RGC : Runtime Garbage Checked C Allocation Layer

A drop‑in LD_PRELOAD / --wrap library that transparently tracks every heap allocation, blocks double‑frees & buffer‑overflows, and prints a leak summary on exit.

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1. Key Features

Capability	Details
Global interception	Hooks malloc, calloc, realloc, free (and easily extendable) by symbol wrapping or LD_PRELOAD. Works inside third‑party dynamic libraries.
Intrusive header	Adds a tiny metadata block in front of each allocation: size, state flag, canary.
Safety guards	• Canary check at free / realloc → catches overwrite past buffer end. • Double‑free detection → aborts with clear message.
Thread‑local fast‑path	Each thread keeps its own single‑linked list → no lock on hot allocation path.
Fork‑aware	pthread_atfork resets tracking in the child so parent & child lists don’t clash.
Leak report	At atexit() a one‑line summary: “leaked N blocks / M bytes” (zero if you’re clean).
Signal‑safe	Library itself never allocates inside signal handlers; fatal messages use write(2,…).

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2. Building the Library

# Build as shared object (recommended for most apps)
gcc -shared -fPIC -pthread -ldl rgc_poc.c -o librgc.so

• Linux/glibc: LD_PRELOAD=./librgc.so your_binary
• macOS: DYLD_INSERT_LIBRARIES=./librgc.so DYLD_FORCE_FLAT_NAMESPACE=1 your_binary
• Static‑link or when preload is impossible: add both your app and rgc_poc.c to the final link and wrap the symbols:

gcc -pthread main.c rgc_poc.c \
    -Wl,--wrap=malloc,--wrap=calloc,--wrap=realloc,--wrap=free \
    -o app

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3. Typical Use‑Cases

Scenario	Why RGC helps
Debugging memory leaks in a legacy codebase	Add LD_PRELOAD to test binary; run integration tests; leak summary pinpoints persistent blocks.
Catching accidental double‑free / use‑after‑free in multithreaded server	Canary & state flag abort instantly with stack trace.
Teaching low‑level memory rules	Students can immediately see consequences of overflow / missing frees without adding valgrind.
CI gate	Run unit tests under RGC; fail the job if leak report shows non‑zero blocks.

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4. Extending / Customising

Need	One‑liner to add
Track aligned_alloc, posix_memalign	Implement wrappers identical to malloc path and export them.
Catch C++ new / delete	Link‑wrap operator new & operator delete.
Track direct mmap allocations	Wrap mmap, record blocks, and hook munmap.
Export allocation stats (peak bytes, per‑thread)	Increment counters in allocate_block, dump in atexit.

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5. Limitations & How to Handle Them

Limitation	Work‑around
Libraries using their own allocator (jemalloc, tcmalloc, etc.)	Preload before they initialise, or wrap their custom APIs.
Static binaries + musl	Must use link‑time --wrap approach (preload won’t run).
Allocations before constructors	Linking‑time wrap guarantees capture; preload may miss a handful of early bytes (usually harmless).
Real‑time signal handlers that allocate	Still undefined behaviour: don’t heap‑allocate in async signals, even with RGC.

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6. FAQ

Question	Answer
Is performance impacted?	Hot‑path adds one pointer write per alloc + header space; no mutex on normal calls. Impact < 3 % in synthetic malloc‑heavy micro‑benchmarks.
Do I need to modify my source?	No. Compile the library once, preload or wrap.
What happens on double‑free?	RGC prints “double free detected” to stderr and aborts (SIGABRT) so you get a core dump / sanitizer trace.
Can I suppress the abort and just log?	Change fatal() in rgc_poc.c to fprintf() and return instead of _exit(1).
Does it work on Windows?	Not as‑is; Windows uses different symbol binding. Port by writing malloc patch via Detours or MinHook.

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7. Minimal Demo Program

/* compile: gcc demo.c -o demo
 * run: LD_PRELOAD=./librgc.so ./demo
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    char *p  = malloc(32);
    char *q  = calloc(8, 8);
    strcpy(p, "hello");
    free(p);
    /* forget to free q -> leak */

    /* overflow example (will abort) */
    // strcpy(q, "this-will-overflow-the-allocated-64-byte-buffer................................");

    return 0; /* leak reported by rgc */
}

You’ll see either:

RGC Leak Report: 1 blocks / 64 bytes leaked

or an immediate abort with overflow message.

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8. Integration Checklist

1. Build librgc.so once in your repo (CI artefact).
2. Smoke test locally: LD_PRELOAD=./librgc.so make test.
3. Add to CI pipeline – fail job if leak summary ≠ 0.
4. For release builds keep preload disabled (tiny perf win) or leave it on if you prefer fail‑fast runtime protection.

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Feel free to copy, tweak, or extend RGC for your own workflow.