1//===-- Linux implementation of fork --------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "src/unistd/fork.h"
10
11#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
12#include "src/__support/common.h"
13#include "src/__support/macros/config.h"
14#include "src/__support/threads/fork_callbacks.h"
15#include "src/__support/threads/identifier.h"
16#include "src/__support/threads/thread.h" // For thread self object
17
18#include "src/__support/libc_errno.h"
19#include <signal.h> // For SIGCHLD
20#include <sys/syscall.h> // For syscall numbers.
21
22namespace LIBC_NAMESPACE_DECL {
23
24// The implementation of fork here is very minimal. We will add more
25// functionality and standard compliance in future.
26
27LLVM_LIBC_FUNCTION(pid_t, fork, (void)) {
28 invoke_prepare_callbacks();
29 pid_t parent_tid = internal::gettid();
30 // Invalidate parent's tid cache before forking. We cannot do this in child
31 // process because in the post-fork instruction windows, there may be a signal
32 // handler triggered which may get the wrong tid.
33 internal::force_set_tid(0);
34#ifdef SYS_fork
35 pid_t ret = syscall_impl<pid_t>(SYS_fork);
36#elif defined(SYS_clone)
37 pid_t ret = syscall_impl<pid_t>(SYS_clone, SIGCHLD, 0);
38#else
39#error "fork and clone syscalls not available."
40#endif
41
42 if (ret == 0) {
43 // Return value is 0 in the child process.
44 // The child is created with a single thread whose self object will be a
45 // copy of parent process' thread which called fork. So, we have to fix up
46 // the child process' self object with the new process' tid.
47 internal::force_set_tid(syscall_impl<pid_t>(SYS_gettid));
48 invoke_child_callbacks();
49 return 0;
50 }
51
52 if (ret < 0) {
53 // Error case, a child process was not created.
54 libc_errno = static_cast<int>(-ret);
55 return -1;
56 }
57 // recover parent's tid.
58 internal::force_set_tid(parent_tid);
59 invoke_parent_callbacks();
60 return ret;
61}
62
63} // namespace LIBC_NAMESPACE_DECL
64

source code of libc/src/unistd/linux/fork.cpp