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Valgrind-clean solutions for pointer, memory-allocator, and systems-programming assignments, with an ownership comment on every malloc and free. The single biggest deduction in a memory-debugging task is a missing NULL check after malloc that segfaults the moment the grader forces allocation to fail, the exact failure mode our tutors patch with a defensive check and document in the function header. Verified CS graduates with kernel-level depth, from $20 per task, 12-hour average turnaround.

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Why C

C at the university level

Valgrind-clean solutions for pointer, memory-allocator, and systems-programming assignments, with an ownership comment on every malloc and free. The single biggest deduction in a memory-debugging task is a missing NULL check after malloc that segfaults the moment the grader forces allocation to fail, the exact failure mode our tutors patch with a defensive check and document in the function header. Verified CS graduates with kernel-level depth, from $20 per task, 12-hour average turnaround.

Topics covered

What we tutor in C

Pointers & Memory Management

Stack vs heap allocation, malloc and free pairing, dangling pointer prevention, and Valgrind workflows for leak detection.

Data Structures (from scratch)

Linked list, BST, hash table, and heap implementations in pure C with explicit memory hygiene and adversarial test coverage.

Systems Programming (Unix/Linux)

fork, exec, wait, pipe, signal handlers, and the kernel-interface patterns systems courses grade against POSIX conformance.

File I/O & System Calls

Implementation patterns, named pitfalls, and the autograder cases that catch them in C coursework.

Embedded C STM32 & ARM Cortex-M

Implementation patterns, named pitfalls, and the autograder cases that catch them in C coursework.

Network Programming (Sockets)

Implementation patterns, named pitfalls, and the autograder cases that catch them in C coursework.

Related

Pair C with

Operating SystemsPointersConcurrency

Full overview

C in CS curricula

C hands you the machine with no safety net: no classes, no garbage collection, no built-in strings, no exceptions, and no standard containers. A systems programming course leans on raw pointers, manual malloc and free, and the stack-versus-heap distinction while students learn where every byte lives. A data structures course grades linked lists, hash tables, and binary search trees implemented from scratch with pointer surgery, where the grading rubric runs each test under a memory-leak check.

A memory-debugging task hands you a leaking, crashing program and asks you to find the use-after-free, the double-free, and the buffer overflow with Valgrind and AddressSanitizer. A concurrency project moves to pthreads, mutexes, and condition variables, where the course autograder runs each test under a thread sanitizer to surface intermittent races. A socket-programming assignment builds TCP and UDP clients and servers with select, poll, or epoll for concurrent I/O and proper errno handling on every system call.

An operating systems kernel project moves to fork, exec, pipe, dup2, mmap, and page-table walks inside a teaching kernel graded against held-out test inputs. An embedded firmware assignment targets bare-metal ARM Cortex-M or AVR hardware with memory-mapped registers, interrupt handlers, and I2C, SPI, or UART drivers. Our C tutors deliver code compiled with -Wall -Wextra -Wpedantic -fsanitize=address,undefined, every warning addressed, defensive NULL checks on every allocation, and Valgrind output showing zero leaks and zero errors.

The assessment split runs roughly 70-30 between implementation projects graded against held-out test cases and written components that test pointer reasoning and complexity analysis. Both halves reward one skill: tracking the lifetime of every allocation one level above the code. The CSHH bench for C pairs verified CS graduates with kernel-level depth (page tables, system-call dispatch, lock-free patterns) and architecture specialists who read the x86-64 calling convention and the generated assembly when a bug hides below the source.

Where Students Get Stuck

Six named C failure modes

Missing NULL check after malloc

The autograder runs under ulimit -v to force allocation failure. Programs that skip the NULL check segfault. We add if (p == NULL) return -1 (or equivalent) after every allocation and document the error path in the function header.

Off-by-one in null terminator

Allocating strlen(s) bytes instead of strlen(s) + 1 produces a string that prints correctly under most inputs and crashes on the boundary. We use snprintf with explicit buffer sizes or add an assert(buf_size > strlen(src)) check.

Use-after-free in linked list traversal

Freeing a node then dereferencing node->next is the classic bug. The fix: save node->next into a local before calling free(node). AddressSanitizer pinpoints the dereference instantly.

Double free

Calling free twice on the same pointer corrupts malloc metadata silently. We set the pointer to NULL after free, so the second free becomes a safe no-op (free(NULL) is defined).

Undefined behavior the optimizer exploited

Signed integer overflow, strict-aliasing violations, and uninitialized stack variables compile cleanly at -O0 and break at -O2. We compile with -fsanitize=undefined to surface the UB at runtime with file and line numbers.

Race conditions in pthreads

Unsynchronized access between threads passes single-threaded tests. The autograder uses -fsanitize=thread to catch the race. We identify the shared resource, add a pthread_mutex around the critical section, and verify with Helgrind.

Debugging C code step by step with breakpoints, variable inspection, and step controls

How we work

Our C approach

Clean, portable C that compiles without a single warning under -Wall -Wextra -Wpedantic, AddressSanitizer, and UndefinedBehaviorSanitizer. Step 1: read the grading rubric twice and identify the compiler flags and the C standard the autograder uses (-std=c11 by default, -std=c17 for the strictest compatibility). Step 2: write the function signatures with documented ownership before any logic, naming who allocates, who frees, and who only borrows each pointer.

Step 3: implement with a defensive check at every boundary, a NULL check after every malloc, an errno inspection after every system call, and the pointer set to NULL after every free so a stray second free becomes a safe no-op. Step 4: write test cases covering NULL inputs, empty inputs, capacity-1, and adversarial sequences that trigger worst-case behavior. Step 5: run Valgrind Memcheck and AddressSanitizer locally, confirm zero leaks and zero errors, and attach the clean output to the delivery.

Any pointer-heavy code above 100 lines includes an ownership diagram of the allocation lifetimes.

What you receive

Autograder and artifact bundle

Every C delivery ships with the .c and .h source files in the directory layout your course expects, a Makefile matching the grading-harness format (or a CMakeLists.txt for larger cross-platform builds), a SOLUTION.md with the design rationale and complexity analysis per function, and a CHECKLIST.md mapping each rubric item to where the code satisfies it. The bundle adds a Valgrind output file (memcheck --leak-check=full --track-origins=yes), an AddressSanitizer log captured under -O2, and an ownership diagram (ASCII source plus rendered PNG) for any pointer-heavy code above 100 lines, plus a 5-bullet oral-defense brief covering the 3 questions a grader is most likely to ask about your memory model.

Assignment Types

C assignment types we cover

Pointer and dynamic memory tasks

Pointer arithmetic, malloc, calloc, realloc, and free discipline, the stack-versus-heap distinction, and manual ownership tracking, delivered Valgrind-clean with an ownership comment on every allocation. Named pitfall: a use-after-free where a freed node is dereferenced on the next loop iteration, pinpointed to the exact line with AddressSanitizer.

Data structures from scratch

Linked lists, hash tables with chaining and open addressing, binary search trees, heaps, and graph adjacency lists built without any library containers, with Big-O annotated per operation. Named pitfall: a double-free that corrupts heap metadata silently, fixed by setting the pointer to NULL after free so the second free is a safe no-op.

Systems programming (Unix/Linux)

A Unix shell with pipe chains, I/O redirection via dup2, background processes, signal handling for SIGINT and SIGTSTP, and job control using fork, exec, wait, and setpgid, with errno checked after every system call. Named pitfall: a forgotten waitpid that leaves zombie processes, surfaced under strace and reaped with a SIGCHLD handler.

Socket and network programming

TCP and UDP clients and servers, connection state machines, byte-order handling with htonl and ntohl, and select, poll, or epoll for concurrent I/O, with proper EAGAIN, EINTR, and EWOULDBLOCK handling. Named pitfall: assuming a single read returns a whole message, fixed with a length-prefixed framing loop that reads until the full payload arrives.

Concurrency with pthreads

pthread_create, pthread_join, mutexes, condition variables, and a producer-consumer queue, delivered with a documented lock-acquisition order on every shared resource. Named pitfall: a data race that passes single-threaded tests and corrupts shared state under the grader, caught with -fsanitize=thread and confirmed clean under Helgrind.

Operating systems kernel projects

Inside a teaching operating system kernel: process and thread scheduling, system-call dispatch, virtual memory with page tables, copy-on-write fork, and a file system with inodes, graded against held-out test inputs. Named pitfall: leaking page-table entries on process teardown, which panics the kernel after many fork calls and is fixed by freeing every mapped page in the cleanup path.

Embedded and bare-metal firmware

Bare-metal C on ARM Cortex-M and AVR targets with memory-mapped registers, interrupt service routines, non-blocking state machines, and I2C, SPI, or UART drivers, cross-compiled for the board. Named pitfall: a blocking delay loop inside the main loop that starves interrupt servicing, refactored to a timer-driven state machine that keeps the firmware responsive.

Advanced Topics

Graduate-level C we cover

Learning path showing progression from C fundamentals through data structures to advanced topics

Memory Allocator Design

A custom malloc and free built on sbrk() or mmap(), with free-list management, block splitting, coalescing, boundary tags for O(1) coalescing, and buddy-system strategies. We deliver 3 allocator designs: an implicit free list with first-fit (the correctness baseline), an explicit free list with LIFO insertion (the speed win), and segregated free lists with size classes plus best-fit within each class (the space-efficiency winner). Benchmarked against libc malloc on a trace-driven workload. Common bug: forgetting to align the payload to 16 bytes on x86-64 crashes callers that use SSE instructions.

Network Protocol Implementation

A protocol built from a written spec: byte ordering with htonl and ntohl, length-prefixed serialization, a state machine for the handshake and teardown, timeout handling with select, and TCP-like reliability over UDP through sequence numbers, acknowledgements, and retransmission. We deliver sliding-window flow control and a congestion-avoidance backoff. Common bug: treating one recv as one logical message, fixed by reading until the declared payload length arrives.

Linux Kernel Module Development

The kernel-mode versus user-mode distinction (ring 0 vs ring 3 on x86-64, supervisor vs user on ARM), module init and cleanup with the module_init and module_exit macros, /proc entries via proc_create, ioctl handlers with copy_from_user and copy_to_user (never dereference a userspace pointer directly), spinlocks (spin_lock_irqsave for IRQ-safe sections) and mutexes (mutex_lock for sleepable contexts), and RCU (Read-Copy-Update) for lock-free read-heavy structures. We deliver a character-device driver matching the kernel-Makefile build pattern used by Linux 6.x, with insmod plus rmmod tested in a QEMU sandbox to avoid bricking the host. Common bug: returning -EINVAL where -EFAULT is correct hides bad-pointer bugs from the kernel logs.

C Interview Prep and System Design Patterns

C interview prep covers 4 recurring patterns: a thread-safe singleton with pthread_once, a memory pool for a fixed-size object lifecycle (eliminates fragmentation in long-running daemons), a producer-consumer queue with pthread_mutex plus pthread_cond, and bit manipulation for hot paths (population count via __builtin_popcountll, leading zeros via __builtin_clzll). For system design we deliver a write-through cache with LRU eviction (doubly-linked list plus hash table for O(1) access), an event loop with epoll for 10,000 concurrent connections, and a length-prefixed protocol with htonl plus htons byte-order handling. Common interview bug: confusing the return values of read (returns 0 at EOF, -1 on error) and recv (returns 0 on orderly shutdown, -1 on error).

Sample Output

Idiomatic C we ship

C sample 
/* Generic linked list in C with defensive checks */
#include <stdlib.h>

typedef struct Node {
    void *data;
    struct Node *next;
} Node;

Node* list_prepend(Node *head, void *data) {
    Node *new_node = malloc(sizeof(Node));
    if (new_node == NULL) return head;
    new_node->data = data;
    new_node->next = head;
    return new_node;
}

void list_free(Node *head) {
    while (head != NULL) {
        Node *tmp = head;
        head = head->next;
        free(tmp);
    }
}

Diagnostic Walkthrough

One named C fix, before and after

Before: strncpy off-by-one and missing NULL C 
#include <string.h>
#include <stdlib.h>

char* copy_label(const char *src) {
    /* Bug 1: strlen(src) bytes leaves no room for '\0' */
    char *dst = malloc(strlen(src));
    /* Bug 2: no NULL check after malloc */
    strncpy(dst, src, strlen(src));
    /* Bug 3: strncpy does NOT NUL-terminate when len == strlen */
    return dst;  /* caller reads past the buffer */
}
After: defensive C with explicit NUL C 
#include <string.h>
#include <stdlib.h>

char* copy_label(const char *src) {
    if (src == NULL) return NULL;
    size_t len = strlen(src);
    /* +1 for the NUL terminator */
    char *dst = malloc(len + 1);
    if (dst == NULL) return NULL;
    memcpy(dst, src, len);
    dst[len] = '\0';  /* explicit NUL terminator */
    return dst;
}
The strncpy off-by-one combined with a missing NULL check is the classic 3-bug stack: under-allocation by one byte, no malloc failure handling, and strncpy that silently skips NUL termination when the count equals strlen(src). The fix allocates len+1, checks the malloc return, and uses memcpy plus an explicit NUL assignment. AddressSanitizer flags the read-past-buffer instantly with a 6-line trace.
IDE workspace showing C code with file tree, syntax highlighting, and minimap

Tools & Environment

Tools we use for C

GCCClangGDBValgrindAddressSanitizerstraceltraceperfbpftraceclang-static-analyzerCoverityFrama-CMakeCMakeVim/EmacsGitWireshark

Sample Projects

Recent C deliveries

Unix Shell Implementation

Pipe chains across N commands, I/O redirection with dup2, background processes with fork plus setpgid, signal handling for SIGINT and SIGTSTP, built-in commands, and job control, with every system call checked against errno.

Memory Allocator (malloc/free)

An explicit free list with coalescing, a best-fit allocation strategy, boundary tags for O(1) coalescing, 16-byte payload alignment, and throughput plus space-efficiency benchmarks measured against glibc malloc.

HTTP/1.1 Web Server

Multi-threaded with pthreads and a fixed-size thread pool, GET and POST handling, static file serving with sendfile, MIME type detection, persistent connections, and correct error responses, verified Helgrind-clean under a 1000-connection stress test.

File System Implementation

A virtual disk with inodes, direct and indirect blocks, a free-block bitmap, a buffer cache, and CRUD plus mkdir supporting hierarchical directories, with the on-disk layout documented in the SOLUTION.md.

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FAQ

C homework help, frequently asked

Can you help with operating systems kernel assignments?
Yes. Process and thread scheduling, virtual memory with page tables and the TLB, file systems with inodes, synchronization primitives, and copy-on-write fork inside a teaching operating system kernel. We deliver against the held-out test inputs your grader uses and document every page allocation freed on teardown.
Do you debug memory leaks?
Valgrind --tool=memcheck --leak-check=full --track-origins=yes plus AddressSanitizer to trace every allocation. We provide annotated reports and teach the discipline so the next leak is faster to find.
Can you help with embedded C?
Bare-metal programming with memory-mapped registers, interrupt handlers, RTOS support (FreeRTOS, Zephyr), I2C, SPI, UART drivers, and cross-compilation for ARM Cortex-M and AVR targets.
Do you help with socket programming?
TCP and UDP clients and servers, connection management, protocol state machines, and select, poll, or epoll for concurrent I/O. Includes proper EAGAIN, EINTR, and EWOULDBLOCK handling.
Can you implement data structures in C?
Linked lists, hash tables with chaining and open addressing, BSTs, heaps, and graphs with clean APIs, defensive NULL checks, proper error handling, and zero Valgrind errors.
Do you help with Makefiles?
Proper dependency tracking with gcc -MMD, separate compilation, debug and release targets, and pattern rules for portability. Also CMake for larger cross-platform projects.
Can you help build a Unix shell?
Pipe chains, I/O redirection with dup2, signal handling for SIGINT and SIGTSTP, built-in commands, and job control using fork, exec, wait, pipe, and setpgid.
Do you support cross-platform C?
Portable code for Linux, macOS, and Windows with conditional compilation (#ifdef) and abstraction layers for filesystem and threading primitives.
Can you help with a teaching OS kernel lab in C?
A teaching operating system kernel (RISC-V or x86, based on a minimal Unix) is the standard vehicle for kernel-level work. We deliver the recurring labs: extra system calls (a trace facility, a memory-info call), a page-table walker with super-page support, traps with a backtrace and a periodic alarm signal, copy-on-write fork with lazy page allocation and reference-counted physical pages, user-level threads with a context switch and a barrier, and mmap with file-backed on-demand paging. Common bug: forgetting to free the page-table entries on process teardown panics the kernel after many fork calls.
Do you support a teaching OS project with VM and a file system?
Yes. A 32-bit teaching operating system project splits into 4 parts and we deliver each: threads (an alarm clock with priority donation and a 4.4BSD multilevel-feedback scheduler), user programs (argument passing on the stack, system calls backed by a file-descriptor table, and denying writes to a running executable), virtual memory (a frame table with eviction, a supplemental page table, stack growth, and mmap), and the file system (a buffer cache and indexed files with direct, indirect, and doubly-indirect blocks plus subdirectories). Common bug: holding one global lock across the entire system-call dispatch serializes the kernel and fails the out-of-memory stress tests.
Can you help with the malloc lab free-list allocator?
A memory-allocator project implements malloc and free against trace-driven tests scored on space efficiency and throughput. We deliver 3 allocator designs: an implicit free list with first-fit (the correctness baseline), an explicit free list with LIFO insertion (the speed win), and segregated free lists with 16 size classes plus best-fit within each class (the efficiency winner). Boundary tags give O(1) coalescing on free, and the realloc fast path skips the copy when the next block is free. Common bug: forgetting to align the payload to 16 bytes on x86-64 crashes callers that use SSE instructions.
Do you help with Arduino C programming assignments?
Arduino C with the Wiring framework (setup, loop, pinMode, digitalWrite, analogRead, analogWrite for PWM), interrupts via attachInterrupt with FALLING or RISING modes (only on pins 2 and 3 for the Uno), serial communication at 9600 or 115200 baud, and library integration (Servo.h, LiquidCrystal.h, Wire.h for I2C, SPI.h). For sensor-heavy projects we deliver non-blocking state machines (millis() instead of delay()), debouncing for mechanical switches, and PWM motor control with H-bridge drivers (L298N). Common bug: using delay() inside loop() blocks interrupt servicing and breaks responsive UIs.
Can you help with FreeRTOS task scheduling assignments?
FreeRTOS task scheduling with xTaskCreate (3 parameters: priority 0 to configMAX_PRIORITIES-1, stack depth in words, parameter pointer), preemptive priority-based scheduler with optional time-slicing among equal-priority tasks, and the 4 standard sync primitives (xSemaphoreCreateBinary for signaling, xSemaphoreCreateMutex with priority inheritance, xQueueCreate for typed message passing, xEventGroupCreate for bit-flag synchronization). We deliver the priority-inversion-safe mutex pattern (xSemaphoreCreateMutex, not Binary), the queue-set pattern for waiting on multiple queues, and tickless idle for low-power microcontroller targets. Common bug: vTaskDelay vs vTaskDelayUntil confusion produces drift in periodic tasks.
Do you cover C99 vs C11 vs C23 standard selection?
C99 added designated initializers, compound literals, variable-length arrays (VLAs, optional in C11), and inline functions. C11 added _Thread_local (TLS) with threads.h, _Atomic with stdatomic.h, _Generic for type-generic macros, and aligned_alloc. C23 (ratified 2024) adds typeof, nullptr keyword, constexpr (limited form), enum with underlying type, and digit separators (1_000_000). We default to -std=c11 -pedantic for portability across GCC 7+ and Clang 6+, switch to -std=c17 (bug-fix release of C11) for the strictest compatibility, and only adopt -std=c2x when the course explicitly requires it. Common bug: VLAs are optional in C11 and unsupported on MSVC; we replace with malloc plus free for portability.

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