In the following C program using a const pointer to char, where does the error occur and why? #include <stdio.h> int main(void) { char mybuf[] = "Zanzibar"; char yourbuf[] = " Zienckewiz"; char * const ptr = mybuf; *ptr = 'a'; ptr = yourbuf; return 0; }

Introduction / Context:
This question explores the meaning of a const pointer declaration in C: char * const ptr. It checks whether you can distinguish between a constant pointer (whose address value cannot change) and a pointer to const data (whose pointed to data cannot change). Correctly interpreting which part is const is crucial for understanding which operations are allowed.

Given Data / Assumptions:

• mybuf and yourbuf are character arrays initialized from string literals.

• ptr is declared as char * const ptr = mybuf; which means ptr is a constant pointer to non const char.

• The statement *ptr = 'a'; attempts to modify the first character in mybuf through ptr.

• The statement ptr = yourbuf; attempts to change ptr so it points to a different array.

• The code is otherwise syntactically correct.

Concept / Approach:
The declaration char * const ptr means: "ptr is a constant pointer to char". The const applies to the pointer itself, not to the data. So ptr must always point to the same address after initialization, but the characters at that address are modifiable because they are not declared const. Therefore writing through *ptr is allowed, but reassigning ptr to a new address is not.

Step-by-Step Solution:
Step 1: Interpret char * const ptr = mybuf; as making ptr a constant pointer that initially points to mybuf[0]. Step 2: Since the pointee type is plain char (not const char), modifying *ptr is equivalent to modifying mybuf[0], which is legal because mybuf is a modifiable array. Step 3: The statement *ptr = 'a'; simply changes the first character of mybuf from 'Z' to 'a', which is allowed. Step 4: The next statement ptr = yourbuf; attempts to change the pointer value so that ptr no longer points to mybuf but to yourbuf. Step 5: Because ptr is declared const, its value (the stored address) cannot be changed after initialization, so this assignment violates the constness of ptr and causes a compilation error.

Verification / Alternative check:
You can rewrite the declaration as char * const ptr and contrast it with const char *ptr. In the latter case, the data is const but the pointer itself is not, so ptr can point to different strings but you cannot change the characters through ptr. In this question, the const is placed after the *, which means the pointer is const. This confirms that the illegal operation is reassigning ptr, not modifying the character it points to.

Why Other Options Are Wrong:
Option A is wrong because there is indeed an error: ptr = yourbuf; is not allowed. Option B is wrong because *ptr = 'a' is valid; the character data is not const, only the pointer is. Option D is wrong because the array initializations are fine: mybuf and yourbuf are local arrays initialized with string literals, which is standard and legal.

Common Pitfalls:
A frequent confusion arises between "pointer to const" and "const pointer". Remember the rule: const placed to the left of * usually applies to the pointed to data (const char *), while const placed to the right of * usually applies to the pointer itself (char * const). Additionally, some learners mistakenly think anything involving const forbids all modifications; in reality you must be precise about which entity is const. Understanding these distinctions improves your ability to read and write const correct interfaces.

Final Answer:
The illegal action is reassigning the const pointer, so the error occurs at ptr = yourbuf; because you cannot change the value of a const pointer variable.