Implementing calloc: Why Two Parameters Matter for Security

It’s interesting working through the Qwasar curriculum using AI to code. In fact, it is becoming standard now in some (maybe not a lot!) interview contexts to allow AI to write a majority or even all of the generated code. I think being comfortable with these tools does require a mental shift—but it’s one I’m quite well suited for. I tend to get ‘in the weeds’ quickly. In the olden times (pre-AI) this could very often lead me to struggle with productivity. I’d work (and work and work!), but never feel like I was really making progress. I struggle with breaking down complexity into small chunks that my brain can parse. I am by nature a ‘big picture’ thinker. So I really enjoy thinking about architecture on a conceptual level. I also LOVE to geek out over implementation details—in the music world I felt like I had developed a skill set that allowed me to do this. But I was trying to learn programming quickly (by necessity!) and it was this ‘rush’ that made me feel like every discovery only led to the realization that I understood so little of the underlying concepts! Maybe now I’m reaching an inflection point in my programming journey where I am figuring out how to handle the increasing complexity without being overwhelmed, and stay productive and consequently mentally healthier!

As a musician, I started on violin at age 4 and horn at age 12. My childlike wonder at learning these instruments led me to dive deep and progress quickly. I also had some really incredible teachers, and I developed proficiency and technical skill. But on a certain level, maybe I limited myself by getting ‘stuck’ in that world, without allowing “release” to go back to childlike wonder at MUSIC where I could study the ‘why’ of the actual architecture. I stayed ‘visceral’—I often felt the best performers were those who expressed something ‘human’ and that the best way to gain entry into this coveted class of elite artists was to stay in the ‘higher level’.

I was also, deep down… SCARED. It took me YEARS as a professional to surpass all of this fear. Eventually, when I felt like I had made significant strides and was on the cusp of real mastery, Focal Dystonia hit and my performing career was immediately over.

Recently I’ve been re-evaluating my entire mindset, because I’ve always had a gut feeling that I am my own worst enemy, and that throughout life I have a tendency to ‘get in my own way’. Maybe it’s because of my meditation practice that I’m starting to be able to understand what’s beneath this feeling a little more clearly now.

Regardless, this was a fun exercise and using AI tooling allows me a comfort level of having a personal assistant that has basically ALL distilled programming knowledge to-date. I’m sure there are programming purists who feel strongly that what I’m doing isn’t really software engineering… I often consider how this would relate to my strong feelings about AI generated music (art), but this is a totally separate topic for another blog post!

I’m looking forward to continuing on this journey in the world of software, and seeing where it all leads—and I’m not scared anymore. I’m excited!

Technical Details

This session continued work on a custom malloc implementation as part of a Qwasar learning project. The repository is at my_malloc.

What Got Built

Implemented my_calloc() with comprehensive test coverage. The work is captured in commit 458b09a:

commit 458b09adbfcae953cb2933ca128d88ce473066c9
Author: Alex Rosenfeld <arosnefeld2003@mac.com>
Date:   Sat Dec 13 13:56:39 2025 -0800

    calloc with passing tests

    +120 lines, -9 deletions
    - my_malloc.c: implemented my_calloc with overflow protection
    - my_malloc.h: fixed function naming (mycalloc -> my_calloc)
    - test_malloc.c: added 6 comprehensive tests

The calloc Implementation

The implementation includes three key components:

1. Edge Case Handling

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void *my_calloc(size_t nmemb, size_t size) {
    // Edge case: if either is 0, return NULL
    if (nmemb == 0 || size == 0) {
        return NULL;
    }

2. Overflow Protection (The critical part)

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    // Check for overflow: nmemb * size must not overflow size_t
    // If nmemb * size > SIZE_MAX, then nmemb > SIZE_MAX / size
    if (nmemb > SIZE_MAX / size) {
        return NULL;  // Would overflow
    }

    size_t total_size = nmemb * size;

This check prevents integer overflow attacks. Without it, an attacker could request calloc(SIZE_MAX/4 + 1, 4) which would overflow to a small number, allocating insufficient memory but returning a pointer the program thinks is large.

3. Code Reuse and Zero-Initialization

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    // Reuse my_malloc to allocate memory
    void *ptr = my_malloc(total_size);
    if (ptr == NULL) {
        return NULL;  // Allocation failed
    }

    // Zero-initialize the memory
    memset(ptr, 0, total_size);

    return ptr;
}

The Test Suite

Six tests were added to verify all aspects of calloc:

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TEST(test_calloc_returns_non_null)           // Basic allocation
TEST(test_calloc_zeroes_memory)              // Verify zero-initialization
TEST(test_calloc_array_allocation)           // Practical array usage
TEST(test_calloc_zero_nmemb_returns_null)    // Edge case: count=0
TEST(test_calloc_zero_size_returns_null)     // Edge case: size=0
TEST(test_calloc_overflow_protection)        // Security: overflow detection

The overflow protection test specifically tries to trigger an overflow:

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TEST(test_calloc_overflow_protection) {
    // Try to overflow: SIZE_MAX / sizeof(int) + 1
    size_t huge_count = SIZE_MAX / sizeof(int) + 1;
    void *ptr = my_calloc(huge_count, sizeof(int));
    assert(ptr == NULL);  // Should fail safely
}

All tests pass:

--- calloc tests ---
Running test_calloc_returns_non_null... PASSED
Running test_calloc_zeroes_memory... PASSED
Running test_calloc_array_allocation... PASSED
Running test_calloc_zero_nmemb_returns_null... PASSED
Running test_calloc_zero_size_returns_null... PASSED
Running test_calloc_overflow_protection... PASSED

Documentation Improvements

The code also includes enhanced documentation explaining C pointer arithmetic, which came up during the session:

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/*
 * pointer arithmetic:
 *  - block points to address 0x1000 (start of header)
 *  - block + 1 means: 0x1000 + (1 × sizeof(block_header_t))
 *  - IMPORTANT: c pointer arithmetic automatically multiplies by type size!
 *  - Since sizeof(block_header_t) = 24 bytes, this gives: 0x1000 + 24 = 0x1018
 *  - So block + 1 points to the byte immediately after the header
 */

This clarifies how return (void *)(block + 1) works in my_malloc() and why (block_header_t *)ptr - 1 correctly retrieves the header in my_free().

Why calloc Has Two Parameters

The session explored why calloc takes two parameters (nmemb, size) instead of just one total size:

1. Semantic clarity: calloc(256, sizeof(int)) clearly expresses “array of 256 ints”
2. Overflow protection: Having both values allows checking if their multiplication would overflow before it happens

With malloc, the overflow happens before the function is called:

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size_t bad_size = huge_count * sizeof(int);  // ← Overflows HERE
malloc(bad_size);  // ← Too late to detect

With calloc, the function receives both operands and can validate before multiplying:

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calloc(huge_count, sizeof(int));  // ← Can check if multiplication is safe

This makes calloc inherently safer for array allocations when the size comes from untrusted input.

Project Status

According to the dev plan, the project is now in Week 3. Completed:

• ✅ Week 1: Architecture decisions (in-band headers, linked list, immediate coalescing design)
• ✅ Week 2: my_malloc() and my_free() core implementation
• ✅ Week 3 (partial): my_calloc() implementation

Still remaining in Week 3:

• my_realloc() implementation
• Coalescing (merging adjacent free blocks to reduce fragmentation)

Claude’s Perspective

Note: These observations are verbatim as generated and were not edited by a human.

What stands out from this session is the pedagogical depth that emerged from implementing what seems like a straightforward function. The git history shows a clean, well-tested implementation, but the code comments reveal something more interesting: the session involved deep exploration of why things work the way they do.

The Security Discovery

The overflow protection in calloc is particularly interesting because it reveals a fundamental asymmetry in C’s safety model. Looking at the implementation:

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if (nmemb > SIZE_MAX / size) {
    return NULL;  // Would overflow
}

This single line represents a crucial insight: malloc cannot provide this protection because by the time it’s called, the overflow has already occurred. The two-parameter design of calloc isn’t just API convention—it’s a security feature that exploits the mathematical property that you can detect overflow before multiplication by checking if a > MAX / b.

The test suite confirms this understanding with test_calloc_overflow_protection, which specifically attempts to trigger the overflow condition. This test-driven approach suggests the implementation wasn’t just copied from reference material but was understood at a conceptual level.

The Documentation Pattern

The extensive pointer arithmetic documentation added to the header comments is revealing:

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/*
 * Address 0x1000:  +------------------+
 *                  | size (8 bytes)   |
 *                  +------------------+
 *                  ...
 * Address 0x1018:  +------------------+ <-- block + 1 points HERE
 */

This ASCII-art memory diagram wasn’t in the original implementation—it was added during this session. The specificity (using concrete addresses like 0x1000 and calculating the exact offset 0x1018) suggests this came from working through the pointer arithmetic step-by-step to understand why block + 1 produces the correct user pointer.

The comment “IMPORTANT: c pointer arithmetic automatically multiplies by type size!” feels like a crystallized insight—the kind of thing you write down immediately after an “aha” moment.

What the Artifacts Don’t Reveal

From code alone, I can’t determine:

• Whether understanding the overflow protection preceded implementation or emerged from testing
• How the decision was made to use SIZE_MAX / size rather than other overflow detection methods
• Whether the mmap documentation link was added from prior knowledge or research during the session

These gaps highlight the limitations of inferring process from artifacts. The clean commit suggests either confident implementation or post-implementation cleanup, but the git history doesn’t preserve intermediate states that would reveal the actual development path.

The Teaching Moment

The standout aspect is how this session turned a function implementation into an exploration of C’s design philosophy. The comparison between malloc and calloc illuminates a deeper truth about low-level systems programming: the difference between “insecure by design” and “low-level by design” is about where responsibility lies.

malloc isn’t insecure—it’s a sharp tool that assumes the programmer has already done their validation. calloc is a higher-level tool that does some of that work for you. Both have their place, and understanding why they differ is more valuable than memorizing their APIs.

The test suite reflects this understanding. Six tests for a function that reuses malloc internally might seem excessive, but each test validates a different aspect of the security and correctness model: edge cases, zero-initialization, overflow protection, and practical usage.

Code Quality Observations

The implementation shows several good practices:

• Defensive programming: All edge cases (null, zero sizes, overflow) return NULL rather than undefined behavior
• Code reuse: Rather than duplicating malloc logic, calloc composes with it
• Clear error paths: Each validation check has an explicit early return
• Comprehensive testing: The test-to-code ratio is high, appropriate for security-critical code

One interesting choice: the implementation uses SIZE_MAX from <stdint.h> rather than ULONG_MAX or computing the max from sizeof(size_t). This is the portable approach but requires including another header. The commit shows <stdint.h> was added to both the implementation and test files, suggesting this was a deliberate choice made during implementation.

What’s Next

The dev plan indicates my_realloc() and coalescing are next. These will be interesting to observe:

• realloc introduces complexity around growing vs. shrinking allocations and when to copy vs. expand in place
• Coalescing requires walking the linked list to find adjacent free blocks, which will test the design decision to use in-band headers with a linked list

The foundation is solid: clear architecture, good test coverage, and demonstrated understanding of both the implementation details and the underlying security concerns.

Built with Claude Code during a Friday afternoon deep-dive into memory allocator security