From d28c5ba100cbc8a4ea6e4f4da16f5c8319a02a72 Mon Sep 17 00:00:00 2001
From: Freya Murphy <freya@freyacat.org>
Date: Mon, 28 Apr 2025 17:28:02 -0400
Subject: fork syscall

---
 kernel/include/comus/user.h |  5 +++++
 kernel/syscall.c            | 18 ++++++++++++++++--
 kernel/user.c               | 31 +++++++++++++++++++++++++++++++
 user/fork.c                 | 20 ++++++++++++++++++++
 4 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 user/fork.c

diff --git a/kernel/include/comus/user.h b/kernel/include/comus/user.h
index 29c978c..f51ada5 100644
--- a/kernel/include/comus/user.h
+++ b/kernel/include/comus/user.h
@@ -17,6 +17,11 @@
  */
 int user_load(struct pcb *pcb, struct disk *disk);
 
+/**
+ * Clone a user process. Used for fork().
+ */
+struct pcb *user_clone(struct pcb *pcb);
+
 /**
  * Clean up all loaded userland data from a pcb
  */
diff --git a/kernel/syscall.c b/kernel/syscall.c
index dd9bc16..2dd6460 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -1,4 +1,4 @@
-#include "lib/kio.h"
+#include <comus/user.h>
 #include <comus/cpu.h>
 #include <comus/syscalls.h>
 #include <comus/drivers/acpi.h>
@@ -44,6 +44,20 @@ static int sys_waitpid(void)
 	dispatch();
 }
 
+static int sys_fork(void)
+{
+	struct pcb *child;
+
+	child = user_clone(pcb);
+	if (child == NULL)
+		return -1;
+
+	child->regs.rax = 0;
+	pcb->regs.rax = child->pid;
+	schedule(child);
+	return 0;
+}
+
 static int sys_write(void)
 {
 	ARG1(int, fd);
@@ -313,7 +327,7 @@ static int sys_ticks(void)
 
 static int (*syscall_tbl[N_SYSCALLS])(void) = {
 	[SYS_exit] = sys_exit,		 [SYS_waitpid] = sys_waitpid,
-	[SYS_fork] = NULL,			 [SYS_exec] = NULL,
+	[SYS_fork] = sys_fork,			 [SYS_exec] = NULL,
 	[SYS_open] = NULL,			 [SYS_close] = NULL,
 	[SYS_read] = NULL,			 [SYS_write] = sys_write,
 	[SYS_getpid] = sys_getpid,	 [SYS_getppid] = sys_getppid,
diff --git a/kernel/user.c b/kernel/user.c
index fc1167d..f9b541c 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -230,6 +230,37 @@ fail:
 	return 1;
 }
 
+struct pcb *user_clone(struct pcb *pcb)
+{
+	struct pcb *child;
+
+	if (pcb_alloc(&child))
+		return NULL;
+
+	// copy context
+	memcpy(&child->regs, &pcb->regs, sizeof(struct cpu_regs));
+	child->memctx = mem_ctx_clone(pcb->memctx, true);
+	if (child->memctx == NULL)
+		return NULL;
+
+	// set metadata
+	child->parent = pcb;
+	child->state = PROC_STATE_READY;
+	child->priority = pcb->priority;
+	child->ticks = 0;
+
+	// copy heap
+	child->heap_start = pcb->heap_start;
+	child->heap_len = pcb->heap_len;
+
+	// copy elf data
+	memcpy(&child->elf_header, &pcb->elf_header, sizeof(Elf64_Ehdr));
+	memcpy(&child->elf_segments, &pcb->elf_segments, sizeof(Elf64_Ehdr));
+	child->n_elf_segments = pcb->n_elf_segments;
+
+	return child;
+}
+
 void user_cleanup(struct pcb *pcb)
 {
 	if (pcb == NULL)
diff --git a/user/fork.c b/user/fork.c
new file mode 100644
index 0000000..d534fcf
--- /dev/null
+++ b/user/fork.c
@@ -0,0 +1,20 @@
+#include <unistd.h>
+#include <stdio.h>
+
+int main(void)
+{
+	printf("im going to print some a's and b's. get ready\n");
+
+	int pid = fork();
+	if (pid < 0) {
+		fprintf(stderr, "fork failed!\n");
+		return 1;
+	}
+
+	for (int i = 0; i < 10; i++) {
+		putchar(pid == 0 ? 'a' : 'b');
+		putchar('\n');
+	}
+
+	return 0;
+}
-- 
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