QUESTION 31 · String Basics

Why use const char * for a function that only reads a string?

2 marks

Answer

It prevents modification through that parameter’s character access.

The caller still supplies a valid null-terminated string.
The qualifier does not automatically prove the pointer or its lifetime is valid.

Explain character pointers and const string parameters in functions.

5 marks

Answer in brief

A character pointer can identify the first byte of a string. Functions can use it to read characters or, for a writable target and suitable interface, modify them.

Read-only interface

void show(const char *text);

Working

The caller supplies a pointer to the first character. show reads text until its null terminator through puts. The const-qualified character access prevents the function from writing text[0] through this parameter. The local pointer parameter itself is a copied pointer value.

String literal and array

A literal such as "Welcome" must not be modified. A writable array such as char name[] = "Student"; contains its own copy and can be changed within bounds. Both can be passed to a suitable reading function. Use const char * for a literal pointer so the intended reading-only use is clear.

Modification interface

A function that writes a string normally needs char * and a destination capacity. The pointer alone does not communicate how many positions are available. A function must not store beyond the capacity or leave a result unterminated when a string result is promised.

Return and lifetime

Returning a pointer is valid only when the target remains alive for its later use. Do not return the address of an ordinary automatic local character array. A pointer to caller-owned storage or a suitable lasting object requires a clearly stated lifetime and ownership agreement.

Complexity

Displaying n bytes takes O(n) output work. The pointer argument is small fixed-size state and does not copy the whole string. No new variable-length buffer is created by this example.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
void show(const char *text)
{
    puts(text);
}

int main(void)
{
    show("Welcome");
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

Welcome