💥 LET’S UNLEASH A CLASSIC UNIX TRAP

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🧠 06_malloc_after_fork_glitch

💣 “What happens when a multithreaded process forks… then uses malloc() in the child?”
✅ Short answer: 🧨 Undefined behavior. Heap corruption. Crash potential. Memory allocator meltdown.
✅ Long answer: You’re about to build a glitch simulator that weaponizes fork() vs malloc().

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❗ Background: The Fork + Malloc Glitch

🔁 Context:

• In POSIX, fork() in a multithreaded process only duplicates the calling thread

• The other threads vanish 🚫

• But locks they held might remain locked

• Now child calls malloc() → tries to grab mutex → 💀 deadlock or corruption

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🔒 Danger:

The malloc() implementation (e.g. glibc ptmalloc) uses internal global locks

So after fork(), if a thread had locked one → it remains locked in the child
→ But that thread no longer exists to unlock it
→ malloc() deadlocks or segfaults in child.

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📄 06_malloc_after_fork_glitch.c

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
 
void	*thread_fn(void *arg)
{
	while (1)
	{
		void *p = malloc(1024);
		usleep(1000);
		free(p);
	}
	return (NULL);
}
 
int	main(void)
{
	pthread_t t;
	void *child_mem;
 
	printf("🚀 Launching malloc thread...\n");
	if (pthread_create(&t, NULL, thread_fn, NULL) != 0)
	{
		perror("pthread_create");
		exit(EXIT_FAILURE);
	}
 
	usleep(10000); // Let thread run a bit
 
	printf("🔪 Forking now...\n");
	pid_t pid = fork();
 
	if (pid == -1)
	{
		perror("fork");
		exit(EXIT_FAILURE);
	}
	else if (pid == 0)
	{
		printf("🧒 Child: trying to malloc...\n");
		child_mem = malloc(128); // 🔥 This may deadlock or crash
		if (!child_mem)
			perror("child malloc");
		else
			printf("✅ Child malloc succeeded (unexpected?)\n");
		exit(0);
	}
	else
	{
		wait(NULL);
		printf("👨 Parent: done.\n");
	}
	return (0);
}

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🔬 Behavior You Might See

Platform	Behavior
🔥 Linux glibc	Random crash or freeze in child malloc()
🍎 macOS	May work because of different allocator
🧪 musl	More consistent — often avoids deadlock
🧠 valgrind	Will scream in agony about heap corruption
😈 TSAN/ASAN	Will flag data race / malloc lock misuse

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🧠 What You Just Simulated:

Effect	Description
🔒 Fork-time lock state	Child process inherited a locked heap mutex
🚷 Thread inconsistency	Dead threads held state; child can’t recover
💥 Kernel obeys POSIX, but malloc doesn’t	POSIX says “only fork if safe” — you just showed why

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💣 Want to Turn This Into a Diagnostic Infrastructure?

• Add pthread_atfork() to simulate fixing it

• Log getpid() before and after each allocation

• Create a cleanup hook to compare heap consistency

• Wrap malloc() to log FD table before/after

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🧱 Bonus Vault Upgrade Options

Project	Mutation
06b_fork_after_malloc_inside_signal.c	Trigger fork from inside SIGALRM handler and malloc in child
06c_thread_pool_fork_test.c	Fork inside a thread pool running live jobs
06d_malloc_wrapped_logger.c	Wrap malloc with logging and getpid() comparison to detect fork glitch

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✅ Ready to go next?

Say:

• "wrap this into full diagnostic infrastructure"

• "drop 06b now"

• or "bring the next glitch: [[07_shared_mmap_allocator/]]"

You’re now hunting what breaks after the program is “correct” —
Welcome to the asymmetric glitch dimension.
🧠💥🧬🧨