Fork() in the road paper [LWN.net]

Fork() in the road paper

Posted Jun 5, 2026 22:57 UTC (Fri) by malmedal (subscriber, #56172)
In reply to: Fork() in the road paper by gutschke
Parent article: Moving beyond fork() + exec()

If you want hacky. Why not just CLONE_VM without CLONE_VFORK?

It's somewhat annoying because you have to allocate a stack in the parent process and you can't reuse that area until you are sure the child has called exec but not insurmountable.

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Fork() in the road paper

Posted Jun 5, 2026 23:11 UTC (Fri) by gutschke (subscriber, #27910) [Link] (1 responses)

It's been ages since I last experimented with CLONE_VM, so I probably don't remember all the details. But I think it had really awkward calling conventions that made it pretty much impossible to use from C code.

If my cover ever was productized in some form, adding yet another assembly wrapper is obviously doable. But I intentionally tried to keep things as high level and portable as it's possible with this sort of low level code.

And yes, it's ugly, and very far from portable without at least some effort. That's the nature of these APIs

Fork() in the road paper

Posted Jun 5, 2026 23:35 UTC (Fri) by malmedal (subscriber, #56172) [Link]

I was curious as to what it would look like, so I coded it up. It's not too bad, unless I'm missing something.

The stack for each process can be reused as soon at it has called exec. Not sure how to best detect that. Options include things like FD_CLOEXEC or when /proc/<pid>/exe of the child has changed.

#include <fcntl.h>
#include <linux/sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <unistd.h>

int clone(int (*fn)(void *), void *stack, int flags, void *arg, ...
/* pid_t *parent_tid, void *tls, pid_t *child_tid */);

int target(void *arg) {
char fname[256];
int i = *(int *)arg;
snprintf(fname, sizeof(fname), "file%d.txt", i);
int fd = open(fname, O_CREAT | O_RDWR, 0777);
dup2(fd, 1);
dup2(fd, 2);
execl("/usr/bin/bash", "bash", "-c", "sleep 100 ; date", NULL);
return -1;
}

double dtime() {
struct timeval tv;
gettimeofday(&tv, NULL);
return (double)tv.tv_sec + tv.tv_usec / 1000000.0L;
}

int main(int argc, char **argv, char **envp) {
const int STACK_SIZE = 65536;
const int STACKS = 200;
int pids[STACKS];
int args[STACKS];
char *stack = malloc(STACK_SIZE * STACKS);
const long SIZE = 20 * 1024L * 1024 * 1024;
char *buffer = malloc(SIZE);
memset(buffer, 1, SIZE);
double start = dtime();
for (int i = 0; i < STACKS; i++) {
args[i] = i + 1;
pids[i] = clone(target, stack + STACK_SIZE * (i % STACKS + 1), CLONE_VM,
&args[i]);
}
fprintf(stderr, "Avg %d %f\n", 0, (dtime() - start) / 200.0);
for (int i = 0; i < STACKS; i++) {
int status;
int ret = waitpid(pids[i], &status, 0);
if (status != 0 || ret < 0) {
printf("status %d %d %d %d\n", i, pids[i], status, ret);
}
}
printf("done\n");
}