- The document discusses the process of forking and creating new processes in an operating system. It describes the key steps like allocating memory for the child process, copying resources from the parent, and starting the new process. - Code examples are provided to demonstrate how fork is implemented at the system call level and how it is used in C programs to create new threads. - The document also explains the data structures and functions involved in process switching and context switching between threads.

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#include <stdio.h>
#include <unistd.h>
int main() {
pid_t pid = fork();
// forkA
if (pid == -1) {
printf("Fork failed¥n");
//
} else if (pid == 0) {
printf("Child process, pid: %d¥n",
getpid());
//
} else {
printf("Parent process, pid: %d¥n",
getpid());
}
}
$ gcc fork_sample.c && ./a.out
Parent process, pid: 72390
Child process, pid: 72391

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#ifdef __ARCH_WANT_SYS_FORK
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
return _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0);
#else
/* can not support in nommu mode */
return -EINVAL;
#endif
}
#endif

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#include <stdio.h>
#include <pthread.h>
void* print_doubled(void *x) {
int *ix = (int *)x;
printf("%d¥n", *ix * 2);
}
int main() {
pthread_t th;
int x = 100;
int ret = pthread_create(
&th, NULL, print_doubled, (void *)&x);
if (ret != 0) {
printf("thread execution failed");
return 255;
}
pthread_join(th, NULL);
return 0;
}
$ gcc -lpthread pthread_sample.c
$ ./a.out
200

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int
__pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg)

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int
__pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg)
{
const struct pthread_attr *iattr = (struct pthread_attr *) attr;
struct pthread *pd = NULL;
int err = ALLOCATE_STACK(iattr, &pd);
/* */
pd->start_routine = start_routine;
pd->arg = arg;
/* */
*newthread = (pthread_t) pd;
/* */
int retval = create_thread(pd, iattr, ...);
return retval;
}

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static int
create_thread (struct pthread *pd, const struct pthread_attr *attr,
bool *stopped_start, STACK_VARIABLES_PARMS, bool *thread_ran)
{
/* */
const int clone_flags = (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SYSVSEM
| CLONE_SIGHAND | CLONE_THREAD
| CLONE_SETTLS | CLONE_PARENT_SETTID
| CLONE_CHILD_CLEARTID
| 0);
/* */
ARCH_CLONE(&start_thread, STACK_VARIABLES_PARMS, clone_flags, pd, &pd->tid, tp,
&pd->tid);
/* */
return 0;
}

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SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
int __user *, parent_tidptr,
int __user *, child_tidptr,
unsigned long, tls)
{
return _do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr, tls);
}

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/*
* cloning flags:
*/
#define CSIGNAL 0x000000ff /* signal mask to be sent at exit */
#define CLONE_VM 0x00000100 /* set if VM shared between processes */
#define CLONE_FS 0x00000200 /* set if fs info shared between processes */
#define CLONE_FILES 0x00000400 /* set if open files shared between processes */
#define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */
/* */
#define CLONE_NEWPID 0x20000000 /* New pid namespace */
#define CLONE_NEWNET 0x40000000 /* New network namespace */
#define CLONE_IO 0x80000000 /* Clone io context */
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long _do_fork(unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr,
unsigned long tls)

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struct task_struct {
struct thread_info thread_info;
volatile long state;
void *stack;
struct mm_struct *mm;
struct mm_struct *active_mm;
pid_t pid;
pid_t tgid;
/* Filesystem information */
struct fs_struct *fs;
/* Open file information */
struct files_struct *files;
/* c */
}

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long _do_fork(unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr,
unsigned long tls)
{
struct task_struct *p;
/* */
p = copy_process(
clone_flags, stack_start, stack_size,
child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
/* */
struct pid *pid = get_task_pid(p, PIDTYPE_PID);
long nr = pid_vnr(pid);
wake_up_new_task(p);
/* */
return nr;
}

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static struct task_struct *copy_process(
unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *child_tidptr, ...)
{
struct task_struct *p; int retval;
/* s */
p = dup_task_struct(current, node);
/* s */
retval = copy_fs(clone_flags, p);
if (retval)
goto bad_fork_cleanup_files;
retval = copy_mm(clone_flags, p);
if (retval)
goto bad_fork_cleanup_signal;
/* s */
return p;
}

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static int copy_mm(unsigned long clone_flags,
struct task_struct *tsk)
{
struct mm_struct *mm, *oldmm;
/* */
oldmm = current->mm;
/* */
if (clone_flags & CLONE_VM) {
mmget(oldmm);
mm = oldmm;
goto good_mm;
}
mm = dup_mm(tsk);
good_mm:
tsk->mm = mm;
tsk->active_mm = mm;
return 0;
/* */
}

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static __always_inline struct rq *
context_switch(struct rq *rq,
struct task_struct *prev,
struct task_struct *next, /* h */)
{
struct mm_struct *mm, *oldmm;
mm = next->mm;
old_mm = prev->active_mm;
/* h */
if (!mm) {
/* h */
} else
switch_mm_irqs_off(oldmm, mm, next);
/* h */
switch_to(prev, next, prev);
barrier();
return finish_task_switch(prev);
}

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void switch_mm_irqs_off(struct mm_struct * prev, struct mm_struct *next,
struct task_struct *tsk)
{
struct mm_struct *real_prev = this_cpu_read(cpu_tlbstate.loaded_mm);
if (real_prev == next) {
/* */
return;
} else {
/* */
}
/* */
}

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