing a pointer change C into pass by reference?

2 marks

Answer

No. The pointer value itself is passed by value.

The copied pointer can identify the same caller-owned object.
Assigning through it can change that object.

Explain how pointer arguments allow a function to modify caller data.

5 marks

Answer in brief

A function can receive a pointer to a caller’s object and modify the object through that pointer.

Function interface

void set_value(int *p, int value);

Working

The caller passes &number and the new integer value.
The parameter p receives a copy of the address.
The assignment *p = value changes the caller’s number object.

Example trace

number begins at 10. set_value(&number, 30) gives p the address of number and gives value the integer 30. The function checks p against NULL, then writes through it. After returning, main prints 30 because number itself was changed.

Value versus target

Changing p to NULL inside the function would only change the local pointer parameter. It would not replace a caller’s pointer variable. Changing *p updates the target. To update a caller’s pointer object, an additional level of indirection can be used, as shown in the pointer-to-pointer topic.

Validity

A non-null test is useful but does not prove every pointer is valid. The caller must still provide a pointer to a live, writable int object. A dangling pointer can be non-null. The example passes the address of a local object whose lifetime includes the complete call.

Complexity

The function performs a fixed comparison and at most one fixed-size assignment. Time and auxiliary space are O(1). Passing an address does not require copying every byte of a larger object that might be reached through it.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
void set_value(int *p, int value)
{
    if (p != NULL) { *p = value; }
}

int main(void)
{
    int number = 10;
    set_value(&number, 30);
    printf("Number = %d\n", number);
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

Number = 30